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.

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 .

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.

Answer/Explanation:

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.

If there is no dominant allele for a trait present on a homologous chromosome to mask the recessive allele, the recessive phenotype will be expressed. In this case, the gene is located on the Z chromosome. In an individual with a ZW genotype (female) there is no homologous Z partner, meaning whatever allele is present on the solitary Z chromosome will be expressed.

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 .

ZW females X ZZ males at 24°C

Half of the progeny are female. This is likely determined by genetics, as there is a 50/50 chance that the offspring inherit Z or W from mother, and always Z from father

ZZ females X ZZ males at 24°C

None of the progeny are female. This is determined by genetics and temperature, as there are no W chromosomes to pass on to the offspring, meaning all offspring will always be ZZ. At this temperature, that means no ZZ females are produced.

ZW females X ZZ males at 35°C

Almost all of the progeny are female. This is likely temperature dependent, because there is equal possibility genetically that offspring could be male or female (inherit W or Z). However, the proportions are different between 24 and 36C, suggesting higher temperatures influence the sex determination. At higher temperatures, ZW and ZZ individuals develop into females

ZZ females X ZZ males at 35°C

Almost all of the progeny are female. This is likely temperature dependent, because there is equal possibility genetically that offspring could be male or female (inherit W or Z). However, at higher temperatures, ZZ offspring develop into females over males.

Predict the effect of increasing global temperatures on the continued presence of the W chromosome in this species of lizard.

At temperatures over 33C, there is a sharp shift towards female sex of XX individuals rather than males. The W chromosome determines a female, but is not required to do so. Increasing temperatures can also produce females. Therefore, the presence of the W chromosome will likely reduce or even disappear entirely, and the species could just rely on temperature to determine sex.

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.

This is supported by the data showing that the presence of the W chromosome is not at all required to develop into a female. If you look at the ZZ x ZZ crosses, you can get all males or or females or a mix completely dependent on temperature, without a W chromosome in sight.

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