Sex determination

   Sex  determination is the process of sex differentiation which utilises various genetical concepts to decide whether a particular individual will develop into male or female sex.

Mechanisms of sex determination

1.Chromosomal sex determination

2.Environmental sex determination

3.Genic mechanism of sex determination

Chromosomal sex determination.

Chromosomes which do not differ in morphology and number in male and female sex are refered as autosomes.

Chromosomes which differ in morphology and number in male and female sex and contain genes determining sex are known as allosomes or sex chromosome

Chromosomal sex determination

3 types

1.Allosomal sex determination

2.Haplodiploidy system of sex determination

3.Genic balance system

Allosomal sex determination
4 different systems

XX-XY system

XY-XX system

XX-XO system

XO-XX system

XX-XY system of sex determination

Found in Humans, fruit fly, Melandrium album etc.

–XX –female; homogametic

–XY –male; heterogametic

                                                         Sex Determination -Humans

•XX-XY sex determination

•SRY gene on Y chromosome determines maleness

                                                          XY-XX sex determination

Birds, snakes, amphibians, butterfiles, isopods, some fish

–XX –male; homogametic

–XY–female; heterogameic

                                                          XX-XO sex determination

Grasshoppers, many orthopterans and hemipterans

–XX –female; homogametic

–XO male; heterogametic

(O = absence of chromosome)

                                                        XO-XX sex determination

Insects like Fumea

–XO –female; homogametic

–XX male; heterogametic

2. Haplodiploidy system of sex determination

Unfertilized eggs, haploid set –male

–Fertilized eggs, diploid set –female

3. Genic balance system

Drosophila melanogaster–fruit fly

•XX-XY system

–But presence of Y does not determine maleness

–Genetic balance system –sex determined by a balance between genes on autosomes and genes on X

•X : A ratio

–X = number of X chromosomes

–A = number of haploid sets of autosomes

                    Environmental sex determination.

Sea worm(Bonellia and Dinophilus)

 Bonellia: Larvae which remain free in sea water and settle on the sea bottom-Female

Larvae which settle on the proboscis of female-

Male Dinophilus: Larger eggs after fertilization- Female, Smaller eggs- Male

Horse tail plant(Equisetum) Grown under good condition- Female ,Grown under stress condition- Male

Cucumber and Musk melon- Treatment of ethylene enhances production of female flowers.

Temperature at which eggs are incubated has a decisive effect on the sex of the animal that hatch from them.

ØOnly females at high and only males at low temperature eg. turtles

ØOnly males at high and only females at low temperature eg. Crocodiles, alligator

ØOnly females at high and low temperatures eg. Australian crocodiles

                  MONOGENIC SEX DETERMINATION

Expression of sex influenced by a single gene.

In Drosophila, a transformer gene (tra) which is present on the autosome plays an important role in sex expression.

Transformer gene leads to the transformation of female sex into male.

Transformer gene is recessive;so no effect in heterozygous(Tra/tra) condition in either sex.

In homozygous(tra/tra) condition this gene transform diploid females into sterile males.

In human also a similar gene is found

It transform normal male(XY) into female.

They have faminine character

Also have internal degenerated testis and are sterile- condition called testicular feminisation.

Monogenic control of sex-in plants like asparagus, maize, papaya, spinach etc.

In papaya, 3 alleles governs the sex ie;

 m, M1and M2of single gene.

Homozygous recessive(mm)-produce female parent

Heterozygous- (M1m and M2m)-produce male and hermaphrodite plants

Dominant allees in homozygous (M1M1 and M2M2)as well as heterozygous condition (M1M2)-in viable plants

Crosses between females and male produce females and males in 1:1 ratio.

Crosses between females and hermaphrodite produce females and hermaphrodite in 1:1 ratio.

Seeds obtained from hermaphrodite plants will produce hermaphrodite and female progeny in 2:1 ratio.

                 Sexual mosaics or Gynandromorphs

Abnormal chromosomal behaviour in insects can result in the formation of Sexual mosaics or Gynandromorphs in which some part of the animal are female and other part are male

                           Sex chromatin body/Barr body

Barr observed chromatin bodies in the nerve cell of female cat.

Barr body was related to the x chromosome and was not observed in the normal male cells.

It distinguishes female cells from male cells.

Useful for diagonising various kinds of sex chromosomal abnormalities.

Abnormal female with 1 X chromosome turner syndrome- no barr body.

Male with 2 X and 1Y- Klinefelter syndrome-1 barr body.

Abnormal female with 3 X chromosome- 2 barr body.

                                    Dosage compensation

Lyon  hypothesis;an individual may have any number of X chromosome, but only one remains active and others become condensed or inactive.

Lyon  hyothesis and discovery of barr bodies confirmed the concept Dosage compensation

Data compiled and edited by 

Asoontha

CoA, Vellayani

Kerala Agriculture University