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.
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