Tamilnadu Board Class 12 Zoology Chapter 4

Download Tamilnadu Board Class 12 Zoology Chapter 4

Preview text


Principles of Inheritance and Variation

Chapter outline
4.1 Multiple alleles 4.2 Human blood groups 4.3 Genetic control of Rh factor 4.4 Sex determination in human, insects and birds 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.

Drosophila are ideal for the study of genetics and development
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 e.g. 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.

TN_GOVT_XII_Zoology_chapter 4.indd 47

25-02-2019 18:21:56

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 (ABO) the blood antigens and antibodies were discussed in chapter 7 of class XI.
4.2.1 ABO blood types
Multiple allele inheritance of ABO 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 labeled as ‘L’ (after the name of the discoverer, Landsteiner) or I (from isoagglutination). The I gene exists in three allelic forms, IA, IB and IO. IA specifies A antigen. IB allele
Principles Of Inheritance And Variation 48

determines B antigen and IO allele specifies no antigen. Individuals who possess these antigens in their fluids such as the saliva are called secretors.
Each allele (IA and IB) produces a transferase enzyme. IA allele produces N-acetyl galactose transferase and can add N-acetyl galactosamine (NAG) and IB allele encodes for the enzyme galactose transferase that adds galactose to the precursor (i.e. H substances) In the case of IO/IO 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 IA and IB are dominant to IO, but co-dominant to each other (IA=IB). Their dominance hierarchy can be given as (IA=IB> IO). 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 IAIA, IA IO, IBIB, IB IO, IAIB and IO IO.

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.

TN_GOVT_XII_Zoology_chapter 4.indd 48

25-02-2019 18:21:57

Table 4.1 Genetic basis of the human ABO blood groups


ABO blood group

Antigens Antibodies

present on present

red blood in blood




Type A


Anti -B


Type A


Anti -B


Type B


Anti -A


Type B


Anti -A

Neither IAIB Type AB A and B Anti
-A nor Anti-B


Type O


Anti -A and anti

A nor 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 an Rh- (Rh negative) person is exposed to Rh+ (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 an 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.

C or c

C or c

D or d

D or d

E or e

E or e

Fig. 4.1 Fischer and Race hypothesis – Rh Blood Type - Homologous Chromosome pair
(showing 3 loci and 2 alleles per locus)
In the above Fig. 4.1, three pairs of Rh alleles (Cc, Dd and Ee) occur at 3 different loci on homologous chromosome pair-1. The possible genotypes will be one C or c, one D or d, one E or e from each chromosome. For e.g. CDE/cde; CdE/cDe; cde/cde; CDe/CdE etc., All genotypes carrying a dominant ‘D’ allele will produce Rh+positive phenotype and double recessive genotype ‘dd’ will give rise to Rhnegative phenotype.
Wiener Hypothesis
Wiener proposed the existence of eight alleles (R1, R2, R0, Rz, r, r1, r11, ry) at a single Rh locus. All genotypes carrying a dominant ‘R allele’ (R1, R2 ,R0 ,Rz) will produce Rh+positive’ phenotype and double recessive genotypes (rr, rr1, rr11, rry) will give rise to Rh-negative phenotype.
4.3.1 Incompatibility of Rh – Factor – Erythroblastosis foetalis
Rh incompatability has great significance in child birth. If a woman is Rh negative and
49 Principles Of Inheritance And Variation

TN_GOVT_XII_Zoology_chapter 4.indd 49

25-02-2019 18:21:57

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 IgG 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 Erythoblastosis foetalis or Haemolytic disease of the new born (HDN).
Prevention of Eryhroblastosis 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 mothers 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
Principles Of Inheritance And Variation 50

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).
Y CHROMOSOME 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 and XX-XY type.
XX-XO Type
This method of sex determination is seen in bugs, some insects such as cockroaches and grasshoppers. The female with two X chromosomes are homogametic (XX) while the males with only one X chromosome

TN_GOVT_XII_Zoology_chapter 4.indd 50

25-02-2019 18:21:57

are heterogametic (XO). The presence of an unpaired X chromosomes determines the male sex. The males with unpaired ‘X’ chromosome produce two types of sperms, one half with X chromosome and other half without X chromosome. The sex of the offspring depends upon the sperm that fertilizes the egg (Fig. 4.2).

P1 Gametes



F1 Generation AAXX


Fig. 4.2 XX-XO Type of sex determination
XX-XY type (Lygaeus Type)
This method of sex determination is seen in human beings and in Drosophila. The females are homogametic with XX chromosome, while the males are heterogametic with X and Y chromosome. Homogametic females produce only one kind of egg, each with one X chromosome, while the heterogametic males produce two kinds of sperms some with X chromosome and some with Y chromosome. The sex of the embryo depends on the fertilizing sperm. An egg fertilized by an ‘X’ bearing sperm produces a female, if fertilized by a ‘Y’ bearing sperm, a male is produced (Fig. 4.3).

P1 Gametes



F1 Generation AAXX


Fig. 4.3 XX-XY Type of sex determination

Heterogametic Females
In this method of sex determination, the homogametic male possesses two ‘X’ chromosomes as in certain insects and certain vertebrates like fishes, reptiles and birds producing a single type of gamete; while females produce dissimilar gametes. The female sex consists of a single ‘X’ chromosome or one ‘X’ and one ‘Y’ chromosome. Thus the females are heterogametic and produce two types of eggs. 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 and ZW-ZZ type.
ZO-ZZ Type
This method of sex determination is seen in certain moths, butterflies and domestic chickens. In this type, the female possesses single ‘Z’ chromosome in its body cells and is heterogametic (ZO) producing two kinds of eggs some with ‘Z’ chromosome and some without ‘Z’ chromosome, while the male possesses two ‘Z’ chromosomes and is homogametic (ZZ) (Fig. 4.4).

