Principles Of Inheritance And Variation

Class 12 | Pure Science — Zoology | Chapter 4

Class 12 Unit 4 Pure_Science

UNIT 2 CHAPTER 4
PRINCIPLES OF INHERITANCE AND VARIATION
Drosophila are ideal for the study of genetics and development

CHAPTER OUTLINE

4.1 Multiple alleles
4.2 Human blood groups
4.3 Genetic control of Rh factor
4.4 Sex determination
4.5 Sex linked inheritance
4.6. Karyotyping
4.7. Pedigree analysis
4.8. Mendelian disorders
4.9. Chromosomal abnormalities
4.10. Extra chromosomal inheritance
4.11 Eugenics, euphenics and euthenics

LEARNING OBJECTIVES

• Learns the inheritance of multiple alleles with reference to human blood groups.
• Understands the mechanism of sex determination in human beings, insects and birds.
• Learns about sex linked (X and Y) inherited diseases in human beings.
• Understands the Mendelian disorders and diseases associated with chromosomal abnormalities.
• Gains knowledge on extra chromosomal inheritance.
• Realises the significance of the applications of genetics in the improvement of human race.

Genetics is a branch of biology that deals with the study of heredity and variations. It describes how characteristics and features pass on from the parents to their offsprings in each successive generation. The unit of heredity is known as the gene. Gene is the inherited factor that determines the biological character of an organism. A variation is the degree by which the progeny differs from their parents. In this chapter, we are going to learn about multiple alleles with reference to the human blood groups, sex determination in man, insects and birds, sex linked inherited traits, genetic disorders and extra chromosomal inheritance. The betterment of human race can be achieved by methods like eugenics, euthenics and euphenics.

4.1 Multiple Alleles

The genetic segregations in Mendelian inheritance reveal that all genes have two alternative forms dominant and recessive alleles example. tall versus dwarf (T and t). The former is the normal allele or wild allele and the latter the mutant allele. A gene can mutate several times producing several alternative forms. When three or more alleles of a gene that control a particular trait occupy the same locus on the homologous chromosome of an organism, they are called multiple alleles and their inheritance is called multiple allelism.

4.2 Human Blood Groups

Multiple allelism occurs in humans, particularly in the inheritance of different types of blood groups. The blood group inheritance in human can be understood by learning about antigens and antibodies. The composition of blood, different types of blood groups (Blood group A, Blood group B, Blood group O) the blood antigens and antibodies were discussed in chapter 7 of class XI.

4.2.1 A, B, O Blood Types

Multiple allele inheritance of A, B, O blood groups

Blood differs chemically from person to person. When two different incompatible blood types are mixed, agglutination (clumping together) of erythrocytes (RBC) occurs. The basis of these chemical differences is due to the presence of antigens (surface antigens) on the membrane of RBC and epithelial cells.

Karl Landsteiner discovered two kinds of antigens called antigen 'A' and antigen 'B' on the surface of RBC's of human blood. Based on the presence or absence of these antigens three kinds of blood groups, type 'A', type 'B', and type 'O' (universal donor) were recognized. The fourth and the rarest blood group 'AB' (universal recipient) was discovered in 1902 by two of Landsteiner's students Von De Castelle and Sturli.

Bernstein in 1925 discovered that the inheritance of different blood groups in human beings is determined by a number of multiple allelic series. The three autosomal alleles located on chromosome 9 are concerned with the determination of blood group in any person. The gene controlling blood type has been labelled as 'L' (after the name of the discoverer, Landsteiner) or I (from isoagglutination). The I gene exists in three allelic forms, I^A, I^B and I^O.

I^A specifies A antigen. I^B allele determines B antigen and I^O allele specifies no antigen. Individuals who possess these antigens in their fluids such as the saliva are called secretors. I^A allele produces N-acetyl galactose transferase and can add N acetyl galactosamine (NAG) and I^B allele encodes for the enzyme galactose transferase that adds galactose to the precursor (that is, H substances). In the case of I^O allele no terminal transferase enzyme is produced and therefore called "null" allele and hence cannot add NAG or galactose to the precursor.

From the phenotypic combinations it is evident that the alleles I^A and I^B are dominant to I^O, but co-dominant to each other (I^A = I^B). Their dominance hierarchy can be given as (I^A = I^B > I^O).

A child receives one of three alleles from each parent, giving rise to six possible genotypes and four possible blood types (phenotypes). The genotypes are I^AI^A, I^AI^O, I^BI^B, I^BI^O, I^AI^B and I^OI^O.

Do You Know?

• Antigens similar to those found among human beings have been recognized in the blood of other organisms. A type antigens have been found in chimpanzees and in gibbons, A, B and AB antigen in orangutans.
• New world monkeys (Platyrrhina) and lemurs have a substance similar but not identical with B antigen in humans.
• Three blood groups have been distinguished in cats with a genetic system similar to those in humans.
• The secretors (antigens found in the body fluids) can be detected in tears, saliva, urine, semen, gastric juice and in the milk of animals.

Table 4.1 Genetic basis of the human A, B, O blood groups

Genotype	ABO Blood Group Phenotype	Antigens present on red blood cell	Antibodies present in blood plasma
IAIA	Type A	A	Anti B
IAIO	Type A	A	Anti B
IBIB	Type B	B	Anti A
IBIO	Type B	B	Anti A
IAIB	Type AB	A and B	Neither Anti A nor Anti B
IOIO	Type O	Neither A nor B	Anti A and Anti B

Rhesus or Rh Factor

The Rh factor or Rh antigen is found on the surface of erythrocytes. It was discovered in 1940 by Karl Landsteiner and Alexander Wiener in the blood of rhesus monkey, Macaca rhesus and later in human beings. The term 'Rh factor' refers to immunogenic D antigen of the Rh blood group system. An individual having D antigen are Rh D positive (Rh+) and those without D antigen are Rh D negative (Rh−).

Rhesus factor in the blood is inherited as a dominant trait. Naturally occurring Anti D antibodies are absent in the plasma of any normal individual. However if a Rh negative person is exposed to Rh positive blood cells (erythrocytes) for the first time, anti D antibodies are formed in the blood of that individual. On the other hand, when a Rh positive person receives Rh negative blood no effect is seen.

