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The sex of an organism is typically determined genetically, but environmental factors can also play a role.

Scientists examined the roles of genes and environmental temperature in the sex determination of a lizard species. Individuals with a ZW pair of chromosomes are always female (ZWf), while individuals with a ZZ pair of chromosomes can be either male (ZZm) or female (ZZf). Scientists mated ZWf or ZZf females with ZZm males and incubated the eggs produced in 20 clutches at temperatures ranging from 23°C to 36°C. When the young hatched, the scientists determined their sexes (Table 1). Among the eggs that did not hatch, the mortality of embryonic lizards was approximately the same for both males and females.

Table 1. Fraction of female progeny produced as a function of egg incubation temperature

Egg Incubation Temperature (°C) ZZm×ZWf Crosses: Fraction of Female Progeny ±2SEX¯¯¯ ZZm×ZZf Crosses: Fraction of Female Progeny ±2SEX¯¯¯

23 0.5±.08 0

24 0.5±.02 0

26 0.5±.07 0

28 0.5±.03 0

30 0.5±.01 0.03±.01

32 0.5±.03 0.2±.02

33 0.58±.03 0.52±.05

34 0.74±.04 0.88±.04

35 0.93±.03 0.97±0.2

36 1.0±0.0 1.0±0.0

(a) If a particular gene is located on the Z chromosome of this lizard species, describe why a lizard with a ZW genotype has a greater probability of expressing the recessive phenotype for the trait than a lizard with the ZZ genotype does.

(b) For each cross, describe whether genetics or temperature determines the sex of progeny produced from eggs incubated at 35°C. For each cross, describe whether genetics or temperature determines the sex of progeny produced from eggs incubated at 24°C.

(c) Predict the effect of increasing global temperatures on the continued presence of the W chromosome in this species of lizard. Scientists claim that, in this species, the W chromosome is unimportant in sex determination. Instead, proteins that are encoded by a gene or genes on the Z chromosome, and that are maximally expressed only at certain temperatures, are responsible for determining whether embryos will develop as females or males. Use the data to support this claim.


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