P1 Gametes



F1 Generation AAZZ


Fig. 4.4 ZO-ZZ type of sex determination
ZW-ZZ type
This method of sex determination occurs in certain insects (gypsy moth) and in vertebrates such as fishes, reptiles and birds. In this method the female has one ‘Z’ and one ‘W’ chromosome (ZW) producing two types of eggs, some carrying the Z chromosomes and some carry the W chromosome. The male sex
51 Principles Of Inheritance And Variation

TN_GOVT_XII_Zoology_chapter 4.indd 51

25-02-2019 18:21:57

has two ‘Z’ chromosomes and is homogametic (ZZ) producing a single type of sperm (Fig .4.5).

P1 Gametes



F1 Generation AAZZ


Fig. 4.5 ZW-ZZ type of sex determination

Sex determintion 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 (44A) and one pair of sex chromosomes (XX or XY). Females are homogametic producing only one type of gametes (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 exist in Drosophila. (Fig. 4.6).

Male (Heterogametic) Parents 44A + XY
Gametes Sperms (22A+X) (22A+Y)

Female (Homogametic) 44A + XX
Ova (22A+X) (22A+X)


Progeny (44A+XX) (44A+XY) (44A+XX) (44A+XY)

Female Male

Female Male

Fig. 4.6 Sex determination in human beings

The Y Chromosome and Male Development
Current analysis of Y chromosomes has revealed numerous genes and regions with
Principles Of Inheritance And Variation 52

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 non functional 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 and is expressed as:
Sex index = Number of X Chromosomes X Number of Sets of Autosomes A

TN_GOVT_XII_Zoology_chapter 4.indd 52

25-02-2019 18:21:57

Change in this ratio leads to a changed sex phenotype. The results obtained from a cross between triploid female Drosophila (3A:3X) with a diploid male (2A: XY) is shown in tables 4.2. and 4.3.
Table: 4.2 Bridges classical cross of a triploid (3A+XXX) female fly and a diploid (2A+XY) male fly

Parent Gametes

Triploid 3A + XXX (2A + XX) (A + X) (2A + X) (A + XX)

Diploid 2A + XY (A + X) (A + Y)


A+X 3A + XXX Triploid Female
3A + XX Triploid Intersex
2A + XXX Super female
2A + XX Diploid Female

A + Y 3A + XXY Triploid Intersex
3A + XY Super Male
2A + XXY Diploid Female
2A + XY Diploid Male

When the X : 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/A = 0.33) and metafemales (X/A=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 sexswitch gene. However, the Y- chromosome of Drosophila is required for male fertility.
• X-Chromosome was discovered by Henking (1891)
• Y-Chromosome was discovered by Stevens (1902)

Table: 4.3 Different doses of X chromosomes and autosome sets and their effect on sex determination in Drosophila


Meta female / Super female

Normal Female

Tetraploid Triploid Diploid


Inter sex

Normal male

Meta male / Super male

Number of ‘X’ Chromosomes (X)
3 4 3 2 1 2 1 1

Number of Autosome sets
2 4 3 2 1 3 2

Number of X chromosome Sex Index = Number of autosome sets
3/2 = 1.5
4/4 = 1.0
3/3 = 1.0
2/2 = 1.0
1/1 = 1.0
2/3 = 0.67
½ = 0.50
1/3 = 0.33

53 Principles Of Inheritance And Variation

TN_GOVT_XII_Zoology_chapter 4.indd 53

25-02-2019 18:21:57

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.
• The number of Barr bodies follows N-1 rule (N minus one rule), where N is the total number of X chromosomes present.
Haplodiploidy in Honeybees
In hymenopteran insects such as honeybees, ants and wasps a mechanism of sex determination called haplodiploidy mechanism
Principles Of Inheritance And Variation 54

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 haplodiplody 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.

TN_GOVT_XII_Zoology_chapter 4.indd 54

25-02-2019 18:21:57

The X – linked and Y – linked genes in the differential region (non–homologus 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 (XcXc) and hemizygous recessive males (XcY) are unable to distinguish red and green colour. The inheritance of colour blindness can be studied in the following two types of marriages.
(i) 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. 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 (Fig. 4.7).





;F <







;< ; <








Fig. 4.7 Marriage between colour blind man and normal visioned woman

ii) Marriage between normal visioned man and colour blind woman
If a colour blind woman (XcXc) 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 (Fig. 4.8).

55 Principles Of Inheritance And Variation

TN_GOVT_XII_Zoology_chapter 4.indd 55

25-02-2019 18:21:57












;F <








Fig. 4.8 Marriage between normal visioned man and colour blind woman

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.

Principles Of Inheritance And Variation 56

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,





• 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 (Fig. 4.9).










;< &*

Fig. 4.9 - Human karyotype (male)

TN_GOVT_XII_Zoology_chapter 4.indd 56

25-02-2019 18:21:57

Preparing to load PDF file. please wait...

0 of 0
Tamilnadu Board Class 12 Zoology Chapter 4