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

2. Mendel's Laws of Inheritance

Dihybrid Cross

Genetics

2. Mendel's Laws of Inheritance

Dihybrid Cross

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2:06 minutes

Problem 17c

Textbook Question

In rats, the following genotypes of two independently assorting autosomal genes determine coat color:

A third gene pair on a separate autosome determines whether or not any color will be produced. The CC and Cc genotypes allow color according to the expression of the A and B alleles. However, the cc genotype results in albino rats regardless of the A and B alleles present. Determine the F₁ phenotypic ratio of the following crosses:
AaBbCc×AaBbcc

Verified step by step guidance

Step 1: Identify the genotypes and phenotypes involved in the problem. The three genes are: (1) Gene A, which determines coat color (A— for gray, aa for black); (2) Gene B, which modifies coat color (B— for gray or black, bb for yellow or cream); and (3) Gene C, which determines whether color is expressed (C— for color, cc for albino).

Step 2: Determine the possible gametes for each parent. For the first parent (AaBbCc), the possible gametes are formed by combining one allele from each gene: A or a, B or b, and C or c. This results in 8 possible gametes: ABC, ABc, AbC, Abc, aBC, aBc, abC, and abc. For the second parent (AaBbcc), the possible gametes are formed similarly, but since the C gene is homozygous recessive (cc), the gametes will always carry the c allele. This results in 4 possible gametes: ABc, Abc, aBc, and abc.

Step 3: Set up a Punnett square to determine the genotypes of the offspring. Since the first parent produces 8 types of gametes and the second parent produces 4 types of gametes, the Punnett square will have 8 rows and 4 columns. Fill in the squares by combining the alleles from the gametes of each parent.

Step 4: Analyze the offspring genotypes to determine their phenotypes. For each genotype, consider the following: (1) If the offspring has at least one C allele (C—), the coat color will depend on the A and B alleles. Use the given phenotypic rules (e.g., A—B— for gray, A—bb for yellow, aaB— for black, aabb for cream). (2) If the offspring is cc, it will be albino regardless of the A and B alleles.

Step 5: Count the number of offspring with each phenotype and calculate the phenotypic ratio. Group the offspring into categories based on their phenotypes (e.g., gray, yellow, black, cream, albino) and count how many fall into each category. Express the results as a ratio of phenotypes.

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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 traits segregate independently of one another during gamete formation. This means that the inheritance of one trait (e.g., coat color) does not influence the inheritance of another trait (e.g., albinism). Understanding this concept is crucial for predicting the phenotypic ratios in genetic crosses involving multiple genes.

Genotype and Phenotype

The genotype refers to the genetic makeup of an organism, while the phenotype is the observable expression of that genotype, influenced by both genetic and environmental factors. In the context of the question, the different combinations of alleles (genotypes) for the A, B, and C genes lead to distinct coat colors (phenotypes) in the rats, which must be analyzed to determine the F₁ phenotypic ratio.

Epistasis

Epistasis occurs when the expression of one gene is affected by one or more other genes. In this case, the presence of the cc genotype masks the expression of the A and B alleles, resulting in an albino phenotype regardless of the coat color genes. Recognizing this interaction is essential for accurately calculating the phenotypic ratios in the given genetic crosses.

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Textbook Question

Pigment in mouse fur is only produced when the C allele is present. Individuals of the cc genotype are white. If color is present, it may be determined by the A, a alleles. AA or Aa results in agouti color, while aa results in black coats. What F₁ and F₂ genotypic and phenotypic ratios are obtained from a cross between AACC and aacc mice?

618

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Textbook Question

Mendel crossed peas having round green seeds with peas having wrinkled yellow seeds. All F₁ plants had seeds that were round and yellow. Predict the results of testcrossing these F₁ plants.

592

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Textbook Question

In rats, the following genotypes of two independently assorting autosomal genes determine coat color:

A third gene pair on a separate autosome determines whether or not any color will be produced. The CC and Cc genotypes allow color according to the expression of the A and B alleles. However, the cc genotype results in albino rats regardless of the A and B alleles present. Determine the F₁ phenotypic ratio of the following crosses:

AABbCc×AABbcc

1359

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Textbook Question

In rats, the following genotypes of two independently assorting autosomal genes determine coat color:

649

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Textbook Question

In rats, the following genotypes of two independently assorting autosomal genes determine coat color:

AAbbCC×aaBBcc

518

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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).

544

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Textbook Question

Another recessive mutation in Drosophila, ebony (e), is on an autosome (chromosome 3) and causes darkening of the body compared with wild-type flies. What phenotypic F₁ and F₂ male and female ratios will result if a scalloped-winged female with normal body color is crossed with a normal-winged ebony male?

Work out this problem by both the Punnett square method and the forked-line method.

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Textbook Question

In tomato plants, the production of red fruit color is under the control of an allele R. Yellow tomatoes are rr. The dominant phenotype for fruit shape is under the control of an allele T, which produces two lobes. Multilobed fruit, the recessive phenotype, has the genotype tt. Two different crosses are made between parental plants of unknown genotype and phenotype. Use the progeny phenotype ratios to determine the genotypes and phenotypes of each parent.

Cross 1 progeny:

3/8 two-lobed, Red

3/8 two-lobed, yellow

1/8 multilobed, Red

1/8 multilobed, Yellow

Cross 2 progeny:

1/4 two-lobed, Red

1/4 two-lobed, yellow

1/4 multilobed, Red

1/4 multilobed, yellow

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