Sex Linked Inheritance

1. Sex Linked Inheritance • Chromosomes are the carriers of genes. Genes are located on both the autosomes and sex chromosomes. • Genes on • Autosomes – somatic characters • Sex chromsomes- sex characters • However, certain genes present in the sex chromosomes control the somatic characters. • The characters which are controlled by such genes are called sex linked character and transmission of such characters from one generation to nextis called sex linked inheritance

2. Discovery and types of Sex linked inheritance • Was first discovered by Thomas H. Morgan in 1910 on Drosophila melanogaster. • It can be classified into three types depending upon the chromosomes (X or Y) having sex linked genes. • 1. X – Linked inheritance • 2. Y - Linked inheritance • 3. XY - Linked inheritance

3. 1. X- Linked inheritance • Certain sex linked genes are located only on X chromosomes and their alleles are absent in Y chromosomes. • These genes are called X – linked genes; • Their mode of inheritance is called X-linked inheritance • This pertains to the inheritance of those characters which are controlled by genes located in the non-homologous part of X-chromosomes • Examples: Colour blindness, Hemophilia – human • Eye colour in Drosophila

4. Y – Linked inheritance • The sex linked genes are located on Y chromosomes only are called Y linked genes. • The Y linked genes are confined only male (human). • Hence they are also called Holandric genes (holo – whole; andros –male) • These genes are transmitted directly from the father to the son • Example: Hairy ear rims in Men

5. XY Linked inheritance • Certain sex linked genes are located on both X and Y chromosomes • They are called XY linked genes and their mode of inheritance is called XY linked inheritance.

6. General patterns of Sex linked Inheritance • X – Linked Recessive: • 1. Usually more males than females are affected • 2. No offspring of an affected male are affected, making the trait skip generations in pedigree. • But an exception to this pattern occurs in the rare instance when the affected male mate with a female carrier, producing a affected female offspring. • X – Linked Dominant: • 1. Affected male produce all affected female offspring and no affected male offspring • 2. Approximately half the offspring of affected females are affected, regardless of their sex.

7. Y – Linked Inheritance: • 1. Traits are always passed from father to son • 2. Only males are affected. • Kinds of Sex Linkage: • 1. Diagenic: when the characters pass from father to grandson through his daughter • Father → Daughter → Grandson • 2. Diandric: When the characters pass from mother to grand-daughter through her son • Mother → Son→ Granddaughter

8. 3. Hologenic: • when characters go directly from female to female, i.e., from mother to daughter and then to granddaughter. Mother → Daughter → Granddaughter • 4. Holandric: • When the characters are pass from male to male, i.e, from father to son and then to his grandson. • Father → Son → Grandson

9. Sex linkage in Drosophila: Normal wild type of fruit fly has red eye (RR). In the population of red eyed flies, white eyed (recessive) appeared as mutation (rr). When white eyed male was mated with red eyed female F1 flies had red eyes In F2 on an average of 3 flies had red eyes and 1 had white eyes

10. This results reveal white eye is due to recessive gene. • But if F2 individuals are classified on the basis of their sex as well as eye colour, a peculiar picture is emerged. • All the F2 female had red eyes, but half of the males had red eye and the other half had white eyes. • It appears that eye colour of a fly in F2 depended on its sex

11. But when white eyed female flies were mated with red eyed males, half the flies were red-eyed and the remaining half had white eyed. • In F2 generation, both male and female flies showed the ratio of 1 red: 1 white. • Morgan correctly reasoned that white eye gene was located on X chromosome

12. Colour blindness • We have 3 color receptors in the retinas of our eyes. They respond best to red, green, and blue light. • Each receptor is controlled by a gene. The blue receptor is on an autosome, while the red and green receptors are on the X chromosome (sex-linked).

