Back to 2. Genetics, Environment and Behavior


Side Box 2.3: Sex-linked versus Autosomal Genes

Genetic crosses of the ruff illustrate how tests are made for sex-linked versus autosomal factors. In birds, the female is the heterogametic sex (ZW) whereas the male is homogametic (ZZ). This contrasts with mammals in which males are heterogametic (XY) and females are homogametic (XX). In ZW inheritance, the female has a W-chromosome, which typically carries no genes (analogous to the mammalian Y-chromosome which has very few functional genes).

One allele is dominant (it could be either I or s). A female that receives an sex-linked dominant gene from her own father (e.g., on the W chromosome) would give the gene directly to her sons (sons are WW and get a W from their mother, which the mother inherited from father). Let us assume the following pedigrees in which we mate sires to a mother that had the same morph as the sire (e.g., the cross: satellite fathers X satellite maternal grandfathers and the cross independent fathers X independent maternal grandfathers).

Mothers father is recessive W(s)W(s). The mother must get a recessive W(s) from her own father.
Theoretical Results cross

 mother's genotype: W(s)Z

 W(I) Z

genotype of father

W(s)W(s)

 W(s)  W(s)W(s)  WZ
 W(s)  W(s)W(s)  WZ

All sons show recessive trait.

Mother's father is dominant phenotype so his genotype is W(I)W(s) or W(I)W(I). Mother could get recessive W(s) or W(I) from father.
Theoretical Results cross

 mother's genotype: W(s)Z or W(I)Z

 W(s) or W(I)

Z

genotype of father

W(s)W(s)

 W(s)

 W(s)W(I) or W(s)W(s)

 WZ
 W(s)

 W(I)W(s) or W(s)W(s)

 WZ

Sons could be both types.

One of these special same-morph crosses will always be entirely recessive and this cross should show only a single type of sons. In the actual data both of these crosses show mixed morph broods (e.g., the cross: satellite fathers X satellite maternal grandfathers and the cross independent fathers X independent maternal grandfathers). This result is impossible for a dominant sex-linked trait because at least one cross should breed true. The gene cannot be a sex-linked dominant gene.

Lank and his colleagues then considered two autosomal dominance models: independent is dominant (I) and satellite is recessive (i) or the converse is true (independent is recessive, s, and satellite is dominant, S). Data fit a Satellite dominant model best. They considered only the data with multiple sons. All 5 satellite parent crosses produced mixed broods, whereas all 5 independent parent crosses bred true and produced multiple independent sons. This result is consistent with satellite dominant and independent recessive. In contrast, the Independent dominant is rejected by the data at the P < 0.05 level.


Back to 2. Genetics, Environment and Behavior