Table of contents

1. Introduction to Genetics51m
2. Mendel's Laws of Inheritance3h 37m
3. Extensions to Mendelian Inheritance2h 41m
4. Genetic Mapping and Linkage2h 28m
5. Genetics of Bacteria and Viruses1h 21m
6. Chromosomal Variation1h 48m
7. DNA and Chromosome Structure56m
8. DNA Replication1h 10m
9. Mitosis and Meiosis1h 34m
10. Transcription1h 0m
11. Translation58m
12. Gene Regulation in Prokaryotes1h 19m
13. Gene Regulation in Eukaryotes44m
14. Genetic Control of Development44m
15. Genomes and Genomics1h 50m
16. Transposable Elements47m
17. Mutation, Repair, and Recombination1h 6m
18. Molecular Genetic Tools19m
- Genetic Cloning
  9m
- Methods for Analyzing DNA
  9m
19. Cancer Genetics29m
- Overview of Cancer
  21m
- Cancer Mutations
  7m
20. Quantitative Genetics1h 26m
21. Population Genetics50m
- Hardy Weinberg
  19m
- Allelic Frequency Changes
  31m
22. Evolutionary Genetics29m

6. Chromosomal Variation

Chromosomal Mutations: Aneuploidy

Genetics

6. Chromosomal Variation

Chromosomal Mutations: Aneuploidy

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2:07 minutes

Problem 18

Textbook Question

Can the Lyon hypothesis be tested in a human female who is homozygous for one allele of the X-linked G6PD gene? Why, or why not?

Verified step by step guidance

Understand the Lyon hypothesis: The Lyon hypothesis states that in female mammals, one of the two X chromosomes in each cell is randomly inactivated during early embryonic development. This process, called X-inactivation, ensures dosage compensation between males (XY) and females (XX).

Recognize the genetic context: The G6PD gene is located on the X chromosome and encodes the enzyme glucose-6-phosphate dehydrogenase. If a female is homozygous for one allele of the G6PD gene, it means she has the same allele on both of her X chromosomes.

Consider the implications of homozygosity: Since the female is homozygous for the G6PD allele, both X chromosomes carry the same version of the gene. This means that even if one X chromosome is inactivated, the active X chromosome will still express the same allele of the G6PD gene.

Evaluate the ability to test the hypothesis: The Lyon hypothesis relies on observing differences in gene expression between the two X chromosomes due to random inactivation. However, in a homozygous individual, there is no difference in the alleles present on the two X chromosomes. Therefore, it is not possible to test the Lyon hypothesis in this case because the inactivation of one X chromosome does not result in any observable difference in G6PD expression.

Conclude the reasoning: Testing the Lyon hypothesis requires heterozygosity for an X-linked gene, where different alleles are present on the two X chromosomes. This allows for the detection of differences in gene expression due to X-inactivation. In a homozygous individual, such differences cannot be observed, making it impossible to test the hypothesis in this scenario.

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Key Concepts

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

Lyon Hypothesis

The Lyon hypothesis, proposed by Mary Lyon in 1961, states that in female mammals, one of the two X chromosomes is randomly inactivated during early embryonic development. This process, known as X-inactivation, ensures dosage compensation between males (XY) and females (XX) for X-linked genes. The inactivated X chromosome forms a Barr body and is largely transcriptionally silent, impacting gene expression and phenotypic outcomes.

X-linked Inheritance

X-linked inheritance refers to the pattern of inheritance for genes located on the X chromosome. In females, who have two X chromosomes, one may carry a mutation while the other may not, leading to a mosaic expression of traits. In contrast, males, having only one X chromosome, will express the trait associated with any mutation present on their single X, making X-linked disorders more prevalent in males.

G6PD Gene and Its Function

The G6PD gene encodes the enzyme glucose-6-phosphate dehydrogenase, which plays a crucial role in the pentose phosphate pathway, helping to protect red blood cells from oxidative damage. Mutations in this gene can lead to G6PD deficiency, an X-linked condition that can cause hemolytic anemia under certain stressors. Testing the Lyon hypothesis in a homozygous female for the G6PD gene would not yield informative results, as both X chromosomes would express the same allele.

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Textbook Question

Drosophila may be monosomic for chromosome 4, yet remain fertile. Contrast the F₁ and F₂ results of the following crosses involving the recessive chromosome 4 trait, bent bristles:

monosomic IV, normal bristles x diploid, bent bristles.

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Textbook Question

Drosophila may be monosomic for chromosome 4, yet remain fertile. Contrast the F₁ and F₂ results of the following crosses involving the recessive chromosome 4 trait, bent bristles:

monosomic IV, bent bristles x diploid, normal bristles

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Textbook Question

Define the Lyon hypothesis.

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Textbook Question

Mendelian ratios are modified in crosses involving autotetraploids. Assume that one plant expresses the dominant trait green seeds and is homozygous (WWWW). This plant is crossed to one with white seeds that is also homozygous (wwww). If only one dominant allele is sufficient to produce green seeds, predict the F₁ and F₂ results of such a cross. Assume that synapsis between chromosome pairs is random during meiosis.

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Textbook Question

In humans that are XX/XO mosaics, the phenotype is highly variable, ranging from females who have classic Turner syndrome symptoms to females who are essentially normal. Likewise, XY/XO mosaics have phenotypes that range from Turner syndrome females to essentially normal males. How can the wide range of phenotypes be explained for these sex-chromosome mosaics?

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Textbook Question

A plant breeder would like to develop a seedless variety of cucumber from two existing lines. Line A is a tetraploid line, and line B is a diploid line. Describe the breeding strategy that will produce a seedless line, and support your strategy by describing the results of crosses.

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Textbook Question

The woman in Problem 24 has had two miscarriages. She has come to you, an established genetic counselor, with these questions: Is there a genetic explanation of her frequent miscarriages? Should she abandon her attempts to have a child of her own? If not, what is the chance that she could have a normal child? Provide an informed response to her concerns.

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Textbook Question

In a recent cytogenetic study on 1021 cases of Down syndrome, 46 were the result of translocations, the most frequent of which was symbolized as t(14;21). What does this symbol represent, and how many chromosomes would you expect to be present in t(14;21) Down syndrome individuals?

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