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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2:03 minutes

Problem 16b

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. In three crosses between agouti females whose genotypes were unknown and males of the aacc genotype, the following phenotypic ratios were obtained:
(1) 8 agouti
(2) 9 agouti
(3) 4 agouti, 8 white 10 black, 5 black, 10 white

Verified step by step guidance

Step 1: Understand the genetic basis of the problem. The C allele is necessary for pigment production, so individuals with the cc genotype are white. If the C allele is present, the A allele determines the coat color: AA or Aa results in agouti, while aa results in black. The male in all crosses is aacc, meaning he is white and contributes the recessive alleles a and c to all offspring.

Step 2: Analyze the phenotypic ratios for each cross. For Cross 1, the offspring are 8 agouti, 8 white, and 10 black. For Cross 2, the offspring are 9 agouti, 10 black, and 10 white. For Cross 3, the offspring are 4 agouti, 5 black, and 10 white. These ratios suggest different genotypes for the agouti females in each cross.

Step 3: Determine the genotype of the agouti females in each cross. Since the male is aacc, the female must contribute the C allele for any pigmented offspring (agouti or black). The ratios of agouti to black offspring indicate whether the female is homozygous (AA) or heterozygous (Aa) for the A allele. For example, a 1:1 ratio of agouti to black suggests the female is Aa, while all agouti offspring suggest the female is AA.

Step 4: Use the white offspring to confirm the female's genotype for the C allele. White offspring result from cc genotypes, meaning the female must have contributed the c allele. If white offspring are present, the female must be heterozygous (Cc) for the C allele. If no white offspring are present, the female is likely homozygous (CC).

Step 5: Summarize the genotypes of the agouti females based on the offspring ratios. For Cross 1, the 1:1 ratio of agouti to black suggests the female is Aa, and the presence of white offspring indicates she is Cc. For Cross 2, the 1:1 ratio of agouti to black and white offspring again suggests Aa and Cc. For Cross 3, the lower number of agouti offspring suggests the female may be Aa and Cc, but further analysis of the ratios is needed to confirm.

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Key Concepts

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

Alleles and Genotypes

Alleles are different forms of a gene that can exist at a specific locus on a chromosome. In this context, the C allele is necessary for pigment production in mouse fur, while the c allele results in a white phenotype. The genotype refers to the genetic makeup of an individual, such as CC, Cc, or cc, which determines the phenotype expressed.

Phenotypic Ratios

Phenotypic ratios represent the relative frequencies of different phenotypes observed in the offspring of genetic crosses. In the given crosses, the ratios of agouti, white, and black mice provide insights into the inheritance patterns of the alleles involved. Analyzing these ratios helps in understanding the dominance and interaction of alleles.

Dominance and Epistasis

Dominance refers to the relationship between alleles where one allele masks the expression of another. In this case, the presence of the C allele is dominant for pigment production. Epistasis occurs when the expression of one gene is affected by another gene, as seen with the A and a alleles influencing the color of fur in the presence of the C allele.

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Textbook Question

In Drosophila, gray body color is dominant to ebony body color, while long wings are dominant to vestigial wings. Assuming that the P₁ individuals are homozygous, work the following crosses through the F₂ generation, and determine the genotypic and phenotypic ratios for each generation. gray, vestigial, x-shaped, and long.

How many different types of gametes can be formed by individuals of the following genotypes:

(a) AaBb

(b) AaBB

(d) AaBBcc

(e) AaBbcc

(f) AaBbCcDdEe

What are the gametes in each case?

In a study of black guinea pigs and white guinea pigs, 100 black animals were crossed with 100 white animals, and each cross was carried to an F₂ generation. In 94 of the crosses, all the F₁ offspring were black and an F₂ ratio of 3 black:1 white was obtained. In the other 6 cases, half of the F₁ animals were black and the other half were white. Why? Predict the results of crossing the black and white F₁ guinea pigs from the 6 exceptional cases.

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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?

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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.

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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

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Textbook Question

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

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