4.3 Genetic Control of Rh Factor

Fisher and Race Hypothesis

Rh factor involves three different pairs of alleles located on three different closely linked loci on the chromosome pair. This system is more commonly in use today, and uses the 'CDE' nomenclature. Three pairs of Rh alleles (C/c, D/d and E/e) occur at 3 different loci on homologous chromosome pair. The possible genotypes will be one dominant C or one recessive c, one dominant D or one recessive d, one dominant E or one recessive e from each chromosome.

For example: Dominant alleles CDE or recessive alleles cde; dominant allele CdE or recessive allele cDe; recessive alleles cde or recessive alleles cde; dominant allele CDe or dominant allele CdE, etc. All genotypes carrying a dominant 'D' allele will produce Rh positive phenotype and double recessive genotype 'dd' will give rise to Rh negative phenotype.

Wiener Hypothesis

Wiener proposed the existence of eight alleles (R¹, R², R⁰, R^z, r, r¹, r¹¹, r^y) at a single Rh locus. All genotypes carrying a dominant 'R allele' (R¹, R², R⁰, R^z) will produce Rh positive phenotype and double recessive genotypes (rr, r¹r¹, r¹¹r¹¹, r^yr^y) will give rise to Rh negative phenotype.

4.3.1 Incompatibility of Rh Factor — Erythroblastosis Foetalis

Rh incompatibility has great significance in child birth. If a woman is Rh negative and the man is Rh positive, the foetus may be Rh positive having inherited the factor from its father. The Rh-negative mother becomes sensitized by carrying Rh positive foetus within her body. Due to damage of blood vessels, during child birth, the mother's immune system recognizes the Rh antigens and gets sensitized. The sensitized mother produces Rh antibodies. The antibodies are Immunoglobulin G type which are small and can cross placenta and enter the foetal circulation.

By the time the mother gets sensitized and produce anti 'D' antibodies, the child is delivered. Usually no effects are associated with exposure of the mother to Rh positive antigen during the first child birth. Subsequent Rh positive children carried by the same mother, may be exposed to antibodies produced by the mother against Rh antigen, which are carried across the placenta into the foetal blood circulation. This causes haemolysis of foetal RBCs resulting in haemolytic jaundice and anaemia. This condition is known as Erythroblastosis foetalis or Haemolytic disease of the new born (HDN).

Prevention of Erythroblastosis Foetalis

If the mother is Rh negative and foetus is Rh positive, anti D antibodies should be administered to the mother at 28th and 34th week of gestation as a prophylactic measure. If the Rh-negative mother delivers Rh positive child then anti D antibodies should be administered to the mother soon after delivery. This develops passive immunity and prevents the formation of anti D antibodies in the mother's blood by destroying the Rh foetal RBC before the mother's immune system is sensitized. This has to be done whenever the woman attains pregnancy.

4.4 Sex Determination

Sex determination is the method by which the distinction between male and female is established in a species. Sex chromosomes determine the sex of the individual in dioecious or unisexual organisms. The chromosomes other than the sex chromosomes of an individual are called autosomes. Sex chromosomes may be similar (homomorphic) in one sex and dissimilar (heteromorphic) in the other. Individuals having homomorphic sex chromosomes produce only one type of gametes (homogametic) whereas heteromorphic individuals produce two types of gametes (heterogametic).

Do You Know?

The human Y chromosome is only 60 Mb in size with 60 functional genes whereas X chromosomes are 165 Mb in size with about 1,000 genes.

Chromosomal Basis of Sex Determination

Heterogametic Sex Determination

In heterogametic sex determination one of the sexes produces similar gametes and the other sex produces dissimilar gametes. The sex of the offspring is determined at the time of fertilization.

Heterogametic Males

In this method of sex determination the males are heterogametic producing dissimilar gametes while females are homogametic producing similar gametes. It is of two kinds:

• XX : XO type (example: Bugs, cockroaches and grasshoppers)
• XX : XY type (example: Human beings and Drosophila)

Heterogametic Females

In this method of sex determination the females are heterogametic producing dissimilar gametes while males are homogametic producing similar gametes. To avoid confusion with the XX:XO and XX:XY types of sex determination, the alphabets 'Z' and 'W' are used here instead of X and Y respectively. Heterogametic females are of two types:

• ZO : ZZ type (example: Moths, butterflies and domestic chickens)
• ZW : ZZ type (example: Gypsy moth, fishes, reptiles and birds)

Sex Determination in Human Beings

Genes determining sex in human beings are located on two sex chromosomes, called allosomes. In mammals, sex determination is associated with chromosomal differences between the two sexes, typically XX females and XY males. 23 pairs of human chromosomes include 22 pairs of autosomes (44AA) and one pair of sex chromosomes (XX or XY). Females are homogametic producing only one type of gamete (egg), each containing one X chromosome while the males are heterogametic producing two types of sperms with X and Y chromosomes. An independently evolved XX:XY system of sex chromosomes also exists in Drosophila.

The Y Chromosome and Male Development

Current analysis of Y chromosomes has revealed numerous genes and regions with potential genetic function; some genes with or without homologous counterparts are seen on the X. Present at both ends of the Y chromosome are the pseudoautosomal regions (PARs) that are similar with regions on the X chromosome which synapse and recombine during meiosis. The remaining 95% of the Y chromosome is referred as the Non-combining Region of the Y (NRY). The NRY is divided equally into functional genes (euchromatic) and nonfunctional genes (heterochromatic). Within the euchromatin regions, is a gene called Sex determining region Y (SRY). In humans, absence of Y chromosome inevitably leads to female development and this SRY gene is absent in X chromosome. The gene product of SRY is the testes determining factor (TDF) present in the adult male testis.

4.4.1 Genic Balance in Drosophila

Genic balance mechanisms of sex determination in Drosophila was first studied by C.B. Bridges. In Drosophila, the presence of Y chromosome is essential for the fertility of male sex, but does not determine the male sex. The gene for femaleness is located on the X chromosome and those for maleness are located on the autosomes.

