Sex Linked Inheritance PDF

Sex Linked Inheritance The transmission of body characters from parents to off- spring along with a sex is calBed sex linked inheritance. It is also cailed sex linkage. The genes controlling body characters located on the sex chro- mosomes are called sex linked genes. The body charac...

Sex Linked Inheritance The transmission of body characters from parents to off- spring along with a sex is calBed sex linked inheritance. It is also cailed sex linkage. The genes controlling body characters located on the sex chro- mosomes are called sex linked genes. The body characters (otherthan sex characters) controliedby genes located on the sex chromosomes are called sex-linked characters. Sex iinked inheritance was discovered by TH. Morgan in 1910. The following are the common examples for sex-inked inheritance: 1. Colour blindness 2.Haemophilia 3. Eye colour in Drosophila 4. Hypertrichosis (Hair in the car pinna) 5. Ichthyosis hystrix. The sex linked genes are located on X chromosomeor Ychro- mosome or both X and Ychromosome. chromo- The genes, controiling body characters, located on X some are calied X-linkedgenes. The inheritance of X-linked genes controlled by X- is called X-linked inheritance. The characters linked genes are called X-linked characters. Eg. Haemophilia, colour blindness, eye colour in Drosophila. located on Y chromo- The genes controlling body characters some are calied Y-linked genes. The inheritance of Y-linked genes characters controlled by Y- is calied Y-linked inheritance. The characters. Eg. Hypertrichosis linked genes are calied Y-linked on the body). (hair in the pinna), Ichthyosis hystrix (scales located on both X andY The genes controlling body characters chromosomes are called XYlinked genes. The inheritance of XY inheritance. The characters con- linked genes is called XYlinked  trolled by XY linked genes are called XY linked inheritance. Eg Aeroderma pigmentosum, Retinitis pigmentosa, nephritis, etc. Most of the sex linked characters are recessive. They are more common in man than in woman. Most of the sex linked genes follow criss-cross inheritance Ag-ag inheritance). The inheritance of a character from the Jather to his grandson through his daughter is called criss-cross inheritance. That is, the sex linked character appears only in alter- nate generations. 1. Colour Blindness 1.Colour blindness is a sex linked character discovered by Wilson in 1911. 2. It is a hereditary disease and the affected persons cannot distinguish red colour and green colour. 3. The red blindness is cailed protonopia. These persons can- not see red colour. The green blindness is called deuteronopia. Such persons cannot see green colour. 4. Colour biindness is a recessive character. 5. lt is caused by recessive genes represented by cc. The nor- mal persons contain the genes CC or Cc or C alone (in man). The Tecessive genes prevent the proper development ofcolour sensitive cells in the retina. 6. The genes for colour blindness are located on the X chromo- somes. Their alleles are absent from Y chromosome. So man has onBy one gene. The presence of only one gene fora character is caiied hemizygous. So man is a hemizygote for colour blindness. 7.This character is common in man but rare in woman. 8.Colourblindness follows criss-cross inheritance as this char- acter is transmitted from the fatherto the grandson through the daugh- ter. It appears only in alternate generations. 9.This character is nevertransmitted to the son from the father. 10. The daughter carrying one recessive gene for colour blind- ness is calied carrier. The carriers are normal in their vision. When a normal woman possessing the dominant gene for nor- mal vision(CC) happens to marry a colour blind man (cY), all their daughters get one gene for colour blindness from their father. But they receive a dominant gene C fron their mother. So they are o r - mal. But they carry the recessive gene in one of their X chromo- mal Somes. So the daughters are called carriers because they carry the blindness. The sons never gct the disease because gene for colour they receive their X chromosome from the normal mother and the Y absent. chromosome tiom their father, from which the allelic gene is Parents:Normal mother x Colour blind father XX CC inheritance Crisseross Son F,: Son in law X Daughte XY XX Cc Carrier XX XX Y Cc C CC G.son G. daughter G. daughter G.son carrier Nomal Colourblind Normai Inheritance of colour blindness. When the daughters (carriers) are mnarried to men with normal vision, some colour blind sons are formed. These affected sons re- ceive their one X ciromosome (in wiich the recessive gene is present) from their mother (carrier Cc). Ifa colour blind woman is married to a normal man, all her sons are colour blind. The daughters are normal but they carry the reces- Sive gene in one of their X chromosomes and they are carriers. When these daughters are married to a colour blind man, colourblind grand- sons and grand daughters are produced in equal numbers. So it fol- lows criss-cross inheritance.  2. Haemophilia (Bleeder's disease) discovered by John Cotto 1. It is a hereditary hlood diseuse in 1803. hlood clotting. This This discase is characterized by delayed 2. factor in the blood called IS because of the absence of a an important role in blood antihaemophilic globulin which plays But in nommal thhe blood dots in 2 to 8 minutes. cotting. in persons, for 20 minutes to 24 hours. is delaycd haemophilic patients, clotting is bleed continuously from the wound. So haemophilia Hencc they disease. also called bleeders Victoria and This disease appeared as a mutant in Queen 3. disease is descendants. Hence this hcr it was transmitted to her from Victoria. So this is- common among the Royalfamily of Queen ease is also called Royal disease. linked recessive character. 4. Haemophilia is a s e x 5. lt is caused by recessive genes represented by hh and the dominant gene H. normal condition is due to X Father Mother ParentsS: XX HH Cris eross Normal nheritand SHaemophilic Son-in-law Daughter Son F XY XY H H Normal Normal Carrier F Daughter Daughter Son Son XX XX XY HH Hh H Normal Carrier Nomnal Hacmophilic Inheritance of haemophilia. chro- 6.T enes are located on the X chromosome. The Ycharac- mosome has no gene. So the maie has only one gene for this ter. So the male is called hemiygous. 7. As other sex linked characters, it is common in men but rare in women.  8.Hacmophilia follows criss-cross inheritance. t 1s transmit- ted from the father to his grandson through his daughter 9. Generally haemophilic patients will die before reaching repro ductive stage, ifthey are exposed to severe bleeding When a normal woman marries a haemophilic man, their sons and daughters will be normal, but the daughters are carriers contain- ing a recessive gene for haemophilia. When this carrier daughter mar- ries a normal man, in the F, all grand daughters are normal, but 50% oftheir sons are hacmophilic. When a normal man marries a haemophilic woman, their sons will be haemophilic and the daughters will be normal but carriers When this haemophilic man marries a carrier woman (like that of F), 50%of grand sons and 50% ofgrand daughters will be haemophilic. These crosses show that haemophilic genes are sex linked and X linked. Father Mother XY X Parents: H Normal Haemophilic F1 (XX\ Hh Daughter Son Normal Haemophilic (carrier) F2 XX XY Hh H Grand daughter Grand daughter Grand son Grand son Normal Haemophilic Nomal Haemophilic (carrier) 1 Inheritance of Haemophilia.