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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2:47 minutes

Problem 24b

Textbook Question

A boy with Down syndrome (trisomy 21) has 46 chromosomes. His parents and his two older sisters have a normal phenotype, but each has 45 chromosomes.
How many chromosomes do you expect to see in karyotypes of the parents?

Verified step by step guidance

Understand the problem: Down syndrome (trisomy 21) occurs when there is an extra copy of chromosome 21. The boy has 46 chromosomes, which is unusual because individuals with Down syndrome typically have 47 chromosomes. The parents and sisters have 45 chromosomes, which suggests a balanced translocation involving chromosome 21.

Recall the concept of a balanced translocation: A balanced translocation occurs when two chromosomes exchange segments without any genetic material being lost or gained. Individuals with a balanced translocation typically have a normal phenotype but may pass on an unbalanced chromosome arrangement to their offspring.

Determine the parental karyotype: Since the parents have 45 chromosomes, one of them likely carries a balanced translocation involving chromosome 21 and another chromosome. This means that one chromosome 21 is fused with another chromosome, reducing the total chromosome count to 45.

Explain the inheritance pattern: The boy inherited the extra chromosome 21 from the parent with the balanced translocation. This occurred because the parent passed on the translocated chromosome along with a normal chromosome 21, resulting in trisomy 21 in the boy.

Conclude the expected chromosome count: Both parents are expected to have 45 chromosomes, with one of them carrying the balanced translocation. The other parent likely has a normal karyotype with 46 chromosomes, but this is not explicitly stated in the problem and would require further clarification.

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Key Concepts

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

Karyotype

A karyotype is a visual representation of an individual's chromosomes, organized in pairs and arranged by size. It is used to identify chromosomal abnormalities, such as aneuploidy, which is an abnormal number of chromosomes. In humans, a normal karyotype consists of 46 chromosomes, arranged into 23 pairs.

Aneuploidy

Aneuploidy refers to the presence of an abnormal number of chromosomes in a cell. This can occur due to nondisjunction during meiosis, leading to gametes with an incorrect number of chromosomes. In the case of Down syndrome, which is caused by trisomy 21, there is an extra copy of chromosome 21, resulting in a total of 47 chromosomes in affected individuals.

Inheritance Patterns

Inheritance patterns describe how genetic traits and conditions are passed from parents to offspring. In the context of Down syndrome, it is important to understand that while the boy has 47 chromosomes, his parents and sisters, having 45 chromosomes, suggest a possible chromosomal abnormality in the parents. This could indicate a structural rearrangement or a condition like Turner syndrome, which affects the number of sex chromosomes.

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Related Practice

Textbook Question

An animal heterozygous for a reciprocal balanced translocation has the following chromosomes:

MN • OPQRST

MN • OPQRjkl

cdef • ghijkl

cdef • ghiST

Identify the gametes produced by alternate segregation. Which, if any, of these gametes are viable?

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Textbook Question

An animal heterozygous for a reciprocal balanced translocation has the following chromosomes:

MN • OPQRST

MN • OPQRjkl

cdef • ghijkl

cdef • ghiST

Diagram the pairing of these chromosomes in prophase I.

359

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Textbook Question

A boy with Down syndrome (trisomy 21) has 46 chromosomes. His parents and his two older sisters have a normal phenotype, but each has 45 chromosomes.

What is the probability the next child of this couple will have a normal phenotype and have 46 chromosomes? Explain your answer.

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Textbook Question

A boy with Down syndrome (trisomy 21) has 46 chromosomes. His parents and his two older sisters have a normal phenotype, but each has 45 chromosomes.

What term best describes this kind of chromosome abnormality?

327

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Textbook Question

A boy with Down syndrome (trisomy 21) has 46 chromosomes. His parents and his two older sisters have a normal phenotype, but each has 45 chromosomes.

Explain how this is possible.

362

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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?

A normal chromosome and its homolog carrying a paracentric inversion are shown here. The dot (·) represents the centromere.

Normal ABC • DEFGHIJK

Inversion abc • djihgfe

Diagram the alignment of chromosomes during prophase I.

370

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