Sex limited inheritance (Pedigree analysis)
Definition: Traits that is visible only within one sex due to anatomical differences.
- The genes involved in these traits are typically autosomal, but the expression of these genes is dependent on the gender of the individual.
- Sex hormone and other physiological differences between male and female affect the expression of these genes
Pattern for sex limited inheritance
- These genes occur in both sexes (probably on the autosomes), but are usually only expressed in the sex with the appropriate hormonal determinant (activator).
- The trait appears in only one sex throughout the pedigree, but it does not need to occur in all members of that sex.
- The trait genes can be carried and transmitted by the opposite sex, although due to anatomical or physiological differences they are not shown in that sex.
- Barred coloring in chickens normally is visible only in the roosters.
- Beard growth in humans is limited to men
- In humans, breast development is a trait that is normally limited to females
Example of pedigree:
Clue 1: only females are affected
Exclude the possibility for Y linked trait
Clue 2: Skipping of generation
Clue 3: Affected offspring does not have affected parent
Exclude the possibility for a dominant trait
Assumption: It can be a maternal, X linked recessive or sex – limited autosomal recessive inheritance
Since all the offspring of affected female is not affected we can exclude maternal inheritance
Check whether X linked recessive or sex limited autosomal recessive
- All affected female does not have affected father
- All sons of affected female is not affected
Therefore we can exclude x linked recessive inheritance
Now you can check for sex limited autosomal recessive inheritance pattern
If it is an autosomal trait, it will appear in both sexes with equal frequency. But here only female are affected. It can be a sex limited trait.
Genotypes of affected female: rr
Genotypes of III – 1 and II – 4: Rr (heterozygous)
Sex limited autosomal recessive is the mode of inheritance shown in the given pedigree
- Even though female having the dominant alleles on their autosome, they are not exhibiting the trait.
- This trait is limited to males only. The male individual appear normal only when they are homozygous recessive for the allele responsible for the trait.