The Chromosomal Basis of Inheritance

Sex-Linked Inheritance

V. SEX-LINKED INHERITANCE

1. Some genes on sex chromosomes play a role in sex determination, but these chromosomes also contain genes for other traits.
2. In humans, the term sex-linked traits usually refers to X-linked traits.
   1. The human X-chromosome is much larger than the Y. Thus, there are more X-linked than Y-linked traits.
   2. Most X-linked genes have no homologous loci on the Y chromosome.
   3. Most genes on the Y chromosome not only have no X counterparts, but they encode traits found only in males (e.g. testis- determining factor).
   4. Examples of sex-linked traits in humans are color blindness and hemophilia. (See Campbell, Figure 14.15)
3. Fathers pass X-linked alleles to only and all of their daughters.
   1. Males receive their X chromosome only from their mothers.
   2. Fathers cannot, therefore, pass sex-linked traits to their sons.
4. Mothers can pass sex-linked alleles to both sons and daughters.
   1. Females receive two X-chromosomes, one from each parent.
   2. Mothers pass on one X-chromosome (either the maternal or paternal homologue) to every daughter and son.
5. If a sex-linked trait is due to a recessive allele, a female will express the trait only if she is homozygous.
   1. Females have two X chromosomes, therefore they can be either homozygous or heterozygous for sex-linked alleles.
   2. There are fewer females with sex-linked disorders than males, because even if they have one recessive allele, the other dominant allele is the one that is expressed. A female that is heterozygous for the trait can be a carrier, but not show the recessive trait herself.
   3. A carrier that mates with a normal male will pass the mutation to half her sons and half her daughters.
   4. If a carrier mates with a male who has the trait, there is a 50% chance that each child born to them will have the trait, regardless of sex.
6. Because males have only one X-linked locus, any male receiving a mutant allele from his mother will express the trait.
   1. Far more males than females have sex-linked disorders.
   2. Males are said to be hemizygous.
7. Hemizygous = A condition where only one copy of a gene is present in a diploid organism.
8. Gene Dosage Compensation (Figure 14.16)
   1. How does an organism compensate for the fact that some individuals have a double dosage of sex-linked genes while others have only one?
   2. In female mammals, most diploid cells have only one fully functional X chromosome.
      1. The explanation for this process is known as the Lyon hypothesis, proposed by the British geneticist Mary F. Lyon.
      2. In females, each of the embryonic cells inactivates one of the two X chromosomes.
      3. The inactive X chromosome contracts into a dense object called a Barr body.
   3. Barr body = A densely staining object inside the nuclear envelope, that is an inactivated X chromosome in female mammalian cells.
   4. Female mammals are a mosaic of two types of cells - those with an active maternal X and those with an active paternal X.
      1. Which of the two Xs will be inactivated is determined randomly in embryonic cells.
      2. After an X is inactivated, all mitotic descendants will have the same inactive X.
      3. As a consequence, if a female is heterozygous for a sex-linked trait, about half of her cells will express one allele and the other cells will express the alternate allele.
      4. Examples of this type of mosaicism are coloration in calico cats and normal sweat gland development in humans.
9. Sex-Limited and Sex-Influenced Traits
   1. Some sex-limited traits are determined by autosomal genes present in both sexes.
   2. Sex-limited traits = Traits which appear exclusively in one sex, but are determined by autosomal genes found in both sexes.
      1. Though only one sec normally expresses the trait, both sexes transmit the genes.
      2. This is important in animal breeding. For example, a dairy-cow breeder must consider a bull's family history for milk yield.
   3. Penetrance or expressivity of autosomal genes may be sex- dependent.
   4. Sex-influenced traits = Traits with sex-dependent variation in penetrance and expressivity of autosomal genes.
      1. For example, a form of male baldness is expressed in the presence of only one copy of the allele; whereas, a woman must be homozygous for the allele in order for it to be expressed.
      2. Penetrance and expressivity of both sex-limited and sex-influence traits is influenced by the individual's hormonal condition.