Sex Linked Inheritance
The inheritance of a trait (phenotype) that is determined by a gene present on one of the sex chromosomes is termed as sex linked inheritance. Sex linkage is the phenotypic expression of an allele related to the sex chromosome of the individual.
This mode of inheritance is in contrast to the inheritance of trait on autosomal chromosomes, where both sexes have the same probability of inheritance. Since humans have many more genes on the X than the Y chromosome, there are many more X linked traits than Y linked traits.
The 23rd pair of chromosomes termed as the sex chromosomes determines sex in human. If at conception there are two X chromosomes, organism will be a female. If there is an X and Y chromosome(XY), organism will be a male. Since the X and Y chromosomes differ in the hereditary information they carry, any gene present on the X chromosome refers to a sex linked gene. Females will have two alleles for sex linked genes while the males will have only one.
Sex Linked Diseases
Following are some of the examples of inheritance of sex linked diseases:
Haemophilia (Bleeders Disease)
Haemophilia is an X-linked recessive disorder. Persons suffering from this disease are unable to properly form blood clots. Any small cut or internal hemorrhaging after even a small bruise is fatal. This is therefore, also termed as ‘bleeders disease’. The concerned gene is termed as antihaemophilic globulin (AHG) gene. It suppresses the synthesis of factor VIII, IX or XI. The gene does not have an allele on Y chromosome.
Following are three disorders :
- Haemophilia A. Factor VII deficiency, also termed as Classic haemophilia (X- linked recessive).
- Haemophilia B. Factor IX deficiency also termed as Christmas disease (X- linked recessive).
- Haemophilia C. Factor XI deficiency also termed as Ashkenazi Jews ( autosomal recessive).
Inheritance of Haemophilia
Haemophilia affects males much more frequently than females. It is so because the X chromosome carries the critical blood clotting gene . Since male only carry X chromosome with recessive gene, haemophilia will immediately show up in males. Females on the other hand carry two X chromosomes. If one of them is defective the other normal chromosome can compensate. The blood clotting would be normal but this female would be carrier of the recessive defective gene. Males on the other hand would never be carriers. Female haemophiliacs are rare because it requires two defective X chromosomes to produce haemophiliac condition.
When the father is haemophiliac and not the mother then, none of the sons would be haemophiliac but all the daughters would be carriers with X chromosome carrying defective recessive gene.
When the mother carries haemophilia and not the father, there is 50% chance at each birth that a son will have haemophilia. Similarly that is a 50% chance at each birth that a daughter will carry defective recessive haemophilia gene.
Haemophilia figured predominantly in the history of European royalty. Queen Victoria passed the mutation to her son Leopold and through daughters to the royal families of Spain and Russia. For this reason it was popularly termed as ‘Royal Disease’.
Horner 1876 discovered, for the first time, that some people cannot distinguish between red and green colours. This condition was present only in some families including well known scientists Dalton (1798). This is due to its sex linked recessive gene located on X chromosome. The gene helps in the formation of colour sensitive cells termed as cones in the retina. These cells are necessary for the distinction of red and green colour.
Protan Defect or Red -Green Blindness
According to E B Wilson (1911) gene for colour blindness in man occurs in X chromosome. Later on, kalmus (1965) and keats (1983) demonstrated two genes termed protan and deutan involved in red- green colour blindness present near one end of X chromosome. Hence the defect is termed as protan defect or red green blindness or colour blindness. This recessive gene on X chromosome does not have its allele on gene on the Y chromosome.
This clearly explains that the colour blindness in females can be expressed only if its genes are present on both the chromosomes but when present only on one chromosome, they become carrier. On the other hand in case of males the colour blindness can be expressed by its gene being present only on one chromosome, the Y chromosome being inert for such trait. This also shows that normal colour is dominant over colour blindness.
A pedigree analysis shows that only males are affected by this disease. This indicates that the gene is inherited in different manner when it is connected with sex.
Colour Blindness in the Offspring of Different Parents
The following four cases are worth considering.
- Of the offspring of a normal female (XX) and colour blind male, their daughters receive the gene for colour blindness present on X chromosome. Thus all the females of this generation would be carriers for colour blindness. The sons (males) on the other hand, would be normal with XY chromosomes.
- In another case, the offspring of a colour blind female and a normal male (XY) would be 50% female carriers and the remaining 50% males (i.e., sons) would be colour blind.
- In the third case if a carrier female marries a normal male (XY) the offspring would be normal male 25%, carrier females 25%, normal males (XY) 25% and colour blind males 25%.
- Lastly if a carrier female marries a colour blind male, the progeny would be carrier females (daughters) with genotype XXc – 25% colour blind female (i.e., daughters) with genotype XCXC – 25%, normal males with the genotype XY- 25% and colour blind males with genotype XCY-25%.