When geneticist C.B. Bridges, working with Drosophila, crossed a triploid (3n) female with a normal male, he observed many combinations of autosomes and sex chromosomes in the offspring. From his results Bridges in 1921 suggested that sex in Drosophila is determined by the balance between the genes for femaleness located on the 'X' chromosomes and those for maleness located on the autosomes. Hence the sex of an individual is determined by the ratio of its X chromosome to that of its autosome sets. This ratio is termed sex index.

Table 4.2: Bridges Classical Cross of a Triploid (3A + XXX) Female Fly and a Diploid (2A + XY) Male Fly

Triploid Female Parent: 3A + XXX	Diploid Male Parent: 2A + XY
Gametes: (2A + XX), (A + X), (2A + X), (A + XX)	Gametes: (A + X), (A + Y)

Offspring Combination	Phenotype
3A + XXX	Triploid Female
3A + XXY	Triploid Intersex
3A + XX	Triploid Intersex
3A + XY	Super Male
2A + XXX	Super Female
2A + XXY	Diploid Female
2A + XX	Diploid Female
2A + XY	Diploid Male

Box Item

• X Chromosome was discovered by Henking (1891)
• Y Chromosome was discovered by Stevens (1902)

When the X to A ratio is 1.00 as in a normal female, or greater than 1.00, the organism is a female. When this ratio is 0.50 as in a normal male or less than 0.50 the organism is a male. At 0.67, the organism is an intersex. Metamales (X to A ratio is 0.33) and metafemales (X to A ratio is 1.50) are usually very weak and sterile.

A sex switch gene in Drosophila directs female development. This gene, sex lethal (SxL) located on the X chromosome, has two states of activity. When it is 'on' it directs female development and when it is 'off' maleness ensures. Other genes located on the X chromosome and autosomes regulate this sex switch gene. However, the Y chromosome of Drosophila is required for male fertility.

Gynandromorphs

These individuals have parts of their body expressing male characters and other parts of the body expressing female characters. The organism is made up of tissues of male and female genotype and represents a mosaic pattern.

4.4.2 Dosage Compensation — Barr Body

In 1949, Barr and Bertram first observed a condensed body in the nerve cells of female cat which was absent in the male. This condensed body was called sex chromatin by them and was later referred as Barr body. In the XY chromosomal system of sex determination, males have only one X chromosome, whereas females have two. A question arises: how does the organism compensate for this dosage differences between the sexes?

In mammals the necessary dosage compensation is accomplished by the inactivation of one of the X chromosome in females so that both males and females have only one functional X chromosome per cell. Mary Lyon suggested that Barr bodies represented an inactive chromosome, which in females becomes tightly coiled into a heterochromatin, a condensed and visible form of chromatin (Lyon's hypothesis). The number of Barr bodies observed in cell was one less than the number of X Chromosome. XO females have no Barr body, whereas XXY males have one Barr body.

Box Item

The number of Barr bodies follows N−1 rule (N minus one rule), where N is the total number of X chromosomes present.

Table 4.3: Different Doses of X Chromosomes and Autosome Sets and Their Effect on Sex Determination in Drosophila

Phenotype	Number of 'X' Chromosomes (X)	Number of Autosome Sets (A)	Sex Index (X/A)
Meta female or Super female	3	2	3/2 = 1.5
Tetraploid Normal Female	4	4	4/4 = 1.0
Triploid Normal Female	3	3	3/3 = 1.0
Diploid Normal Female	2	2	2/2 = 1.0
Haploid Normal Female	1	1	1/1 = 1.0
Inter sex	2	3	2/3 = 0.67
Normal male	1	2	1/2 = 0.50
Meta male or Super male	1	3	1/3 = 0.33

Haplodiploidy in Honeybees

In hymenopteran insects such as honeybees, ants and wasps a mechanism of sex determination called haplodiploidy mechanism of sex determination is common. In this system, the sex of the offspring is determined by the number of sets of chromosomes it receives. Fertilized eggs develop into females (Queen or Worker) and unfertilized eggs develop into males (drones) by parthenogenesis. It means that the males have half the number of chromosomes (haploid) and the females have double the number (diploid), hence the name haplodiploidy for this system of sex determination.

This mode of sex determination facilitates the evolution of sociality in which only one diploid female becomes a queen and lays the eggs for the colony. All other females which are diploid having developed from fertilized eggs help to raise the queen's eggs and so contribute to the queen's reproductive success and indirectly to their own, a phenomenon known as Kin Selection. The queen constructs their social environment by releasing a hormone that suppresses fertility of the workers.

4.5 Sex Linked Inheritance

The inheritance of a trait that is determined by a gene located on one of the sex chromosomes is called sex linked inheritance. Genes present on the differential region of X or Y chromosomes are called sex linked genes. The genes present in the differential region of "X" chromosome are called X linked genes. The X linked genes have no corresponding alleles in the Y chromosome. The genes present in the differential region of Y chromosome are called Y linked or holandric genes. The Y linked genes have no corresponding allele in X chromosome. The Y linked genes inherit along with Y chromosome and they phenotypically express only in the male sex.

Sex linked inherited traits are more common in males than females because, males are hemizygous and therefore express the trait when they inherit one mutant allele. The X linked and Y linked genes in the differential region (non homologous region) do not undergo pairing or crossing over during meiosis. The inheritance of X or Y linked genes is called sex linked inheritance.

4.5.1 Inheritance of X Linked Genes

Red-green colour blindness or daltonism, haemophilia and Duchenne's muscular dystrophy are examples of X linked gene inheritance in humans.

1. Haemophilia

Haemophilia is commonly known as bleeder's disease, which is more common in men than women. This hereditary disease was first reported by John Cotto in 1803. Haemophilia is caused by a recessive X linked gene. A person with a recessive gene for haemophilia lacks a normal clotting substance (thromboplastin) in blood, hence minor injuries cause continuous bleeding, leading to death. The females are carriers of the disease and would transmit the disease to 50% of their sons even if the male parent is normal. Haemophilia follows the characteristic criss cross pattern of inheritance.

