Sex limited inheritance (Pedigree analysis)

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.

Examples:

  • 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:

The clues available in the 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.

Here

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

Example 2: Sex limited autosomal dominant 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.

 

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