Very important for ICAR JRF

1. SEX DETERMINATION,
SEX LINKAGE
&
MULTIPLE ALLELES

2. SEX DETERMINATION
■ Sex refers to contrasting features of male and female individual of same species.
■ Sex determination is the process of sex differentiation.
■ In mammals, sex characters are of two types:
 Primary sex characters:- Include sex organ which are distinct since birth and take part in reproduction.
 Secondary sex characters:- Express differently in male and female and doesn't take part in
reproduction. Eg - voice, facial hair, mammary glands etc.
■ Sex chromosome were 1st discovered by McClung in 1901 in grass hopper and postulated that this
chromosome was involved in sex determination.
 This was the 1st demonstration that a character ( in this case – sex ) was associated with specific
chromosome.
■ Each unisexual diploid individual has two types of chromosome – Allosomes ( Sex chromosome ) &
Autosomes.

3. MECHANISM OF SEX DETERMINATION
IN ANIMALS
Three imp. Mechanism :- Environmental, Chromosomal, Monogenic.
Environmental sex determination :- Sexual differentiation is determined by environmental factor.
a. Association with female:- Bonelia ( Sea worm )
 Larva attached to proboscis of female – male.
 Free living larva – female.
b. Egg size :- Dinophilus ( Sea worm )
 Relatively larger size eggs – female
 Smaller eggs – male.
c. Incubation temp. :- Turtles, Alligators,Crocodiles

4. Chromosomal sex determination
The different mechanism of chromosomal sex determination are:-
Allosomal system
■ XX female, XY male :- Drosophila, Humen, Mice, Housefly etc.
■ XX female XO male :- Grass hopper, protenor ( orthoptera ), cockroach.
■ XY female XX male :- Bird, Butterfly, Moth, Silkworm, Fish, Reptiles.
■ XO female XX male :- Fumea.
Monogenic sex determination :- Expression of sex is influenced by single autosomal gene.
■ Autosomal recessive transformer (tra) gene of drosophila :- In homozygous recessive condition (tra/tra)
transform normal diploid female (XX) into sterile male.
■ The tra gene has no any effect in heterozygous condition (Tra/tra) on either sex.
■ SRY gene of humen :- encodesTDF which directs differentiation of testis.

5. Genic BalanceTheory
■ Proposed by Bridge in 1922 working with drosophila.
■ It states that sex determining genes are present on both X chromosome as well as autosome.
■ The male sex determining genes are present on autosome and female sex determining genes are on X
chromosome.
■ The expression of sex depends on ratio of X chromosome to autosome ( X/A ratio).
■ This ratio is called as sex index.
■ Sex index = X/A; X= no. of X chromosome, A= no. of autosomal sets.
■ Sex index
 1→Female
 0.5 →Male
 Between 0.5-1.0 → intersex
 >1 → Supermale
 <1 → Superfemale

6. SEX MOSAIC IN DROSOPHILA/GYNANDOMORPH
■ 1st reported by Curt Stern in 1936 in Drosophila.
■ Some individuals show male characteristic in a part of their body, while their remaining part show
female phenotype; such individuals are known as gynandomorphs.
■ Three pattern of gynandomorphs may be found in drosophila:- bilateral, anterior/posterior, and in
patches.
■ Gynandomorphs are believed to arise from XX zygote.
■ During embryonic development in one or more cells one of two X chromosomes lost leads to
production of one or more daughter cells having single X chromosome.
■ The part with male phenotype are XO while those with female phenotype are XX,

7. DOSAGE COMPENSATION
■ It is the process by which organisms equalize the expression of sex linked genes
between member of different biological sexes.
■ It occurs in such species where male is heterogametic sex.
■ Dosage compensation is achieved by three ways:-
a) Inactivation of one X chromosome in female:- In mammals one X chromosome
in female is heterochromatinized and is present as barr body in somatic
interphase nuclei. Barr body was first discovered by Barr & Bertram (1949) in
nerve call of cat.
b) Hyperactivation of X linked genes in male:- In Drosophila.
c) Hypoactivation of X chromosomes:- In nematode Caenorhabditis elegans.

8. MECHANISM OF SEX DETERMINATION
IN PLANTS
■ Chromosomal Sex Determination :- It may be grouped into two category
1. Homomorphic sex chromosome:- X & Y chromosomes are identical in morphology however do show
functional differentiation.
a) XX female & XY male:- Asparagus & Spinach
b) XY female XX male:- Fragaria
2. Heteromorphic sex chromosome:- X & Y chromosomes show both morphological and functional
differentiation so that they are identified cytologically.
a) XX female & XY male:- Cannabis, Salix, Bryonia, Melandrium, Rumex acetosella.
b) XX female XY1Y2 male:- Rumex acetosa, Humulus japonicus.
c) X1X1X2X2 female X1X2Y1Y2 male:- Humulus lupulus.

