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

Trihybrid Cross

Genetics

4. Genetic Mapping and Linkage

Trihybrid Cross

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1:18 minutes

Problem 16a

Textbook Question

In Drosophila, Dichaete (D) is a mutation on chromosome III with a dominant effect on wing shape. It is lethal when homozygous. The genes ebony body (e) and pink eye (p) are recessive mutations on chromosome III. Flies from a Dichaete stock were crossed to homozygous ebony, pink flies, and the F₁ progeny, with a Dichaete phenotype, were backcrossed to the ebony, pink homozygotes. Using the results of this backcross shown in the table.

Diagram this cross, showing the genotypes of the parents and offspring of both crosses.

Verified step by step guidance

Step 1: Understand the genetic setup. Dichaete (D) is a dominant mutation on chromosome III that affects wing shape and is lethal when homozygous (DD). Ebony body (e) and pink eye (p) are recessive mutations on the same chromosome. Homozygous ebony, pink flies (ee pp) are used in the cross. The goal is to diagram the genotypes of the parents and offspring for both the initial cross and the backcross.

Step 2: Diagram the first cross. The Dichaete stock parent is heterozygous for Dichaete (D d) and may carry other alleles for ebony and pink. The homozygous ebony, pink parent is ee pp. The F₁ progeny will inherit one allele from each parent. For example, the F₁ Dichaete phenotype flies will have the genotype D d e+ p+ (where e+ and p+ represent wild-type alleles).

Step 3: Diagram the backcross. The F₁ Dichaete phenotype flies (D d e+ p+) are backcrossed to the homozygous ebony, pink parent (ee pp). This cross will produce offspring with various combinations of alleles for Dichaete, ebony, and pink. The phenotypes and genotypes of the offspring will depend on the recombination events between these genes on chromosome III.

Step 4: Analyze the phenotypic ratios. The table of offspring phenotypes and their numbers suggests that recombination occurred between the genes. For example, the presence of Dichaete, ebony, pink flies (D d e p) and wild-type flies (d d e+ p+) indicates crossing over. Use the phenotypic ratios to infer the relative positions of the genes on the chromosome.

Step 5: Construct the genetic map. Based on the recombination frequencies (calculated from the offspring numbers), determine the order and distances between the Dichaete, ebony, and pink loci on chromosome III. This will complete the diagram of the cross, showing the genotypes of the parents and offspring for both the initial cross and the backcross.

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Key Concepts

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

Dominant and Recessive Alleles

In genetics, alleles can be classified as dominant or recessive based on their expression in the phenotype. A dominant allele, such as the Dichaete mutation (D), will mask the effect of a recessive allele when present. Conversely, recessive alleles, like ebony (e) and pink (p), only manifest in the phenotype when homozygous. Understanding these concepts is crucial for predicting the outcomes of genetic crosses.

Homozygous and Heterozygous Genotypes

A homozygous genotype consists of two identical alleles for a particular gene, while a heterozygous genotype contains two different alleles. In this scenario, the Dichaete mutation is lethal when homozygous (DD), meaning individuals with this genotype do not survive. The F₁ progeny from the cross between Dichaete and homozygous ebony, pink flies will be heterozygous (De, dp), which is essential for understanding the inheritance patterns observed in the backcross.

Backcrossing and Phenotypic Ratios

Backcrossing involves mating an F₁ individual with one of its parental genotypes to analyze inheritance patterns. The resulting offspring's phenotypic ratios provide insights into the genetic makeup of the parents. In this case, the backcross results yield specific ratios of Dichaete and other phenotypes, which can be used to infer the genotypes of the parents and understand the interactions between dominant and recessive alleles.

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Textbook Question

Researchers cross a corn plant that is pure-breeding for the dominant traits colored aleurone (C1), full kernel (Sh), and waxy endosperm (Wx) to a pure-breeding plant with the recessive traits colorless aleurone (c1), shrunken kernel (sh), and starchy (wx). The resulting F₁ plants were crossed to pure-breeding colorless, shrunken, starchy plants. Counting the kernels from about 30 ears of corn yields the following data.

Calculate the recombination frequencies between the gene pairs.

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Textbook Question

Flower color may be red, white, or pink, and flower shape may be personate or peloric. For the following crosses, determine the P₁ and F₁ genotypes:

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Textbook Question

In a diploid plant species, an F₁ with the genotype Gg Ll Tt is test-crossed to a pure-breeding recessive plant with the genotype gg ll tt. The offspring genotypes are as follows:

Calculate the recombination frequency between each pair of genes.

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Textbook Question

In a diploid plant species, an F₁ with the genotype Gg Ll Tt is test-crossed to a pure-breeding recessive plant with the genotype gg ll tt. The offspring genotypes are as follows:

What is the order of these three linked genes?

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Textbook Question

An organism of the genotype AaBbCc was testcrossed to a triply recessive organism (aabbcc). The genotypes of the progeny are presented in the following table.

What can you conclude from the actual data about the location of the three genes in relation to one another?

833

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Textbook Question

An organism of the genotype AaBbCc was testcrossed to a triply recessive organism (aabbcc). The genotypes of the progeny are presented in the following table.

Answer part (a) again, assuming the three genes are so tightly linked on a single chromosome that no crossover gametes were recovered in the sample of offspring.

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Textbook Question

An organism of the genotype AaBbCc was testcrossed to a triply recessive organism (aabbcc). The genotypes of the progeny are presented in the following table.

If these three genes were all assorting independently, how many genotypic and phenotypic classes would result in the offspring, and in what proportion, assuming simple dominance and recessiveness in each gene pair?

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Textbook Question

A wild-type trihybrid soybean plant is crossed to a pure-breeding soybean plant with the recessive phenotypes pale leaf (l), oval seed (r), and short height (t). The results of the three-point test cross are shown below. Traits not listed are wild type.

Calculate the interference value for these data.

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