Biology
Biology
11th Edition
ISBN: 9781259188138
Author: Peter H Raven, George B Johnson Professor, Kenneth A. Mason Dr. Ph.D., Jonathan Losos Dr., Susan Singer
Publisher: McGraw-Hill Education
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Chapter 13, Problem 1U

Why is the white-eye phenotype always observed in males carrying the white-eye allele?

a. Because the trait is dominant

b. Because the trait is recessive

c. Because the allele is located on the X chromosome and males only have one X

d. Because the allele is located on the Y chromosome and only males have Y chromosomes

Expert Solution & Answer
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Summary Introduction

Introduction:

The sex chromosomes are those chromosomes that determine the sex of the organism. The females have a pair of X chromosomes, whereas, the males have a pair of dissimilar chromosomes (one X chromosome and one Y chromosome).

Answer to Problem 1U

Correct answer:

The white-eye phenotype is always observed in males carrying the white-eye allele because the allele is located on the X chromosome and males only have one X chromosome. Therefore, option c. is correct.

Explanation of Solution

Reason for the correct statement:

The gene responsible for the appearance of the white eye phenotype is present only on the X chromosome and it is not present on the Y chromosome.

Option c. is given as “because the allele is located on the X chromosome and males only have one X”.

As, “the Y chromosome lacks the gene responsible for the expression of the white eye phenotype and X chromosome has it. It is observed only in the males”, it is the right answer.

Hence, option c. is correct.

Reasons for the incorrect statements:

Option a. is given as “Because the trait is dominant”.

The trait will be dominant if it is expressing in the heterozygous or homozygous dominant conditions. Here, it cannot be determined that whether the condition is dominant or not. So, it is a wrong answer.

Option b. is given as “Because the trait is recessive”.

The trait will be recessive if it is expressing in the homozygous recessive conditions. Here, it cannot be determined that whether the condition is homozygous or not. So, it is a wrong answer.

Option d. is given as “Because the allele is located on the Y chromosome and only males have Y chromosomes”.

The allele for the gene responsible is not present on the Y chromosome. So, it is a wrong answer.

Hence, options a., b., and d. are incorrect.

Conclusion

The gene responsible for the white eye phenotype is present on the X chromosome only and males only have X chromosome and the Y chromosome does not have that gene.

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