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.

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.