Principles of Genetics and Breeding

4.2.4 Sex-limited traits

X-linked alleles follow definite genetic patterns, but the phenotypes do not always follow the expected ratios. This is because many X-linked phenotypes (some autosomal phenotypestoo) are sex-limited in that the phenotype is expressed in only one sex.

• For example, the tigrinus phenotype (stripes on the body) in the guppy is controlled by X_Ti dominant sex-linked allele. Under normal conditions, the tigrinus phenotype is not expressed in females, no matter the genotype:

Genotype	Phenotype
XX	grey female
XXTi	grey female
XTi XTi	grey female
XY	grey male
XTiY	tigrinus male

• Many sex-linked phenotypes need testosterone in order to be expressed. Most of the X-linked genes in the guppy produce phenotypes that are sex-limited to the males. The addition of methyltestosterone to the water or feed will allow the phenotypes to be expressed in female also.
• Another guppy trait that shows simple Mendelian inheritance is the “zebra” pattern of two to five bars on the posterior part of the body of males. Though unable to express the pattern, females are nevertheless able to transmit the hereditary factor for it to their sons. Traits such as this one, which are carried by both sexes but are expressed in only the male (or the female), are called sex-limited.
• Mating a zebra male to a non-zebra female results in off spring heterozygous for the character – that is, each has one dominant and one recessive “zebra” gene-but since the character is limited to males, only the sons show it in the phenotype (the daughters are carriers).
• When a sibling cross of the F₁ generation is made, the resultant offspring, counting sons only, show a 3:1 ratio in favour of the zebra pattern. This indicates that the trait represents a simple, dominant Mendelian factor.