2. Colour Blindness

In human beings a dominant X linked gene is necessary for the formation of colour sensitive cells, the cones. The recessive form of this gene is incapable of producing colour sensitive cone cells. Homozygous recessive females (X^cX^c) and hemizygous recessive males (X^cY) are unable to distinguish red and green colour. The inheritance of colour blindness can be studied in the following two types of marriages.

a) Marriage Between Colour Blind Man and Normal Visioned Woman

A marriage between a colour blind man and a normal visioned woman will produce normal visioned male and female individuals in F1 generation but the females are carriers. The marriage between a F1 normal visioned carrier woman and a normal visioned male will produce one normal visioned female, one carrier female, one normal visioned male and one colour blind male in F2 generation. Colour blind trait is inherited from the male parent to his grandson through carrier daughter, which is an example of criss-cross pattern of inheritance.

b) Marriage Between Normal Visioned Man and Colour Blind Woman

If a colour blind woman (X^cX^c) marries a normal visioned male (X⁺Y), all F1 sons will be colourblind and daughters will be normal visioned but are carriers. Marriage between F1 carrier female with a colour blind male will produce normal visioned carrier daughter, colourblind daughter, normal visioned son and a colourblind son in the F2 generation.

4.5.2 Inheritance of Y-Linked Genes

Genes in the non-homologous region of the Y chromosome are inherited directly from male to male. In humans, the Y linked or holandric genes for hypertrichosis (excessive development of hairs on pinna of the ear) are transmitted directly from father to son, because males inherit the Y chromosome from the father. Female inherits only X chromosome from the father and are not affected.

4.6 Karyotyping

Karyotyping is a technique through which a complete set of chromosomes is separated from a cell and the chromosomes are arranged in pairs. An idiogram refers to a diagrammatic representation of chromosomes.

Preparation of Karyotype

Tjio and Levan (1960) described a simple method of culturing lymphocytes from the human blood. Mitosis is induced followed by addition of colchicine to arrest cell division at metaphase stage and the suitable spread of metaphase chromosomes is photographed. The individual chromosomes are cut from the photograph and are arranged in an orderly fashion in homologous pairs. This arrangement is called a karyotype. Chromosome banding permits structural definitions and differentiation of chromosomes.

Applications of Karyotyping

• It helps in gender identification.
• It is used to detect the chromosomal aberrations like deletion, duplication, translocation, nondisjunction of chromosomes.
• It helps to identify the abnormalities of chromosomes like aneuploidy.
• It is also used in predicting the evolutionary relationships between species.
• Genetic diseases in human beings can be detected by this technique.

Human Karyotype

Depending upon the position of the centromere and relative length of two arms, human chromosomes are of three types: Metacentric, sub metacentric and acrocentric. The photograph of chromosomes are arranged in the order of descending length in groups from A to G.

4.7 Pedigree Analysis

Pedigree is a "family tree", drawn with standard genetic symbols, showing the inheritance pathway for specific phenotypic characters. Pedigree analysis is the study of traits as they have appeared in a given family line for several past generations.

Genetic Disorders

A genetic disorder is a disease or syndrome that is caused by an abnormality in an individual DNA. Abnormalities can range from a small mutation in a single gene to the addition or subtraction of an entire chromosome or even a set of chromosomes. Genetic disorders are of two types namely, Mendelian disorders and chromosomal disorders.

4.8 Mendelian Disorders

Alteration or mutation in a single gene causes Mendelian disorders. These disorders are transmitted to the offsprings on the same line as the Mendelian pattern of inheritance. Some examples for Mendelian disorders are Thalassemia, albinism, phenylketonuria, sickle cell anaemia, Huntington's chorea, etc. These disorders may be dominant or recessive and autosomal or sex linked.

Thalassemia

Thalassemia is an autosomal recessive disorder. It is caused by gene mutation resulting in excessive destruction of RBC's due to the formation of abnormal haemoglobin molecules. Normally haemoglobin is composed of four polypeptide chains, two alpha and two beta globin chains. Thalassemia patients have defects in either the alpha or beta globin chain causing the production of abnormal haemoglobin molecules resulting in anaemia.

Thalassemia is classified into alpha and beta based on which chain of haemoglobin molecule is affected. It is controlled by two closely linked genes HBA1 and HBA2 on chromosome 16. Mutation or deletion of one or more of the four alpha gene alleles causes Alpha Thalassemia.

In Beta Thalassemia, production of beta globin chain is affected. It is controlled by a single gene (HBB) on chromosome 11. It is the most common type of Thalassemia and is also known as Cooley's anaemia. In this disorder the alpha chain production is increased and damages the membranes of RBC.

Phenylketonuria

It is an inborn error of Phenylalanine metabolism caused due to a pair of autosomal recessive genes. It is caused due to mutation in the gene PAH (phenylalanine hydroxylase gene) located on chromosome 12 for the hepatic enzyme "phenylalanine hydroxylase". This enzyme is essential for the conversion of phenylalanine to tyrosine. Affected individual lacks this enzyme, so phenylalanine accumulates and gets converted to phenylpyruvic acid and other derivatives. It is characterized by severe mental retardation, light pigmentation of skin and hair. Phenylpyruvic acid is excreted in the urine.

Albinism

Albinism is an inborn error of metabolism, caused due to an autosomal recessive gene. Melanin pigment is responsible for skin colour. Absence of melanin results in a condition called albinism. A person with the recessive allele lacks the tyrosinase enzyme system, which is required for the conversion of dihydroxyphenyl alanine (DOPA) into melanin pigment inside the melanocytes. In an albino, melanocytes are present in normal numbers in their skin, hair, iris, etc., but lack melanin pigment.

Huntington's Chorea

It is inherited as an autosomal dominant lethal gene in man. It is characterized by involuntary jerking of the body and progressive degeneration of the nervous system, accompanied by gradual mental and physical deterioration. The patients with this disease usually die between the age of 35 and 40.

