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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2:57 minutes

Problem 24d

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.

Verified step by step guidance

Understand the genetic basis of the problem: Down syndrome (trisomy 21) occurs due to the presence of an extra copy of chromosome 21. The parents and siblings with 45 chromosomes likely have a balanced Robertsonian translocation involving chromosome 21, which does not affect their phenotype but can lead to unbalanced gametes during reproduction.

Determine the possible gametes produced by the parent with the Robertsonian translocation: A parent with a Robertsonian translocation involving chromosome 21 can produce gametes with (1) a normal chromosome 21, (2) a chromosome 21 fused with another chromosome (the translocation), or (3) no chromosome 21. These gametes can combine with the normal gametes from the other parent.

Analyze the possible zygote outcomes: When the gametes combine, the zygote can have (1) a normal chromosome count and phenotype, (2) trisomy 21 (Down syndrome), (3) monosomy 21 (lethal), or (4) a balanced translocation carrier with 45 chromosomes and a normal phenotype.

Focus on the desired outcome: The problem asks for the probability of the next child having a normal phenotype and 46 chromosomes. This outcome occurs when the gamete from the parent with the translocation contributes a normal chromosome 21, and the other parent contributes a normal chromosome 21.

Calculate the probability: Determine the proportion of gametes from the parent with the translocation that contribute a normal chromosome 21, and combine this with the probability of the other parent contributing a normal chromosome 21. Use this to calculate the overall probability of the desired outcome.

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Key Concepts

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

Chromosomal Abnormalities

Chromosomal abnormalities occur when there is a deviation from the normal number of chromosomes in a cell. In this case, Down syndrome is caused by trisomy 21, where an individual has three copies of chromosome 21 instead of the usual two. Understanding how these abnormalities arise is crucial for analyzing genetic conditions and their inheritance patterns.

Mendelian Inheritance

Mendelian inheritance refers to the patterns of inheritance first described by Gregor Mendel, which include concepts such as dominant and recessive traits. In this scenario, the normal phenotype and chromosome count of the parents suggest that they may be carriers of a genetic trait, but do not express it. This concept helps in predicting the likelihood of traits being passed to offspring.

Probability in Genetics

Probability in genetics involves calculating the likelihood of certain traits or genetic conditions being passed from parents to offspring. In this case, understanding the probability of the next child having a normal phenotype and 46 chromosomes requires knowledge of the genetic makeup of the parents and the potential combinations of alleles. This concept is essential for making informed predictions about genetic outcomes.

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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 adjacent-2 segregation. Which if any of these gametes are viable?

455

views

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 adjacent-1 segregation. Which, if any, of these gametes are viable?

396

views

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?

445

views

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

views

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.

How many chromosomes do you expect to see in karyotypes of the parents?

746

views

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?

495

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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.

Key Concepts

Chromosomal Abnormalities

Mendelian Inheritance

Probability in Genetics

Watch next

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.