Ch. 14 - Mendel and the Gene

Freeman - Biological Science 7th Edition

Freeman7th EditionBiological ScienceISBN: 9783584863285Not the one you use?Change textbook

Freeman 7th Edition

Ch. 14 - Mendel and the Gene

Problem 11a

Chapter 14, Problem 11a

As a genetic counselor, you advise couples about the possibility of genetic disease in their offspring. Today you met with an engaged couple, both of whom are phenotypically normal. The man, however, has a brother who died of Duchenne-type muscular dystrophy, an X-linked recessive condition. His ﬁancée, whose family has no history of the disease, is worried that the couple's sons or daughters might be afflicted. How would you advise this couple?

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Understand that Duchenne muscular dystrophy (DMD) is an X-linked recessive disorder, meaning the gene causing the disease is located on the X chromosome and two copies of the recessive allele are needed for females to express the disease, while only one is needed for males.

Since the man is phenotypically normal, he does not have DMD, which means he does not carry the recessive allele on his X chromosome. However, his brother had DMD, indicating that their mother was a carrier of the recessive allele.

The man could potentially be a carrier of the recessive allele if he inherited the X chromosome carrying the DMD allele from his mother. However, since he is male, he would express the disease if he had the allele, so he does not carry it.

The fiancée has no family history of DMD, suggesting she is unlikely to be a carrier of the recessive allele. However, without genetic testing, it cannot be completely ruled out.

Advise the couple that their sons cannot inherit DMD from the father, as sons inherit the Y chromosome from their father. Daughters could only be carriers if the mother is a carrier, but they would not express the disease unless they inherit the recessive allele from both parents, which is unlikely given the fiancée's family history.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

X-linked Recessive Inheritance

X-linked recessive inheritance refers to genetic conditions associated with mutations in genes on the X chromosome. Males are more frequently affected because they have only one X chromosome, while females have two. A female carrier can pass the mutated gene to her sons, who will be affected if they inherit the X chromosome carrying the mutation.

Carrier Status

Carrier status in genetics indicates that an individual has one copy of a recessive allele that can cause disease in offspring if inherited. In X-linked recessive conditions, females can be carriers without showing symptoms. Understanding carrier status is crucial for assessing the risk of passing genetic disorders to children.

Genetic Counseling

Genetic counseling involves providing information and support to individuals or families about genetic conditions. Counselors assess the risk of inherited diseases, explain inheritance patterns, and discuss potential outcomes and options. This helps couples make informed decisions about family planning and managing genetic risks.

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Textbook Question

In parakeets, two autosomal genes that are located on different chromosomes control the production of feather pigment. Gene B codes for an enzyme that is required for the synthesis of a blue pigment, and gene Y codes for an enzyme required for the synthesis of a yellow pigment. Green results from a mixture of yellow and blue pigments, and recessive mutations that prevent production of either pigment are known for both genes. Suppose that a breeder has two green parakeets and mates them. The offspring are green, blue, yellow, and albino (unpigmented). Based on this observation, what are the genotypes of the green parents? What genotypes produce each color in the offspring? What fraction of the progeny should exhibit each type of color?

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Textbook Question

The smooth feathers on the back of the neck in pigeons can be reversed by a mutation to produce a 'crested' appearance in which feathers form a distinctive spike at the back of the head. A pigeon breeder examined offspring produced by a single pair of non-crested birds and recorded the following: 22 non-crested and 7 crested. She then made a series of crosses using offspring from the first cross. When she crossed two of the crested birds, all 20 of the offspring were crested. When she crossed a non-crested bird with a crested bird, 7 offspring were non-crested and 6 were crested. For these three crosses, provide genotypes for parents and offspring that are consistent with these results.

As a genetic counselor, you advise couples about the possibility of genetic disease in their offspring. Today you met with an engaged couple, both of whom are phenotypically normal. The man, however, has a brother who died of Duchenne-type muscular dystrophy, an X-linked recessive condition. His ﬁancée, whose family has no history of the disease, is worried that the couple's sons or daughters might be afflicted. The sister of this man is planning to marry his fiancée's brother. How would you advise this second couple?

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Textbook Question

Suppose you are heterozygous for two genes that are located on different chromosomes. You carry alleles A and a for one gene and alleles B and b for the other. Draw a diagram illustrating what happens to these genes and alleles when meiosis occurs in your reproductive tissues.

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Textbook Question

Suppose you are heterozygous for two genes that are located on different chromosomes. You carry alleles A and a for one gene and alleles B and b for the other. Label the stages of meiosis, the homologous chromosomes, sister chromatids, nonhomologous chromosomes, genes, and alleles.

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