Sex Determination and Sex Linakge Genetics Course Dr.Shagufta Naz, Lecture notes of Human Genetics

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Sex Determination and Sex

Linakge

Genetics Course

Dr.Shagufta Naz

Sex linkage

• Heredity is equal from both parents-Mendels studies of

garden pea.

• Inheritance is equal from both parents – sex linkage

• It occurs when the gene controlling a trait is located on

a chromosome a so called sex chromosome, that is not

present in morphologically identical pairs (two

homologous copies ) in both sexes.

• Such Traits exhibit unique sex-linked patterns of

inheritance recognized in pedigree studies.

• Roles of sex chromosomes in controlling sex

phenotypes is important and mechanisms of sex

determination will also be reviewed here.

Explanation

• In fruit fly and human, in many other species, especially higher animals individuals normally exhibit one of the two sex phenotypes
• Female (eggs, ovules, or macrospores)
• Male (sperm, pollen, or microspores)
• Species with seperation of sexes in different individuals ----dioecious.e.g all higher animals and some higher plants
• Species in which both male and female gametes produced by each individual --- monoecious.
• Hermaphroditism—production of both eggs and sperms by the same organisms e.g lower animals, hermaphrodites producing both types of gametes by the same organism.
• The two sex phenotypes are usually quite easily distinguished in humans and fruit flies , not universally the case.
• In Isogamy two genetically distinct types of gametes are sometimes morphologically indistinguishable e.g lower eukaryotes.
• In humans, fruit flies and most higher eukaryotes , the gametes produced are of two quite different types, namely, eggs and sperms.

Identification of sex chromosomes

• First investigation relating chromosomes to sex determination at turn of century
• H.Henking a German biologist discovered in 1891 , a particular nuclear structure traced throughout spermatogenesis of certain insects. Half sperm received this structure and half didn’t.
• Henking didn’t speculate on the significance of this body, but identified it as “ X” body and showed that sperm differed by its presence or absence.
• In 1902 C.E. McClung verified and extended such observations and made cytological observations on different species of grasshoppers and demonstrated that
• Somatic cells in female grasshoppers carry different chromosome numbers than do corresponding cells in male. So,
• He followed X body in spermatogenesis but didn’t succeed in tracing oogenesis of female grasshopper. McClung associated X body with sex determination but erroneously considered it to be peculiar to males.

xx- x y Mechanism of sex

Determination

• Wilson observed another chromosome arrangement in the milkweed bug. Same number of chromosomes were present in the cells of both sexes observed in this bug. The homolog of X was distinctly smaller and was called Y chromosome.
• XX zygotes became females, and XY zygotes became males. This was called XX-XY mechanism of sex determination. Examples most higher animals and some plants. It is also found in fruit fly and humans. Both exhibit same pattern of transmission of X and Y chromosomes in normal individuals. In humans X chromosome is considerably longer than the Y chromosome.
• The total complement of human chromosomes includes 44 autosomes
• XX in female
• XY in male
• Eggs produced by the female in oogenesis have usual complement of autosomes ( 22 ) plus an X chromosome. Sperm from the male have the same autosomal number and either an X or a Y. Eggs fertilized with sperm containing a Y chromosome result in zygotes that develop into males: those fertilized with sperm containing an X chromosome develop into females.
• Control of sex determination by a particular chromosome provided a tangible evidence that genes are in chromosomes. Females are called homogametic sex and males are called heterogametic.

Species with Heterogametic Females

• Many species, including most birds, moths, and some

fish, a chromosomal mechanism of sex determination

occurs, basically identical to XX-XY mechanism but with

females being heterogametic( designated as ZW) and

males being homogametic (usually designated as ZZ).

• This mechanism is sometimes called ZZ-ZW. However,

mechanistically this is identical to XX-XY system, but

with relationship between sex chromosomes and sex

phenotypes reversed. Stated differently, in birds, the

chromosome composition of the egg determines the

sex of the offspring, whereas in humans and fruitflies,

the chromosome composition of the sperm determines

the sex of the offspring.

Explanation

• The Y chromosome induces development of the undifferenciated gonadal medulla into a testis, whereas an XX chromosomal complement induces the undifferenciated gonadal cortex to develop ovaries. The gene on the Y chromosome in humans that is responsible for the development of testis is called TDF (for T estis D etermining F actor).
• The TDF gene has been isolated (cloned), characterized structurally ( sequenced), and found to encode a protein with features that suggest that it acts by regulating the expression of other genes. So TDF gene may be the master regulator that triggers the expression of a large number of genes that produce the male phenotype. In the absence of TDF gene , the genes that produce the female sex phenotype would be expressed. TDF gene (and possibly other male determining genes on the Y chromosome ) exhibits a very dominat effect on the development of the sex phenotype. Even in the presence of three or more X chromosomes, a single Y chromosome is usually sufficient to produce testis and male characteristics.