Unit 5 FRQ (Phenotypes) with Feedback

Unit FRQ: Sex Determination in Tongue Sole Fish

AP Biology Free Response Question – Genetics and Environmental Factors in Sex Determination
Welcome to the AP Bio Unit FRQ on sex determination in the tongue sole fish. These questions will help you understand the role of both genetic and environmental factors in determining the sex of offspring in this fascinating species.

Scenario

In the tongue sole fish (Cynoglossus semilaevis), sex is determined by a combination of genetics and environmental temperature. Genetically, male fish have two Z chromosomes (ZZ), while genetically female fish have one Z chromosome and one W chromosome (ZW). When fish are raised at 22°C, ZZ fish develop into phenotypic males, and ZW fish develop into phenotypic females.

However, when fish are raised at 28°C, the Z chromosome is modified (denoted as Z*). ZW individuals develop as phenotypic males that are fertile and can pass on the Z chromosome to their offspring, even when those offspring are raised at 22°C.

A cross between a ZW female and a Z*Z male is represented in the Punnett square below:

Questions

(a) Predict the percent of phenotypic males among the F₁ offspring of the cross shown in the Punnett square if the offspring are raised at 22°C.

Answer (a):

In the Punnett square, the offspring genotypes are ZZ, ZW, ZZ, and ZW. Let’s analyze the phenotypes of each genotype at 22°C:

• Z*Z: These offspring will develop as phenotypic males due to the presence of the Z* chromosome.
• Z*W: These offspring will also develop as phenotypic males because of the Z* chromosome.
• ZZ: These offspring will develop as phenotypic males.
• ZW: These offspring will develop as phenotypic females.

Thus, out of the four possible offspring, three (ZZ, ZW, and ZZ) will be phenotypic males. Therefore, the percent of phenotypic males among the F₁ offspring is:

$$\frac{3}{4} \times 100 = 75\%$$

(b) Identify the genotype of the male parent in a cross where the offspring show a 2:1 ratio of males to females. Describe one fitness cost to the female of mating with this particular male.

Answer (b):

If the offspring show a 2:1 ratio of males to females, it suggests that one of the possible genotypes is nonviable, likely resulting in embryonic lethality. In this scenario, the genotype of the male parent must be Z*Z, because:

• The ZZ parent can produce offspring with the genotypes ZZ, Z*W, ZZ, and ZW.
• The ZW genotype results in a nonviable offspring, leading to the observed ratio of **2 males (ZZ and ZZ) to 1 female (ZW)**.

Fitness Cost:
A fitness cost to the female of mating with a ZZ male is that **some of her offspring will be nonviable (ZW)**, reducing her overall reproductive success. This means fewer offspring will survive to adulthood, thereby decreasing the female’s contribution to the next generation.

Key Concepts Recap

• Environmental factors, such as temperature, can significantly impact sex determination.
• The presence of specific chromosomal modifications, such as Z*, can influence the viability and phenotypic sex of offspring.
• Mating decisions can carry fitness costs based on genetic compatibility and the resulting viability of offspring.

Practice Makes Perfect!
Try practicing similar FRQs to solidify your understanding of genetics, Punnett squares, and the role of environmental factors in determining phenotypes. Mastering these concepts will give you a strong foundation for tackling more challenging genetics problems.