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

3. Extensions to Mendelian Inheritance

Understanding Independent Assortment

Genetics

3. Extensions to Mendelian Inheritance

Understanding Independent Assortment

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1:07 minutes

Problem 32b

Textbook Question

Three independently assorting genes (A, B, and C) are known to control the following biochemical pathway that provides the basis for flower color in a hypothetical plant:

Three homozygous recessive mutations are also known, each of which interrupts a different one of these steps. Determine the phenotypic results in the F₁ and F₂ generations resulting from the P₁ crosses of true-breeding plants listed here:
yellow (AAbbCC) × green (AABBcc)

Verified step by step guidance

Step 1: Identify the genotypes of the parental (P₁) plants. The first parent is homozygous recessive for the B gene (AAbbCC), and the second parent is homozygous recessive for the C gene (AABBcc). Both parents are homozygous dominant for the other genes.

Step 2: Determine the F₁ genotype by performing a cross between the P₁ genotypes. Since the P₁ plants are true-breeding, all F₁ offspring will inherit one dominant allele and one recessive allele for each gene, resulting in the genotype AaBbCc.

Step 3: Predict the phenotype of the F₁ generation. Since all F₁ individuals are heterozygous for all three genes, and assuming that dominant alleles (A, B, C) are required for the pathway to produce the final flower color, all F₁ plants will exhibit the wild-type phenotype (e.g., purple flowers).

Step 4: Determine the F₂ genotypes by performing a dihybrid cross for each gene independently. The F₁ individuals (AaBbCc) will produce gametes with all possible combinations of alleles (ABC, ABc, AbC, Abc, aBC, aBc, abC, abc). Use a Punnett square to calculate the F₂ genotypic ratios.

Step 5: Predict the F₂ phenotypic ratios by analyzing the biochemical pathway. For each step in the pathway, a recessive homozygous genotype (aa, bb, or cc) will block the pathway, resulting in intermediate phenotypes (e.g., yellow or green flowers). Combine the results of the Punnett square with the pathway logic to determine the phenotypic ratios in the F₂ generation.

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Key Concepts

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

Independent Assortment

Independent assortment is a fundamental principle of genetics stating that alleles for different genes segregate independently of one another during gamete formation. This means that the inheritance of one trait will not affect the inheritance of another trait, allowing for a variety of combinations in offspring. In the context of the question, it implies that the genes A, B, and C will assort independently when determining the phenotypes of the F₁ and F₂ generations.

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 the given question, the parental plants are homozygous for their respective traits (AAbbCC and AABBcc), which will influence the genotypes of the F₁ generation. Understanding these terms is crucial for predicting the genetic makeup of the offspring and their resulting phenotypes.

Phenotypic Ratios in Genetic Crosses

Phenotypic ratios describe the relative frequencies of different phenotypes in the offspring resulting from a genetic cross. In this scenario, the phenotypes of the F₁ and F₂ generations can be predicted using a Punnett square, which illustrates the combinations of alleles from the parents. Analyzing these ratios helps in understanding how the recessive mutations affect flower color and how they manifest in the offspring.

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