Sex chromosome

sex chromosome in human beings

1. UNIVERSITY OF JAMMU SEX CHROMOSOME POLYMORPHISM IN ANIMALS Name: Srishty Sudershan Roll no. : Submitted to:

2. SEX CHROMOSOMES A sex chromosome also referred as allosomes, heterochromosome. In organisms that have separate sex a specific pair of chromosomes are present which determines the sex of an organisms. The other chromosomes are referred as autosomes or somatic chromosomes.

3. Discovery of Sex Chromosomes Sex chromosomes in the form of X- was first seen in fire flies by Heinking in 1891 . the chromosome was without any homologous in male . Mcclung in 1902 discovered that the single X-chromosome determines the sex of squash bug. Mc clung in 1902 observed 24 chromosomes in female grasshopper & 23chromosomes in male grasshopper. Miss Stevens in 1902 discovered Y- chromosome. Edmund Beecher Willson also contributed towards the discovery of sex chromosomes.

4. Polymorphism in sex chromosomes Homogametic and Heterogametic The organism having homomorphic sex chromosome i.e. XX , ZZ produces only one kind of gamates all with X or Z and these organisms are said to HOMOGAMETIC. The organism having heteromorhic sex chromosomes i.e. XY or ZO produces two types of gametes half with X and half with Y . The sex chromosomes have two region : a. Homologous b. Non homologous or Differential The differential region carry completely sex linked gene and doesn’t undergoes crossing over but the homologous region carry same genes and goes under crossing over.. The genes present in the differential region of the Y- chromosome are called as Holoandric genes ( Holo means whole & Andros means male ) .

5. Mechanism of sex determination Non allosomic sex determination : in this type of sex determination of an individual factors other than the allosomes are taken into consideration. Hormonal Haplo Diploidy mechanism Environmental Parasitic mode of life There are different methods operate in the determination of sex of an individual. Allosomic sex determination: in this type of sex determination method sex chromosomes are taken into consideration for the determination of sex of an individual. eg . XX-XY, ZZ-ZW, ZZ-ZO

6. Allosomic determination of sex Heterogametic Male: Males produces two types of gametes . XX-XY , XX-XO XX-XY Type : found in mammals, including human beings, fruitflies . Parents : Father Mother Genotype : 44A+XY 44A+XX Gamates 22A+X +22A+Y 22A+X+22A+X

7. XX-XO type : this type is usually seen in some insects. Parents: Male Female Genotype: AA+XO AA+XX Gametes: 22A+X+22A+O 22A+X+22A+X

8. ZZ-ZW type: found in certain insects, and vertebrates like fishes, reptiles, birds and also in some plants. Parents: Father Mother Genotype: AA+ZZ AA+ZW Gamates : A+Z,A+Z A+Z,A+W Hetrogametic Female: Usually seen in birds and in reptiles. In thus case females are heteromorphic and males are homomorphic . ZZ+ZO , ZZ+ZW

9. ZZ-ZO type: usually seen in moths and butterflies. Parents : Male Female Genotype: AA+ZZ AA+ZO Gamates : A+Z , A+Z A+Z , A+O

10. Genic Balance Theory: Proposed by Calvin bridges in 1926

11. Hormonal control of sex determination Formation of free martin: in cattles, when twin calves of different sex occurs the female is usually sterile known as freemartin. This usually occurs due to the hormones of other calf which is of different sex.

12. Haplodiploidy mechanism of sex determination Hymenopterans like honey bee, wasps, sawflies, &ants shows a unique phenomena in which unfertilized eggs develops into male. The female is diploid(2n) and the male is haploid(n). Eggs are formed by meiosis and sperm by mitosis. Fertilization restores the diploid number of chromosomes in zygote. In honey bee quality of food determine the fate of diploid individual ; if the larvae feed on royal jelly it will became the fertile queen and if on the nectar it will became sterile female or worker. Development of a haploid egg into a haploid individual (male) without fertilization is known as ARRHENOTOKUS PARTHENOGENESIS or PARTHENOGENESIS or ARRHENOTOKY (ARHEN means MALE & TOKUS means BIRTH)

13. DIFFERENT FACTORS INVOLVED IN SEX DETERMINATION SIZE OF ANIMAL : The marine annelid Ophryotrocha differentiate into a sperm producing male as a young animal and then changes into an egg laying female when it gets older. SIZE OF EGG : Marine archannelida Dinophilus. Small egg – Male Large egg - Female AGE OF AN ORGANISM : seen in a mollusc belonging to genus Crepidula. young organism is functional male while it becomes female when it matures. RATE OF METABOLISM : A.F.Shull & D.D.Whitney explain that by increasing the metabolic rate in rotifers the occurrence of male increases than female.

14. Temperature determining sex of an organism Temperature plays an imp role in sex determination. For turtles eggs incubate at temperature less than 28 degree Celsius – males above 31 degree Celsius – females for Snapping turtles, Macroclemys females develop if the temp is below 22 degree or above 28 degree Celsius. males will develop if the temp lies between 22—28 degree Celsius. in American alligators a temp below 28 degree celsius produces males while above 33 degree celsius produce female and temp between 28—33 degree produces male and female in equal proportion. For lizard the organism in cool nest are male irrespective of their genetic composition. European pond turtle, Emys obicularis Temp below 25 degree develops into males temp above 30 degree develops into females

15. Parasitic sex determination An invasive barnacle ( Loxothylacus panopaei ) that hijacks mud’s crab reproductive system and impregnates it with parasitic larvae even if the crab is male.

16. Sex determination in C. elegans Two sexual phenotypes are seen in this worm- the male having only one functional testis and hermaphrodite bearing two gonads producing both egg and sperm. The signal that determines maleness is provided by the expression of gene present on X- chromosome & autosomes. Hermaphrodite have two X chromosomes while males have only one X-chromosome. Ratio of one results in the organism being hermaphrodite while that of 0.5 result in male worm.

17. Sex determination in humans Maleness is due the dominant effect of Y chromosome. The effect of Y chromosome is manifested Early in the development when it directs the primordial gonads to develop into testis. Testis-Determining factor(TDF) product of gene called SRY(Sex determining region Y ). A syndrome called Testicular feminization results from a mutation in an X-linked gene , Tfm which encodes the testosterone receptor.

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