13. Features of Colour blindness • Colour blindness is a sex linked character discovered by Wilson • It is a hereditary disease and the affected person cannot distinguish green and red colour. • The red blindness is called Protonopia, these persons cannot see red colour • While, green blindness is called deuteronopia, such a persons cannot see green colour. • Colour blindness is a recessive character, represented by cc • The genes are for colour blindness is located on X chromosome. It is common in male but rare female. • Colour blindness follows criss- cross inheritance as transmitted from father to grandson through daughter. • It is never transmitted from father to son

14. XCXC - Normal female XCXc - Carrier female XCY - Normal Male XcY - Affected male XcXc - Affected female

16. Hemophilia • Hemophilia is another sex-linked trait. • Discovered by John Cotto. • Hemophilia is characterized by delayed blood clotting. People with hemophilia can easily bleed to death from very minor wounds. • Otherwise called as “Bleeders disease” • Hemophilia is sex linked recessive character (hh) and the genes are located on X chromosomes • It is common in men and rare in women

17. This disease is appeared as a mutant in Queen Victoria and from her it was transmitted to her descendants. • “Royal disease” • XH XH - Normal female • XH Xh - Carrier female • XH Y - Normal Male • Xh Y - Affected male • Xh Xh - Affected female

18. Sex linked Inheritance in Cat • Sex linkage has been noticed in Tortoise shell cats. • This involves the inheritance of coat colour. • Tortoiseshell cats have patches of black and orange fur. Almost all tortoiseshells are female. Heterozygous for the X-linked coat color gene, one allele black and the other allele orange. • When a black coloured female is matted with yellow coloured male, tortoise shell female is obtained. • Even reciprocal also tortoise are female ♀ ♂ XB XB X Xb Y (Black) (Yellow) XBXb (Tortoise shell)

19. Incomplete Sex Linkage • The genes located on homologous regions of sex chromosomes do not inherit together because crossing over occur may occur in these region. • So these genes are incompletely sex linked genes and their mode of inheritance are incomplete sex inheritance. • Xeroderma pigmentosum is an abnormal condition of skin in man, in which the skin becomes extremely sensitive to light. • Even low light can produce pigmentation. In extreme cases, develops cancerous growth in body. • Retinitis pigmentosa is another case of incomplete sex linkage caused by the interaction of a number of dominant and recessive genes. • In this case, the affected person develops night blindness leading to the formation of pigmentation in retinal wall.

20. Sex Limited Traits • Some genes fail to produce their phenotypic effects in the presence of certain hormones and are described as sex limited. • In human being several characters are restricted with particular sex. • For e.g, facial hair in male only (beard )

21. Sex-Influenced Traits • In contrast to Sex limited gene where one expression of a trait is limited to only one sex only; sex influenced genes are those whose dominance is influenced by the sex of the bearer. • Such characters are called Sex influenced traits. • Good examples: male pattern baldness in humans and horns in sheep. • Pattern baldness is found in both sexes, but is rarer in females, but it is dominant in males and recessive in females. Thus, male heterozygotes are bald but female heterozygotes have normal hair.

22. The Adams family and Baldness

23. Dominant sex influenced trait • Autosomal genes responsible for horns in some breeds of sheep may behave differently in the presence of male and female sex hormones. • In suffolk sheep neither sex is horned, and the genotype is “hh”. • Among the heterozygous F1 progeny crosses between these breeds, horned males and hornless female are formed. • Because both the sex are genotypically alike (h+h), the genes behave as dominance in male and recessive in females, i.e., only one allele is required for male but the allele must be homozygous expression in female.

24. Deleterious recessive Sex Linked Genes in Human • 1. Lesch –Nyhan Syndrome: • Characterized by excess of uric acid, is inherited through sex linked recessive gene. • This means that mother contributes the X chromosome with defective gene to a male zygote. • Half of the male children of carrier mothers may be expected to inherit this disease. • These are deficient for the enzyme hypoxanthine-guanine phosphoribiosyltransferase.(HPRT) • Infant who receive this disease appears normal at birth and for several months. • By about 10 months of age, they become abnormal and mental retardation, and patient may literally bite of his own fingers and lips.

25. 2. Duchene type Muscular Dystrophy: • Is also depends on sex linked recessive gene. • If mother is carrier, about half of the male children are expected to be affected. • Can be identified by chromosome study. • It affects male before they reach teens, with muscular deterioration . • Muscles of leg and shoulders become stiff and the children usually become paralyzed and crippled during their middle or late teens. • Virtually all die before age of 21.

26. 3. Hunter syndrome: • It is characterized by mental retardation, coarse features, hirsutism (abnormal hairiness), and a characteristic broad nose and a large protruding tongue. • Symptoms appear in childhood, and a chemical procedure for diagnosis has been developed. • Mucopolysaccharides (Containing an amino sugars as well as uronic acid) accumulate in skin cells of these patients. • Stained with O-toluidine blue, will results in pink in colour.