- fail to appear in females unless their father also possesses the same and the mother is a carrier.
- When present in heterozygous condition in a female, she functions as carrier.
- When present in homozygous condition in a female, she transfers the trait to all the sons.
Traits governed by sex-linked dominant genes :
- Produce disorders in females more often than in males.
- are present in all the female offsprings if father possesses the same.
- Do not get transmitted to son if mother does not exhibit them.
x-linkage in Drosophila
In 1910, Morgan fortuitously discovered a single fruitfly with white eyes
that did not result from any of his treatments. (Normal Drosophila have
red eyes, not white eyes.) Morgan immediately crossed this white-eyed male
Drosophila to its red-eyed sisters.
Interestingly, when Morgan later inbred the heterozygous f 1 red-eyed flies,
the traits of the f 2 progeny did not assort independently. Morgan expected
a 1:1:1:1 ratio of red-eyed females, red-eyed males, white-eyed males, and
white-eyed females. Instead, he observed the following phenotypes in his f 2
generation:
- 2,459 red-eyed females
- 1,011 red-eyed males
- 782 white-eyed males
There were no white-eyed females, Morgan wondered whether this was because the trait was sex-limited and only expressed
in male flies. To test whether this was indeed the case, Morgan did a reciprocal cross between a red-eyed male and white-eyed
female, the f 1 offsprings instead of being all red-eyed consisted of 50% red-eyed and 50% white-eyed and all the red eyed offsprings
were females and all the male offsprings were white-eyed. When these f 1 offsprings were interbred, their f 2 offsprings consisted of
following phenotypes:
- 129 red-eyed females
- 132 red-eyed males
- 88 white-eyed females
- 86 white-eyed males
Thus, the results of this cross did produce white-eyed females, and the groups had approximately equal numbers. Morgan therefore
hypothesised that the eye-colour trait was connected with the sex factor. This in turn led to the idea of genetic linkage, which means
that when two genes are closely associated on the same chromosome, they do not assort independently.
x-linkage in humans
In humans, many genes and the traits controlled by them are recognised as being linked to the X chromosome. These X-linked traits
can be easily identified in pedigrees, characterised by a criss-cross pattern of inheritance. Many X-linked human genes have been
identified, like the genes controlling two forms of hemophilia and two forms of muscular dystrophy are located on the X chromosome.
because of the way in which X-linked genes are transmitted, unusual circumstances may be associated with recessive X-linked
disorders in comparison to recessive autosomal disorders. for example, if an X-linked disorder debilitates or is lethal to the affected
individual prior to reproductive maturation, the disorder occurs exclusively in males.
This is the case because the only sources of the lethal allele in the population are heterozygous females who are “carriers” and do
not express the disorder. They pass the allele to one-half of their sons who develop the disorder because they are hemizygous who
rarely, if ever, reproduce.