4.9 Chromosomal Abnormalities

Each human diploid (2n) body cell has 46 chromosomes (23 pairs). Chromosomal disorders are caused by errors in the number or structure of chromosomes. Chromosomal anomalies usually occur when there is an error in cell division. Failure of chromatids to segregate during cell division resulting in the gain or loss of one or more chromosomes is called aneuploidy. It is caused by nondisjunction of chromosomes. Group of signs and symptoms that occur together and characterize a particular abnormality is called a syndrome. In humans, Down's syndrome, Turner's syndrome, Klinefelter's syndrome, Patau's syndrome are some of the examples of chromosomal disorders.

a. Autosomal Aneuploidy in Human Beings

Several autosomal aneuploidies have been reported in human beings. Examples: Down's syndrome (21 Trisomy), Patau's syndrome (13 Trisomy).

1. Down's Syndrome or Trisomy 21

Trisomic condition of chromosome 21 results in Down's syndrome. It is characterized by severe mental retardation, defective development of the central nervous system, increased separation between the eyes, flattened nose, ears are malformed, mouth is constantly open and the tongue protrudes.

2. Patau's Syndrome or Trisomy 13

Trisomic condition of chromosome 13 results in Patau's syndrome. Meiotic nondisjunction is thought to be the cause for this chromosomal abnormality. It is characterized by multiple and severe body malformations as well as profound mental deficiency. Small head with small eyes, cleft palate, malformation of the brain and internal organs are some of the symptoms of this syndrome.

b. Allosomal Abnormalities in Human Beings

Mitotic or meiotic non-disjunction of sex chromosomes causes allosomal abnormalities. Several sex chromosomal abnormalities have been detected. Example: Klinefelter's syndrome and Turner's syndrome.

1. Klinefelter's Syndrome (XXY Males)

This genetic disorder is due to the presence of an additional copy of the X chromosome resulting in a karyotype of 47, XXY. Persons with this syndrome have 47 chromosomes (44AA + XXY). They are usually sterile males, tall, obese, with long limbs, high pitched voice, under developed genitalia and have feeble breast (gynaecomastia) development.

2. Turner's Syndrome (XO Females)

This genetic disorder is due to the loss of a X chromosome resulting in a karyotype of 45, X. Persons with this syndrome have 45 chromosomes (44 autosomes and one X chromosome) (44AA + XO) and are sterile females. Low stature, webbed neck, under developed breast, rudimentary gonads, lack of menstrual cycle during puberty, are the main symptoms of this syndrome.

4.10 Extra Chromosomal or Cytoplasmic Inheritance

Certain characters are controlled by non-nuclear genomes found in chloroplast, mitochondria, infective agents and plasmids. These characters do not reveal Mendelian pattern of inheritance. The inheritance of the extra chromosomal genes is found to exhibit maternal influence. Maternal effect is due to the asymmetric contribution of the female parent to the development of zygote. Although both male and female parents contribute equally to the zygote in terms of chromosomal genes, the female parent usually contributes the zygote's initial cytoplasm and organelles, since the sperms contain very little cytoplasm.

If there are hereditary units in the cytoplasm, these will be transmitted to the offsprings through the egg, so the offsprings exhibit maternal effect. The cytoplasmic extranuclear genes have a characteristic pattern of inheritance which do not resemble the genes of nuclear chromosomes and is known as extra chromosomal or extra nuclear or cytoplasmic inheritance and exhibit maternal influence.

In extra nuclear inheritance, male and female parents contribute equally their nuclear genes to the progeny but do not make equal contribution of extra chromosomal genes hence, the crosses can yield different (or) non-Mendelian results. Cytoplasmic inheritance in animals can be studied with reference to shell coiling in Limnaea and kappa particles in Paramecium.

4.11 Eugenics, Euphenics and Euthenics

Eugenics

Application of the laws of genetics for the improvement of human race is called eugenics. The term eugenics means "well born" and was coined by Francis Galton in 1885. For the betterment of future generations it is necessary to increase the population of outstanding people and to decrease the population of abnormal and defective people by applying the principles of eugenics.

Two Methods of Eugenics

(1) Constructive method or Positive eugenics
(2) Restrictive method or Negative eugenics

(1) Positive Eugenics

Positive eugenics attempts to increase consistently better or desirable germplasm and to preserve the best germplasm of the society. The desirable traits can be increased by adopting the following measures:

1. Early marriage of those having desirable traits
2. Subsiding the fit and establishing sperm and egg banks of precious germplasm
3. Educating the basic principles of genetics and eugenics
4. Improvement of environmental conditions
5. Promotion of genetic research

(2) Negative Eugenics

Negative Eugenics attempts to eliminate the defective germplasm of the society by adopting the following measures:

1. Sexual separation of the defectives
2. Sterilization of the defectives
3. Control of immigration
4. Regulation of marriages

Euphenics

The symptomatic treatment of genetic disease of man is called Euphenics or Medical engineering. In 1960 Joshua Lederberg coined the term Euphenics. It means normal appearing. It deals with the control of several inherited human diseases especially the inborn errors of metabolism. Example: Phenylketonuria (PKU).

Euthenics

The science of improvement of existing human race by improving the environmental conditions is called euthenics. It can be achieved by subjecting them to better nutrition, better unpolluted ecological conditions, better education and sufficient medical facilities.