9. ■ Monogenic Sex Determination
■ In papaya, a single gene with three alleles ( m, M1, M2 ) is suggested to control the sex of in
individual.
■ mm→ female
■ M1m → male
■ M2m → Hermaphrodite
■ M1M1, M1M2, M2M2 → inviable
■ In spitting cucumber ( Ecballium elaterium ), gene a has three allele
■ ad for femaleness
■ aD for maleness
■ a+ for monoecy
■ The dominance relationship among these alleles is as follows :- a+ > aD > ad

10. SEX LINKAGE
■ Such genes which are located on X chromosomes are known as sex linked genes and linkage of such
genes is referred as sex linkage.
■ Inheritance of such genes or character is known as sex linked inheritance.
■ Features of sex linked inheritance
■ Sex linked genes are located on X chromosome only.
■ Heterogametic sex is hemizygous for a sex linked genes.
■ Expression of recessive sex linked character is higher in heterogametic sex than homogametic sex.
■ Sex linked genes show criss-cross inheritance.
■ Inheritance of sex linked characters doesn’t show normal segregation pattern.
■ It was noted that a region of Y chromosome is homologous to X chromosome. Genes located in such
region of X chromosome do not show sex linked inheritance.

11. Demonstration of sex linked inheritance
■ Inheritance of white eye in drosophila:-
■ Sex linkage was 1st discovered byT.H Morgan (1910) in drosophila. He demonstrated that gene for
eye colour of Drosophila was located on X chromosome.
■ X+ → Red eye (wild)
■ Xw → white eye (mutant)
■ Nondisjunction of X chromosome:- By Bridge (1916) while studying the inheritance of vermilion eye
colour in drosophila
■ Vermilion eye colour is produced by a sex linked recessive gene v.
■ Attached X chromosome:- By L.V Morgan (1922) while studying the inheritance of yellow body colour
in drosophila
■ Yellow body is recessive to normal grey body. It is determined by gene Y located on X chromosome.

12. Sex linkage in man
■ Haemophilia:-
■ X linked recessive disorder in which individual lack something in blood which is
essential for normal clotting of blood.
■ XH → normal allele
■ Xh → haemophilic allele
■ XH Xh → carrier women
■ XH Y→ normal male
■ Xh Y→ haemophilic male
■ Colour blindness:-
■ X linked recessive disorder.
■ A person having such defect can not differentiate between red and green
colour.
■ XN → normal allele
■ Xn → Colour-blind allele

13. SEX LIMITEDTRAITSVS SEX INFLUENCEDTRAITS
■ Sex limited trait:-
■ Characters which are expressed in one sex only are referred as sex limited character.
■ They express in one sex only and not in other sex.
■ Sex limited genes may be located either in sex chromosome or autosomes.
■ Sex limited genes control the expression of primary and secondary sex character.
■ Eg:- Breast development in women, Beard in man.
■ Sex influenced trait:-
■ The dominance expression of some genes depends on the sex of individual. Such genes are known as sex influenced genes.
■ Such genes are located on autosomes.
■ They express more frequently in one sex than other.
■ Expression of such characters appear to be governed by sex hormones.
■ Eg:- bladness in humans – dominant in male and recessive in female.
horn in sheep – dominant in male and recessive in female.

14. MULTIPLE ALLELES
■ Main features of multiple alleles:-
 Multiple alleles map at the same locus.
 Multiple alleles always control the same character of an individual.
 There is no crossing over between members of multiple allelic series…..this is based on the classical
concept of genes according to which crossing over takes place between gene but not within gene.
(But now it is well known that recombination between multiple alleles does take place.)
 The wild type (normal) allele of multiple allelic series is always dominant over mutant allele.The
mutant alleles may show partial or complete dominance among themselves.
 The cross between two mutant alleles will always produce mutant phenotype. Such cross will never
produce wild phenotype…. i.e. multiple allele do not show complementation.
 Presence of multiple alleles adds to the variability for a character having such alleles.

15. COMPLEMENTATIONTEST-TEST FOR ALLELISM
■ Complementation refers to appearance of wild phenotype when two mutants are crossed.
■ Complementation test is used to determine whether two mutant alleles belong to same gene or two
different genes.
■ When the trans heterozygote for two mutant alleles (affecting same trait) have the mutant phenotype,
they are placed in the same gene. But if they have wild phenotype, they mutant alleles are considered to
be located in two different genes.
 F1 from two mutant strain → wild type → complementation occur → mutant alleles located in different gene.
 F1 from two mutant strain → mutant type → complementation doesn’t occur → mutant alleles located in same
gene.
■ Complementation test can not be used in case of
 Dominant mutation
 Polar mutation

16. Examples of Multiple Alleles
■ ABO Blood group in man:-
 Three alleles:- IA , IB and i
 Blood group A → IA IA, or IA i
 Blood group B → IB IB or IB I
 Blood group AB → IA IB
 Blood group O → i i
 Fur colour in Rabbit:-
 Four Alleles:- C (Agout), cch (Chinchilla), ch (Himalayan), c (Albino)
 Order of dominance :- C > cch > ch > c
 Wing type in drosophila
 Eye colour in Drosophila
 Self incompatibility alleles in plant.

17. PSEUDOLLELES & ISOALLELES
■ PSEUDOALLELES:- It refers to closely linked and functionally related genes. A cluster
of pseudoalleles is known as pseudoallelic series or complex locus or complex region.
 They govern different expression of same genes.
 Pseudoalleles are considered to occupy a complex locus.
 They exhibit low frequency of genetic recombination.
 Eg . Lozenge eye in drosophila, Star asteroid in drosophila
■ ISOALLELES:- An allele which is similar in its phenotypic expression to that of other
independently occurring allele is known as isoallele.