In one of the species of butterflies, the heterogametic sex is female. A red male with a club-shaped antennae was crossed with a yellow female with a filiform antennae. Half of the offspring were yellow males with a filiform tendrils, the other half were red females with a filiform tendrils. How are body color and antennae type inherited? What signs dominate? Identify the genotypes of the crossed forms and their offspring.
Let us analyze the inheritance of each trait separately.
When a female with a filiform tendril was crossed with a male with a club-shaped tendril, all offspring inherited the filiform shape of the antennae. Consequently, the filiform tendrils completely dominate the club-shaped ones.
We introduce the notation of genes:
B – filiform tendrils;
b – club-shaped antennae.
Assume that this symptom is coupled to the floor. Then the female genotype is ZBW, the male is ZbZb. In parental individuals with such genotypes in the offspring, all males would have a filamentous antennae: ZBZb, and females – a club-shaped antennae: ZbW. This contradicts the condition of the problem; therefore, the type of antennae is determined by autosomal genes.
The presence of a uniform offspring (all with a filiform tendrils) allows us to conclude that the parental individuals were homozygous. Thus, the female genotype is BB, the male is bb.
– As a result of the crossing of a yellow female with a red male, in the offspring all the females inherited the paternal trait (red color), and the males inherited the maternal trait (yellow color). This type of inheritance indicates that body color is a sex-linked trait.
If the red color (A) dominates the yellow one (a), then the female has the ZaW genotype. For the male, the phenotypic radical ZAZ– can be written. Regardless of whether it is homozygous or heterozygous, red males – ZAZa should have appeared in the offspring. However, all the males were yellow.
Therefore, the assumption that the red color dominates the yellow turned out to be wrong. In fact, the opposite is true: the yellow color dominates the red.
– We introduce the notation of genes:
A – yellow color;
a – red color.
The yellow female has the ZAW genotype, the red male has ZaZa. Therefore, in the offspring, all females should be red: ZaW, and males should be yellow: ZAZa. This satisfies the condition of the problem.
Answer: Body coloration is a sex-linked sign; antennae type is an autosomal sign. The yellow color of the body completely dominates over the red, and the filiform tendrils over the club-shaped. Parents have the following genotypes: female – ZAWBB, male – ZaZabb. In the offspring, all females have the ZaWBb genotype, males – ZAZaBb.