Summary

• Genetics is a branch of biology that deals with the study of heredity and variation. It describes how characteristics and features pass on from the parents to their offsprings in successive generations.
• Variation is the degree by which progeny differ from their parents.
• A set of three or more alleles of the same gene occupying the same locus in a given pair of homologous chromosomes controlling a particular trait is called Multiple allele. ABO blood grouping in man is a good example for multiple allelism.
• Apart from A and B antigens, the RBC's of humans contain a special type of antigen called Rh antigen or Rh factors.
• Erythroblastosis foetalis, also called haemolytic disease of the newborn, in which the red blood cells of a foetus are destroyed due to maternal immune reaction resulting from a blood group incompatibility between the foetus and the mother.
• The mechanism of determination of male and female individuals in a species is called sex determination. The chromosomes are different in two sexes and referred to as allosomes; the remaining chromosomes are named autosomes.
• The inheritance of a trait that is determined by a gene located on one of the sex chromosomes is called sex linked inheritance. Haemophilia, colourblindness, muscular dystrophy are some examples for X linked inheritance in human beings.
• Pedigree analysis is the study of traits as they have appeared in a given family line for several generations.
• The genetic disorders are of two types — Mendelian and chromosomal.
• Alterations or mutation in single gene causes Mendelian disorders like thalassemia, albinism, phenylketonuria, and Huntington's chorea.
• Chromosomal abnormalities arise due to chromosomal non-disjunction, translocation, deletion, duplication and inversion.
• Down's syndrome, Klinefelter's syndrome, Turner's syndrome and Patau's syndrome are some of the chromosomal disorders.
• Down's syndrome is due to trisomy of chromosome 21.
• Presence of trisomic condition of chromosome 13 results in Patau's syndrome.
• In Turner's syndrome the sex chromosome is XO and in Klinefelter's syndrome the condition is XXY.
• An idiogram refers to a diagrammatic representation of chromosomes.
• The cytoplasmic extra nuclear genes have a characteristic pattern of inheritance which does not resemble genes of nuclear chromosomes and are known as Extrachromosomal or Cytoplasmic inheritance.
• The betterment of human race can be achieved by methods like Eugenics, Euthenics and Euphenics.

Evaluation

MCQ Multiple Choice Questions

1. Haemophilia is more common in males because it is a

a) Recessive character carried by Y chromosome b) Dominant character carried by Y chromosome c) Dominant trait carried by X chromosome d) Recessive trait carried by X chromosome

2. ABO blood group in man is controlled by

a) Multiple alleles b) Lethal genes c) Sex linked genes d) Y linked genes

3. Three children of a family have blood groups A, AB and B. What could be the genotypes of their parents?

a) I^AI^B and I^oI^o b) I^AI^o and I^BI^o c) I^BI^B and I^AI^A d) I^AI^A and I^oI^o

4. Which of the following is not correct?

a) Three or more alleles of a trait in the population are called multiple alleles. b) A normal gene undergoes mutations to form many alleles. c) Multiple alleles map at different loci of a chromosome. d) A diploid organism has only two alleles out of many in the population.

5. Which of the following phenotypes in the progeny are possible from the parental combination A cross with B?

a) A and B only b) A, B and AB only c) AB only d) A, B, AB and O

6. Which of the following phenotypes is not possible in the progeny of the parental genotypic combination I^AI^O cross with I^AI^B?

a) AB b) O c) A d) B

7. Which of the following is true about Rh factor in the offspring of a parental combination Dd × Dd (both Rh positive)?

a) All will be Rh positive b) Half will be Rh positive c) About three fourth will be Rh negative d) About one fourth will be Rh negative

8. What can be the blood group of offspring when both parents have AB blood group?

a) AB only b) A, B and AB c) A, B, AB and O d) A and B only

9. If the child's blood group is 'O' and father's blood group is 'A' and mother's blood group is 'B', the genotype of the parents will be

a) I^AI^A and I^BI^o b) I^AI^o and I^BI^o c) I^AI^o and I^oI^o d) I^oI^o and I^BI^B

10. XO type of sex determination and XY type of sex determination are examples of

a) Male heterogamety b) Female heterogamety c) Male homogamety d) Both (b) and (c)

11. In an accident there is great loss of blood and there is no time to analyse the blood group. Which blood can be safely transferred?

a) O and Rh negative b) O and Rh positive c) B and Rh negative d) AB and Rh positive

12. Father of a child is colourblind and mother is carrier for colourblindness, the probability of the child being colourblind is

a) 25% b) 50% c) 100% d) 75%

13. A marriage between a colourblind man and a normal woman produces

a) All carrier daughters and normal sons b) 50% carrier daughters and 50% normal daughters c) 50% colourblind sons and 50% normal sons d) All carrier offsprings

14. Down's syndrome is a genetic disorder which is caused by the presence of an extra chromosome number

a) 20 b) 21 c) 4 d) 23

15. Klinefelter's syndrome is characterized by a karyotype of

a) XYY b) XO c) XXX d) XXY

16. Females with Turner's syndrome have

a) Small uterus b) Rudimentary ovaries c) Underdeveloped breasts d) All of these

17. Patau's syndrome is also referred to as

a) 13 Trisomy b) 18 Trisomy c) 21 Trisomy d) None of these

18. "Universal Donor" and "Universal Recipient" blood groups are _____ and _____ respectively

a) AB, O b) O, AB c) A, B d) B, A

19. ZW : ZZ system of sex determination occurs in

a) Fishes b) Reptiles c) Birds d) All of these

20. Co-dominant blood group is

a) A b) AB c) B d) O

21. Which of the following is incorrect regarding ZW : ZZ type of sex determination?

a) It occurs in birds and some reptiles b) Females are homogametic and males are heterogametic c) Male produce two types of gametes d) It occurs in gypsy moth

22. Who is the founder of Modern Eugenics movement?

a) Mendel b) Darwin c) Francis Galton d) Karl Pearson

23. Improvement of human race by encouraging the healthy persons to marry early and produce large number of children is called

a) Positive eugenics b) Negative eugenics c) Positive euthenics d) Positive euphenics

24. The _____ deals with the control of several inherited human diseases especially inborn errors of metabolism

a) Euphenics b) Eugenics c) Euthenics d) All of these Exercise Short Answer Questions

25. What is haplodiploidy?

26. Distinguish between heterogametic and homogametic sex determination systems.

27. What is Lyonisation?

28. What is criss-cross inheritance?

29. Why are sex linked recessive characters more common in the male human beings?

30. What are holandric genes?

31. Mention the symptoms of Phenylketonuria.

32. Mention the symptoms of Down's syndrome.

35. What is male heterogamety?

Exercise Long Answer Questions

33. Explain the genetic basis of ABO blood grouping in man.

34. How is sex determined in human beings?

36. Brief about female heterogamety.

37. Give an account of genetic control of Rh factor.

38. Explain the mode of sex determination in honeybees.

39. What are the applications of Karyotyping?

40. Explain the inheritance of sex linked characters in human being.

41. What is extra chromosomal inheritance?

42. Differentiate Intersexes from Supersexes.

43. Discuss the genic balance mechanism of sex determination with reference to Drosophila.

44. Comment on the methods of Eugenics.

Book-back Exercises — Questions & Answers

Answers generated strictly from the Samacheer Kalvi textbook content. Attempt each question first, then click "Show Answer" to check.

