Textbook Question
Having correctly established the F₂ ratio in Problem 18, predict the F₂ ratio of a 'dihybrid' cross involving two independently assorting characteristics (e.g., P₁ = WWWWAAAA x wwwwaaaa).
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Ch. 6 - Chromosome Mutations: Variation in Number and Arrangement
Klug10th EditionEssentials of GeneticsISBN: 9780135588789
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Table of contents
- Ch. 1 - Introduction to Genetics16
- Ch. 2 - Mitosis and Meiosis28
- Ch. 3 - Mendelian Genetics37
- Ch. 4 - Modification of Mendelian Ratios53
- Ch. 5 - Sex Determination and Sex Chromosomes32
- Ch. 6 - Chromosome Mutations: Variation in Number and Arrangement37
- Ch. 7 - Linkage and Chromosome Mapping in Eukaryotes36
- Ch. 8 - Genetic Analysis and Mapping in Bacteria and Bacteriophages24
- Ch. 9 - DNA Structure and Analysis38
- Ch. 10 - DNA Replication29
- Ch. 11 - Chromosome Structure and DNA Sequence Organization22
- Ch. 12 - The Genetic Code and Transcription36
- Ch. 13 - Translation and Proteins27
- Ch. 14 - Gene Mutation, DNA Repair, and Transposition34
- Ch. 15 - Regulation of Gene Expression in Bacteria22
- Ch. 16 - Regulation of Gene Expression in Eukaryotes37
- Ch. 17 - Recombinant DNA Technology27
- Ch. 18 - Genomics, Bioinformatics, and Proteomics30
- Ch. 19 - The Genetics of Cancer21
- Ch. 20 - Quantitative Genetics and Multifactorial Traits37
- Ch. 21 - Population and Evolutionary Genetics45
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Klug 10th EditionCh. 6 - Chromosome Mutations: Variation in Number and ArrangementProblem 19b
Klug 10th EditionCh. 6 - Chromosome Mutations: Variation in Number and ArrangementProblem 19bChapter 6, Problem 19b
A couple planning their family are aware that through the past three generations on the husband's side a substantial number of stillbirths have occurred and several malformed babies were born who died early in childhood. The wife has studied genetics and urges her husband to visit a genetic counseling clinic, where a complete karyotype-banding analysis is performed. Although the tests show that he has a normal complement of 46 chromosomes, banding analysis reveals that one member of the chromosome 1 pair (in group A) contains an inversion covering 70 percent of its length. The homolog of chromosome 1 and all other chromosomes show the normal banding sequence.
What can you predict about the probability of abnormality/normality of their future children?
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Understand that the husband carries a large paracentric inversion on one chromosome 1, meaning a segment of the chromosome is reversed but the overall chromosome number remains normal (46 chromosomes).
Recall that during meiosis, homologous chromosomes pair and undergo crossing over. In individuals heterozygous for an inversion, the inverted segment forms a loop to align properly with the normal homolog.
Recognize that crossing over within the inversion loop can produce abnormal chromatids with duplications and deletions, which often lead to inviable gametes or zygotes with developmental abnormalities.
Note that because the inversion covers 70% of the chromosome length, the chance of crossover within this large inverted segment is relatively high, increasing the risk of producing unbalanced gametes.
Conclude that the couple's future children have a significant risk of chromosomal abnormalities or miscarriage due to unbalanced gametes from the father's inversion, but some gametes will be normal or carry the inversion without imbalance, so normal offspring are also possible.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Chromosomal Inversions
A chromosomal inversion occurs when a segment of a chromosome breaks off, flips, and reinserts in the reverse orientation. Inversions can be pericentric (including the centromere) or paracentric (excluding the centromere). While carriers often have a normal phenotype, inversions can disrupt gene function or cause problems during meiosis, leading to abnormal gametes.
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Inversions
Meiotic Consequences of Inversions and Recombination
During meiosis, homologous chromosomes pair and exchange genetic material via crossing over. In inversion heterozygotes, crossing over within the inverted segment can produce unbalanced gametes with duplications or deletions, increasing the risk of miscarriages, stillbirths, or congenital abnormalities in offspring.
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Genetic Counseling and Risk Prediction
Genetic counseling uses family history and chromosomal analysis to assess reproductive risks. In cases of large inversions, counselors estimate the likelihood of producing abnormal gametes and offspring. Although the carrier may be phenotypically normal, the risk of abnormal children depends on inversion size, location, and recombination frequency.
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Related Practice
Textbook Question
A couple planning their family are aware that through the past three generations on the husband's side a substantial number of stillbirths have occurred and several malformed babies were born who died early in childhood. The wife has studied genetics and urges her husband to visit a genetic counseling clinic, where a complete karyotype-banding analysis is performed. Although the tests show that he has a normal complement of 46 chromosomes, banding analysis reveals that one member of the chromosome 1 pair (in group A) contains an inversion covering 70 percent of its length. The homolog of chromosome 1 and all other chromosomes show the normal banding sequence.
How would you explain the high incidence of past stillbirths?
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Textbook Question
A couple planning their family are aware that through the past three generations on the husband's side a substantial number of stillbirths have occurred and several malformed babies were born who died early in childhood. The wife has studied genetics and urges her husband to visit a genetic counseling clinic, where a complete karyotype-banding analysis is performed. Although the tests show that he has a normal complement of 46 chromosomes, banding analysis reveals that one member of the chromosome 1 pair (in group A) contains an inversion covering 70 percent of its length. The homolog of chromosome 1 and all other chromosomes show the normal banding sequence.
Would you advise the woman that she will have to bring each pregnancy to term to determine whether the fetus is normal? If not, what else can you suggest?
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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.
What kind of chromosomal aberration is shown?
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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.
Using a drawing, demonstrate how these chromosomes would pair during meiosis. Be sure to label the different segments of the chromosomes.
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