In a species of fish, a black spot on the dorsal fin is observed in males and females. A fish breeder carries out a pair of reciprocal crosses and observes the following results.

    Cross I      Parents: black-spot male x nonspotted female
                      Progeny: 22 black-spot male
                                     24 black-spot females
                                     25 nonspotted males
                                     21 nonspotted females
     Cross II     Parents: nonspotted male x black-spot female
                       Progeny: 45 black-spot males
                                      53 nonspotted females

Identify which sex is heterogametic. Give genotypes for the parents in each cross, and explain the progeny proportions in each cross.

Verified step by step guidance

1. Identify the pattern of inheritance by analyzing the results of the reciprocal crosses. Note that the progeny ratios differ between the two crosses, suggesting a sex-linked trait.
2. Determine which sex is heterogametic. In many species, males are XY (heterogametic) and females are XX (homogametic), but in some species, like birds and some fish, females are ZW (heterogametic) and males are ZZ (homogametic).
3. Analyze Cross I: A black-spot male (assumed to be ZZ or XY) crossed with a nonspotted female (assumed to be ZW or XX) results in both black-spot and nonspotted progeny of both sexes. This suggests that the black spot trait is not sex-linked or is autosomal.
4. Analyze Cross II: A nonspotted male crossed with a black-spot female results in all black-spot males and all nonspotted females. This suggests that the black spot trait is linked to the sex chromosomes, with the black-spot allele being dominant and located on the Z or X chromosome.
5. Assign genotypes: In Cross I, the black-spot male could be Z^B Z^b or X^B Y, and the nonspotted female could be Z^b W or X^b X^b. In Cross II, the nonspotted male could be Z^b Z^b or X^b Y, and the black-spot female could be Z^B W or X^B X^b.

Key Concepts

Sex Determination and Heterogamety

In many species, sex is determined by specific chromosomes, with males and females having different combinations. Heterogametic sex refers to the sex that has two different sex chromosomes (e.g., XY in many animals), while the homogametic sex has two of the same (e.g., XX). In this fish species, identifying which sex is heterogametic is crucial for understanding inheritance patterns and progeny outcomes.

Genotype and Phenotype

Genotype refers to the genetic constitution of an organism, while phenotype is the observable physical or biochemical characteristics resulting from the genotype and environmental influences. In the context of the crosses, determining the genotypes of the parent fish (e.g., BB for black-spot and bb for nonspotted) helps explain the phenotypic ratios observed in the progeny.

Reciprocal Crosses

Reciprocal crosses involve switching the sexes of the parents in genetic experiments to determine the influence of sex on inheritance. By analyzing the results of both crosses in this fish study, one can assess whether the presence of the black spot is influenced by the sex of the parent, which aids in understanding the genetic basis of the trait.