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 Rearrangements: Translocations

Genetics

6. Chromosomal Variation

Chromosomal Rearrangements: Translocations

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2:40 minutes

Problem 24c

Textbook Question

A woman who sought genetic counseling is found to be heterozygous for a chromosomal rearrangement between the second and third chromosomes. Her chromosomes, compared to those in a normal karyotype, are diagrammed to the right. This woman is phenotypically normal. Does this surprise you? Why or why not? Under what circumstances might you expect a phenotypic effect of such a rearrangement?

Verified step by step guidance

Step 1: Understand the concept of chromosomal rearrangements. Chromosomal rearrangements involve structural changes in chromosomes, such as inversions, translocations, duplications, or deletions. These changes can affect gene expression or disrupt genes, potentially leading to phenotypic effects.

Step 2: Analyze the woman's genetic condition. The woman is heterozygous for a chromosomal rearrangement, meaning one of her chromosome pairs has undergone a structural change while the other remains normal. Despite this, she is phenotypically normal, which suggests that the rearrangement has not disrupted any critical genes or regulatory regions.

Step 3: Consider why the woman is phenotypically normal. Chromosomal rearrangements may not cause phenotypic effects if they do not disrupt essential genes, regulatory sequences, or gene dosage. Additionally, if the rearrangement is balanced (no net gain or loss of genetic material), it is less likely to result in a phenotype.

Step 4: Explore circumstances where phenotypic effects might occur. Phenotypic effects could arise if the rearrangement disrupts a gene's coding sequence, alters regulatory regions, or creates a fusion gene with abnormal function. Unbalanced rearrangements, which involve a gain or loss of genetic material, are also more likely to cause phenotypic abnormalities.

Step 5: Conclude that the lack of phenotypic effect in this woman is not surprising, given that balanced rearrangements often do not result in observable traits. However, phenotypic effects could occur in her offspring if the rearrangement leads to unbalanced gametes during meiosis, potentially causing developmental or health issues in the next generation.

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Key Concepts

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

Heterozygosity

Heterozygosity refers to the presence of two different alleles at a specific gene locus on homologous chromosomes. In the context of genetic counseling, a heterozygous individual may carry one normal allele and one mutated or rearranged allele. This genetic variation can sometimes lead to a normal phenotype, especially if the normal allele is sufficient to produce the necessary gene product.

Chromosomal Rearrangements

Chromosomal rearrangements involve structural changes to chromosomes, such as deletions, duplications, inversions, or translocations. These alterations can disrupt gene function or regulation, potentially leading to phenotypic changes. However, not all rearrangements result in observable traits, particularly if they do not affect essential genes or if the individual has compensatory mechanisms.

Phenotypic Expression

Phenotypic expression is the observable physical or biochemical characteristics of an organism, determined by both genetic makeup and environmental influences. In cases of chromosomal rearrangements, phenotypic effects may depend on factors such as the specific genes involved, the nature of the rearrangement, and whether the individual has a second functional copy of the affected gene. Thus, a phenotypically normal individual with a chromosomal rearrangement may not exhibit any traits associated with that rearrangement.