Chapter 11, Problem 1TYU

One of the autosomal loci controlling eye color in fruit flies has two alleles: one for brown eyes and the other for red eyes. Fruit flies from a true-breeding line with brown eyes were crossed with flies from a true-breeding line with red eyes. The F₁ flies had red eyes. What conclusion can be drawn from this experiment? (a) these alleles underwent independent assortment (b) these alleles underwent segregation (c) these genes are X-linked (d) the allele for red eyes is dominant to the allele for brown eyes (e) all the preceding are true

Summary Introduction

Introduction: Locus (pl:loci) is the specific location of the gene in the chromosome. Alleles are variants of a gene. The allele that masks the expression of the other allele in heterozygous form is called the dominant allele.

Answer to Problem 1TYU

Correct answer: When fruits flies of true-breeding line with brown eyes are crossed with fruits flies of true-breeding line with red eyes, the F1 had red eyes that show that the allele for red eyes is dominant to the allele for brown eyes. Therefore, option (d) is correct.

Explanation of Solution

Reason for the correct answer.

When true breeding parents are crossed where one parent is homozygous for one allele and the other parent is homozygous for another allele, the trait expressed in the F1 generation is the dominant allele. When applying the above principle for the given question, where fruits flies of true-breeding line with brown eyes are crossed with fruits flies of true-breeding line with red eyes, the F1 will have red eyes. This shows that the allele for red eyes is dominant to the allele for brown eyes.

Option (d) is given as “the allele for red eyes is dominant to the allele for brown eyes”.

When fruits flies of true-breeding line with brown eyes are crossed with fruits flies of true-breeding line with red eyes, the F1 will have red eyes that show that the allele for red eyes is dominant to the allele for brown eyes. Therefore, option (d) is correct.

Reasons for the incorrect statements.

Option (a) is given as “these alleles underwent independent assortment”.

Independent assortment can be explained with two traits only. Here, only one trait is mentioned, that is eye color, therefore, they can show only segregation, not independent assortment. Hence, option (a) is incorrect.

Option (b) is given as “these alleles underwent segregation”.

This option is correct, yet the most suitable option will be option (d) as the law of segregation has proved right for single traits. This experiment reveals the dominant allele more than segregation. Hence, option (b) is incorrect.

Option (c) is given as “these genes are X-linked”.

The question mentions that these alleles are present in autosomal loci and therefore, they cannot be X-linked. Hence, option (c) is incorrect.

Option (e) is given as “all the preceding are true”.

Option (d) is the most suitable answer followed by option (b). The other options are incorrect. Hence, option (e) is incorrect.

Hence, options (a), (b), (c), and (e) are incorrect.

Conclusion

When fruits flies of true-breeding line with brown eyes are crossed with fruits flies of true-breeding line with red eyes, the F1 will have red eyes that show that the allele for red eyes is dominant over the allele for brown eyes.