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Exam 4: Sex Determination and Sex-Linked Characteristics

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Question 1

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Species in which an individual organism has both male and female reproductive structures are called:

A) monoecious.
B) haploid.
C) diploid.
D) dioecious.
E) heterogametic.

F) A) and B)
G) A) and C)

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A

Question 2

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Red-green color blindness is X-linked recessive. A woman with normal color vision has a father who is red-green color blind. The woman has four sons, none of whom are colorblind. In this family there are no instances of chromosome loss or gain such as occurs due to nondisjunction in meiosis. Following are three explanations for why none of the sons are color blind. For each, state if color blindness is possible or not possible; then give the reason for your choice. (a) Explanation 1: None of the sons are color blind because the mother does not carry the color-blindness allele. (b) Explanation 2: None of the sons are color blind because none of them inherited the color-blindness allele from the mother. (c) Explanation 3: None of the sons are color blind because the mother inactivated the X chromosome with the recessive color-blindness allele, and that is the one each son inherited.

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Explanation 1: Color blindness is not possible. The reason for this is that since the mother has a father who is red-green color blind, she must be a carrier of the color-blindness allele on one of her X chromosomes. As a carrier, she has a 50% chance of passing on the color-blindness allele to each of her sons. Explanation 2: Color blindness is not possible. The reason for this is that since the mother is a carrier of the color-blindness allele, each of her sons has a 50% chance of inheriting the color-blindness allele from her. Therefore, it is unlikely that none of the sons inherited the color-blindness allele from the mother. Explanation 3: Color blindness is possible. The reason for this is that the mother could have inactivated the X chromosome with the recessive color-blindness allele, and each son could have inherited the other X chromosome. This would result in none of the sons being color blind, despite the mother being a carrier of the color-blindness allele.

Question 3

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In a few sentences, describe what effect removal of the Xist gene would have. Include a description of what Xist encodes and why a mutation would have this effect.

A Barr body is a(n) :

Explain the genders of human and diploid DrosophilaXXY individuals.

Human males with XY chromosomes are _____ and produce two different kinds of gametes, whereas females with XX chromosomes are _____ and produce only one kind.

Joan is phenotypically normal but had a child with the autosomal recessive disease cystic fibrosis (CF) from a previous marriage. Joan's father has hemophilia A, a sex-linked recessive condition where the blood fails to clot properly. Her father has survived due to recent treatment advances. Joan now intends to marry Bill, who is also phenotypically normal but who has a sister, Jill, with CF. Bill's parents are phenotypically normal, and there is no history of hemophilia A in his family. Assume that Joan and Bill do marry and have a child. What is the probability that this child will have CF but will not have hemophilia A? (Hint: This problem requires that you utilize concepts from Chapter 3 as well as Chapter 4.)

A man and a woman are trying to have children but are unsuccessful. The man's autosomes appear normal, but his sex chromosomes, shown in the following diagram, are not. The diagram also shows a normal male's sex chromosomes for reference. In two to three sentences, explain the man's situation, including the type of chromosome mutation he carries, the specific regions of specific chromosomes involved, and why he is male.

How might an XY person with a deletion of the SRY gene be distinguished from a person with androgen-insensitivity syndrome?

The boy in questions 61 and 62 has an X-linked recessive condition that is not seen in either parent. With this additional information, what can you conclude about the allelic composition of his parents and how he got this condition?

If a female Drosophila that is heterozygous for a recessive X-linked mutation is crossed to a wild-type male, what proportion of female progeny will have the mutant phenotype?

In humans, occasionally a baby is found that has the XX chromosomal karyotype but is phenotypically male. Which of the following statements might be a CORRECT explanation for at least some of these unusual cases?

In Drosophila yellow body color is caused by a sex-linked recessive allele and brown eye color is an autosomal recessive trait. A phenotypically normal female fly, heterozygous for both genes, is crossed to a male with normal body color and brown eyes. What proportion of their offspring should have yellow body color and brown eyes?

A new mutation inDrosophilais found called peach, which causes a peach-like body color. A peach male is crossed to a homozygous wild-type female and the offspring consists of peach females and normal males. If a heterozygous peach female is crossed to a normal male, the offspring consists of ¹/₄ peach females, ¹/₄ normal females, ¹/₄ peach males, and ¹/₄ normal males. What is the MOST likely mode of inheritance for peach?

Which statement BEST summarizes our current understanding of the origin of the Y chromosome?

A female with androgen-insensitivity syndrome, a sex-linked recessive condition, has:

Suppose that an apparently female athlete fails a gender test and is not allowed to compete in her event. The gender test is based on examination of cheek cells for the presence of one or more Barr bodies. Later, it is discovered that the athlete has androgen-insensitivity syndrome. Explain why the athlete failed the gender test. What did the technician see in the test and how was it interpreted?

Human females with XY chromosomes and a phenotype that includes the absence of a uterus and ovaries and the presence of testes are likely to have which of the following mutations?

What is the apparent purpose for X inactivation in humans and other mammals?

Explain how dosage balance is achieved between X-linked genes and autosomal genes in mammals.