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

4. Genetic Mapping and Linkage

Mapping Genes

Genetics

4. Genetic Mapping and Linkage

Mapping Genes

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3:46 minutes

Problem 7

Textbook Question

Genes A, B, and C are linked on a chromosome and found in the order A–B–C. Genes A and B recombine with a frequency of 8%, and genes B and C recombine at a frequency of 24%. For the cross a⁺b⁺c/abc⁺ × abc/abc, predict the frequency of progeny genotypes. Assume interference is zero.

Verified step by step guidance

Understand the problem: Genes A, B, and C are linked on the same chromosome, and their recombination frequencies are given. The recombination frequency between A and B is 8% (0.08), and between B and C is 24% (0.24). The goal is to predict the frequency of progeny genotypes for the given cross, assuming no interference (i.e., double crossovers occur at the expected frequency).

Determine the parental and recombinant gametes: The parental gametes are a⁺b⁺c and abc⁺, as these are the original combinations of alleles. Recombinant gametes arise from crossing over between A and B, B and C, or both (double crossover).

Calculate the frequency of single crossovers: The frequency of single crossovers between A and B is 8% (0.08), and between B and C is 24% (0.24). These frequencies represent the proportion of gametes that will result from crossing over in these regions.

Account for double crossovers: Double crossovers involve recombination in both the A–B and B–C regions. Since interference is zero, the frequency of double crossovers is the product of the single crossover frequencies: (0.08) × (0.24). This represents the proportion of gametes that result from double crossovers.

Distribute the frequencies among the progeny genotypes: Use the recombination frequencies to calculate the proportion of each progeny genotype. Parental genotypes will have the highest frequencies, single crossover genotypes will have intermediate frequencies, and double crossover genotypes will have the lowest frequencies. Ensure that the total frequency sums to 1 (100%).

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Key Concepts

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

Genetic Linkage

Genetic linkage refers to the tendency of genes located close to each other on a chromosome to be inherited together during meiosis. This is due to their physical proximity, which reduces the likelihood of recombination events occurring between them. Understanding linkage is crucial for predicting the inheritance patterns of linked genes, as it affects the ratios of offspring genotypes.

Recombination Frequency

Recombination frequency is a measure of the likelihood that two genes will be separated during meiosis due to crossing over. It is expressed as a percentage, indicating the proportion of recombinant offspring produced. In the given question, the recombination frequencies between genes A and B (8%) and between genes B and C (24%) are essential for calculating the expected genotype frequencies in the progeny.

Interference

Interference is a phenomenon in genetics where the occurrence of one crossover event affects the likelihood of another crossover happening nearby. When interference is zero, as stated in the question, it implies that crossovers occur independently, allowing for straightforward calculations of expected genotype frequencies based on the recombination frequencies. This simplifies the analysis of linked genes in genetic crosses.

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Related Practice

Multiple Choice

The genetic distances between three genes (ab, nm, kf) were determined using a two-point mapping cross. Determine the sequence of the three genes.

True or False:Recombination frequencies are never greater than 50%

Using the following data collected from a test cross, calculate the recombination frequency.

In a diploid species of plant, the genes for plant height and fruit shape are syntenic and separated by 18 m.u. Allele D produces tall plants and is dominant to d for short plants, and allele R produces round fruit and is dominant to r for oval fruit.

Give the same information for a plant with the genotype Dr/dR.

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Textbook Question

Gene G recombines with gene T at a frequency of 7%, and gene G recombines with gene R at a frequency of 4%.

Draw two possible genetic maps for these three genes, and identify the recombination frequencies predicted for each map.

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Textbook Question

Gene G recombines with gene T at a frequency of 7%, and gene G recombines with gene R at a frequency of 4%.

Assuming that organisms with any desired genotype are available, propose a genetic cross whose result could be used to determine which of the proposed genetic maps is correct.

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Textbook Question

Genes A, B, C, D, and E are linked on a chromosome and occur in the order given.

The test cross Ae/aE x ae/ae indicates the genes recombine with a frequency of 28%. If 1000 progeny are produced by this test cross, determine the number of progeny in each outcome class.

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Textbook Question

Genes A, B, C, D, and E are linked on a chromosome and occur in the order given.

Previous genetic linkage crosses have determined that recombination frequencies are 6% for genes A and B, 4% for genes B and C, 10% for genes C and D, and 11% for genes D and E. The sum of these frequencies between genes A and E is 31%. Why does the recombination distance between these genes as determined by adding the intervals between adjacent linked genes differ from the distance determined by the test cross?

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