I. Choose the Correct Answer

1. Haemophilia is more common in males because it is a

a) Recessive character carried by Y chromosome b) Dominant character carried by Y chromosome c) Dominant trait carried by X chromosome d) Recessive trait carried by X chromosome

Answer: d) Recessive trait carried by X chromosome

Explanation: Haemophilia is caused by a recessive X linked gene. Males are hemizygous and therefore express the trait when they inherit one mutant allele.

2. ABO blood group in man is controlled by

a) Multiple alleles b) Lethal genes c) Sex linked genes d) Y linked genes

Answer: a) Multiple alleles

Explanation: Multiple allelism occurs in humans, particularly in the inheritance of different types of blood groups. The I gene exists in three allelic forms, I^A, I^B and I^O.

3. Three children of a family have blood groups A, AB and B. What could be the genotypes of their parents?

a) I^AI^B and I^OI^O b) I^AI^O and I^BI^O c) I^BI^B and I^AI^A d) I^AI^A and I^OI^O

Answer: b) I^AI^O and I^BI^O

Explanation: A child receives one of three alleles from each parent. Parents with genotypes I^AI^O and I^BI^O can produce children with blood groups A, B, AB and O.

4. Which of the following is not correct?

a) Three or more alleles of a trait in the population are called multiple alleles. b) A normal gene undergoes mutations to form many alleles. c) Multiple alleles map at different loci of a chromosome. d) A diploid organism has only two alleles out of many in the population.

Answer: c) Multiple alleles map at different loci of a chromosome.

Explanation: Multiple alleles occupy the same locus on the homologous chromosome, not different loci. When three or more alleles of a gene occupy the same locus they are called multiple alleles.

5. Which of the following phenotypes in the progeny are possible from the parental combination A cross with B?

a) A and B only b) A, B and AB only c) AB only d) A, B, AB and O

Answer: d) A, B, AB and O

Explanation: If parent A has genotype I^AI^O and parent B has genotype I^BI^O, the offspring can have blood groups A, B, AB and O.

6. Which of the following phenotypes is not possible in the progeny of the parental genotypic combination I^AI^O cross with I^AI^B?

a) AB b) O c) A d) B

Answer: b) O

Explanation: From the cross I^AI^O × I^AI^B, possible genotypes are I^AI^A, I^AI^B, I^AI^O, I^BI^O — giving blood groups A, AB, A and B respectively. Blood group O (I^OI^O) is not possible.

7. Which of the following is true about Rh factor in the offspring of a parental combination Dominant D cross with recessive d × (both Rh positive)?

a) All will be Rh positive b) Half will be Rh positive c) About three fourth will be Rh negative d) About one fourth will be Rh negative

Answer: d) About one fourth will be Rh negative

Explanation: Rhesus factor is inherited as a dominant trait. In a Dd × Dd cross, 3/4 offspring will be Rh positive (DD or Dd) and 1/4 (dd) will be Rh negative.

8. What can be the blood group of offspring when both parents have AB blood group?

a) AB only b) A, B and AB c) A, B, AB and O d) A and B only

Answer: b) A, B and AB

Explanation: Both parents are I^AI^B. The cross I^AI^B × I^AI^B produces I^AI^A (blood group A), I^AI^B (blood group AB) and I^BI^B (blood group B). Blood group O cannot be produced.

9. If the child's blood group is 'O' and father's blood group is 'A' and mother's blood group is 'B', the genotype of the parents will be

a) I^AI^A and I^BI^O b) I^AI^O and I^BI^O c) I^AI^O and I^OI^O d) I^OI^O and I^BI^B

Answer: b) I^AI^O and I^BI^O

Explanation: For a child to have blood group O (I^OI^O), each parent must contribute one I^O allele. So the father (blood group A) must be I^AI^O and the mother (blood group B) must be I^BI^O.

10. XO type of sex determination and XY type of sex determination are examples of

a) Male heterogamety b) Female heterogamety c) Male homogamety d) Both (b) and (c)

Answer: a) Male heterogamety

Explanation: In XX-XO type (e.g., bugs, cockroaches) and XX-XY type (e.g., humans and Drosophila), males are heterogametic producing dissimilar gametes while females are homogametic.

11. In an accident there is great loss of blood and there is no time to analyse the blood group, which blood can be safely transferred?

a) O and Rh negative b) O and Rh positive c) B and Rh negative d) AB and Rh positive

Answer: a) O and Rh negative

Explanation: Blood group O is the universal donor. Rh negative blood can be given to both Rh positive and Rh negative individuals safely, making O Rh negative the safest choice.

12. Father of a child is colourblind and mother is carrier for colourblindness, the probability of the child being colourblind is

a) 25% b) 50% c) 100% d) 75%

Answer: b) 50%

Explanation: The carrier mother (X⁺X^c) × colourblind father (X^cY) produces X⁺X^c (carrier female), X^cX^c (colourblind female), X⁺Y (normal male) and X^cY (colourblind male). Thus 50% of the offspring will be colourblind.

13. A marriage between a colourblind man and a normal woman produces

a) All carrier daughters and normal sons b) 50% carrier daughters and 50% normal daughters c) 50% colourblind sons and 50% normal sons d) All carrier offsprings

Answer: a) All carrier daughters and normal sons

Explanation: A marriage between a colour blind man and a normal visioned woman will produce normal visioned male and female individuals in F1 generation but the females are carriers.

14. Down's syndrome is a genetic disorder which is caused by the presence of an extra chromosome number

a) 20 b) 21 c) 4 d) 23

Answer: b) 21

Explanation: Trisomic condition of chromosome 21 results in Down's syndrome. It is characterized by severe mental retardation and defective development of the central nervous system.

15. Klinefelter's syndrome is characterized by a karyotype of

a) XYY b) XO c) XXX d) XXY

Answer: d) XXY

Explanation: Klinefelter's syndrome is due to the presence of an additional copy of the X chromosome resulting in a karyotype of 47, XXY. Persons with this syndrome have 47 chromosomes (44AA + XXY).

