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

Why does this evidence support the hypothesis that a black spot is sex linked?

Verified step by step guidance

1. Identify the pattern of inheritance by analyzing the progeny from both crosses.
2. In Cross I, observe that both black-spot and nonspotted males and females are produced, suggesting that the trait is not simply autosomal dominant or recessive.
3. In Cross II, note that all males have black spots while all females are nonspotted, indicating a potential sex-linked pattern.
4. Consider the possibility of the black spot trait being linked to the sex chromosomes, specifically the X chromosome, since the trait appears to be passed from the mother to the son in Cross II.
5. Conclude that the evidence supports the hypothesis of sex linkage because the trait's expression differs between the sexes and is consistent with X-linked inheritance patterns.

Key Concepts

Sex Linkage

Sex linkage refers to the association of a gene with a sex chromosome, which can lead to traits being expressed differently in males and females. In many species, including fish, genes located on the X or Y chromosomes can result in sex-specific phenotypes. Understanding sex linkage is crucial for interpreting inheritance patterns, especially when analyzing traits that appear to be influenced by the sex of the organism.

Reciprocal Crosses

Reciprocal crosses involve mating individuals in both possible combinations to determine the inheritance of traits. By crossing a black-spot male with a nonspotted female and vice versa, the breeder can observe how the trait is passed on through generations. This method helps clarify whether a trait is dominant, recessive, or linked to sex chromosomes, providing essential data for genetic analysis.

Phenotypic Ratios

Phenotypic ratios represent the relative frequencies of different observable traits in the offspring of a genetic cross. In this case, the ratios of black-spot to nonspotted fish in the progeny can indicate whether the black spot trait is inherited in a manner consistent with sex linkage. Analyzing these ratios helps in understanding the genetic basis of traits and supports or refutes hypotheses about inheritance patterns.