16. Females with Turner's syndrome have

a) Small uterus b) Rudimentary ovaries c) Underdeveloped breasts d) All of these

Answer: d) All of these

Explanation: Turner's syndrome symptoms include low stature, webbed neck, underdeveloped breast, rudimentary gonads and lack of menstrual cycle during puberty.

17. Patau's syndrome is also referred to as

a) 13 Trisomy b) 18 Trisomy c) 21 Trisomy d) None of these

Answer: a) 13 Trisomy

Explanation: Trisomic condition of chromosome 13 results in Patau's syndrome. It is characterized by multiple and severe body malformations as well as profound mental deficiency.

18. "Universal Donor" and "Universal Recipient" blood groups are ___ and ___ respectively

a) AB, O b) O, AB c) A, B d) B, A

Answer: b) O, AB

Explanation: Blood group O is the universal donor and blood group AB is the universal recipient as stated in the chapter describing the discovery of blood groups by Landsteiner.

19. ZW-ZZ system of sex determination occurs in

a) Fishes b) Reptiles c) Birds d) All of these

Answer: d) All of these

Explanation: ZW-ZZ type of sex determination is seen in gypsy moth, fishes, reptiles and birds as mentioned in the chapter.

20. Co-dominant blood group is

a) A b) AB c) B d) O

Answer: b) AB

Explanation: The alleles I^A and I^B are dominant to I^O, but co-dominant to each other (I^A = I^B). When both are present, the blood group is AB.

21. Which of the following is incorrect regarding ZW-ZZ type of sex determination?

a) It occurs in birds and some reptiles b) Females are homogametic and males are heterogametic c) Male produce two types of gametes d) It occurs in gypsy moth

Answer: b) Females are homogametic and males are heterogametic

Explanation: In ZW-ZZ type, females are heterogametic (ZW) producing dissimilar gametes and males are homogametic (ZZ). So the statement that females are homogametic is incorrect.

22. Who is the founder of Modern Eugenics movement?

a) Mendel b) Darwin c) Francis Galton d) Karl Pearson

Answer: c) Francis Galton

Explanation: The term eugenics means "well born" and was coined by Francis Galton in 1885.

23. Improvement of human race by encouraging the healthy persons to marry early and produce large number of children is called

a) Positive eugenics b) Negative eugenics c) Positive euthenics d) Positive euphenics

Answer: a) Positive eugenics

Explanation: Positive eugenics attempts to increase consistently better or desirable germplasm, including early marriage of those having desirable traits and subsiding the fit to preserve the best germplasm of society.

24. The ___ deals with the control of several inherited human diseases especially inborn errors of metabolism

a) Euphenics b) Eugenics c) Euthenics d) All of these

Answer: a) Euphenics

Explanation: Euphenics or Medical engineering deals with the control of several inherited human diseases especially the inborn errors of metabolism. The term was coined by Joshua Lederberg in 1960.

II. Answer Briefly

25. What is haplodiploidy?

Answer: Haplodiploidy is a mechanism of sex determination common in hymenopteran insects such as honeybees, ants and wasps. In this system, fertilized eggs develop into females (diploid) and unfertilized eggs develop into males (haploid) by parthenogenesis. Males have half the number of chromosomes and females have double the number, hence the name haplodiploidy.

26. Distinguish between heterogametic and homogametic sex determination systems.

Answer: Individuals having homomorphic sex chromosomes produce only one type of gametes and are called homogametic (e.g., XX females in humans). Individuals having heteromorphic sex chromosomes produce two types of gametes and are called heterogametic (e.g., XY males in humans). In heterogametic sex determination, the sex of the offspring is determined at the time of fertilization.

27. What is Lyonisation?

Answer: Mary Lyon suggested that Barr bodies represented an inactive X chromosome, which in females becomes tightly coiled into heterochromatin — a condensed and visible form of chromatin. This inactivation of one X chromosome in females, so that both males and females have only one functional X chromosome per cell, is known as Lyonisation (Lyon's hypothesis). The number of Barr bodies is one less than the number of X chromosomes.

28. What is criss-cross inheritance?

Answer: Criss-cross inheritance is the pattern where a trait is inherited from the male parent to his grandson through a carrier daughter. In colour blindness, the colour blind trait is inherited from the father to his grandson through a carrier daughter, which is a classic example of criss-cross pattern of inheritance. Haemophilia also follows this characteristic criss-cross pattern.

29. Why are sex linked recessive characters more common in the male human beings?

Answer: Sex linked recessive characters are more common in males because males are hemizygous for X-linked genes — they have only one X chromosome. Therefore, males express the trait when they inherit even one mutant allele on their single X chromosome. Females, having two X chromosomes, need two copies of the recessive allele to express the trait.

30. What are holandric genes?

Answer: Genes present in the differential (non-homologous) region of the Y chromosome are called Y-linked or holandric genes. These genes have no corresponding allele in the X chromosome and are inherited directly from father to son. In humans, the gene for hypertrichosis (excessive development of hairs on pinna of the ear) is a Y-linked holandric gene.

📝 Summary

1. Genetics is the branch of biology dealing with heredity and variations, where the gene is the basic unit of heredity that determines the biological character of an organism and variations describe how progeny differ from their parents.
2. Multiple allelism occurs when three or more alleles of a gene occupy the same locus on homologous chromosomes; a key example is the inheritance of human ABO blood groups, controlled by three autosomal alleles (I^A, I^B, and I^O) located on chromosome 9.
3. Karl Landsteiner discovered antigen 'A' and antigen 'B' on the surface of RBCs; blood group 'O' is the universal donor and blood group 'AB' (discovered in 1902 by Von De Castelle and Sturli) is the universal recipient.
4. The chapter covers sex determination mechanisms in humans, insects, and birds, as well as sex-linked inheritance, karyotyping, pedigree analysis, Mendelian disorders, and chromosomal abnormalities.
5. The betterment of the human race is explored through the applications of genetics, specifically through the methods of eugenics, euthenics, and euphenics, along with an understanding of extra chromosomal inheritance.