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

Variations of Dominance

Genetics

3. Extensions to Mendelian Inheritance

Variations of Dominance

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1:29 minutes

Problem 4

Textbook Question

In foxes, two alleles of a single gene, P and p, may result in lethality (PP), platinum coat (Pp), or silver coat (pp). What ratio is obtained when platinum foxes are interbred? Is the P allele behaving dominantly or recessively in causing (a) lethality; (b) platinum coat color?

Verified step by step guidance

Step 1: Understand the genetic basis of the problem. The gene in question has two alleles: P and p. The genotype PP results in lethality, Pp results in a platinum coat, and pp results in a silver coat. This indicates that the P allele has different effects depending on whether it is homozygous (PP) or heterozygous (Pp).

Step 2: Set up the Punnett square for the cross. Since platinum foxes have the genotype Pp, the cross is Pp × Pp. Write the possible gametes for each parent: P and p for each parent.

Step 3: Fill in the Punnett square by combining the gametes from each parent. The possible offspring genotypes are PP, Pp, Pp, and pp. This gives a genotypic ratio of 1 PP : 2 Pp : 1 pp.

Step 4: Interpret the phenotypic outcomes. The PP genotype results in lethality, so these individuals do not survive. The Pp genotype results in a platinum coat, and the pp genotype results in a silver coat. Therefore, the phenotypic ratio among the surviving offspring is 2 platinum : 1 silver.

Step 5: Analyze the dominance behavior of the P allele. (a) For lethality, the P allele behaves as a recessive lethal allele because lethality occurs only in the homozygous PP genotype. (b) For platinum coat color, the P allele behaves as a dominant allele because the heterozygous Pp genotype expresses the platinum phenotype.

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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 versions of a gene that can exist at a specific locus on a chromosome. In this case, the alleles P and p represent different traits in foxes. The genotype of an organism, which is the combination of alleles it possesses (e.g., PP, Pp, pp), determines its phenotype, or observable traits, such as coat color and lethality.

Dominance and Recessiveness

Dominance refers to the relationship between alleles where one allele (dominant) masks the expression of another (recessive) in a heterozygous genotype. In this scenario, the P allele is dominant over the p allele in determining coat color, as Pp results in a platinum coat, while pp results in a silver coat. However, the PP genotype leads to lethality, indicating a unique case of dominance where the dominant allele can also cause death.

Punnett Squares and Genetic Ratios

A Punnett square is a tool used to predict the genotypic and phenotypic ratios of offspring from a genetic cross. When interbreeding platinum foxes (Pp x Pp), the Punnett square reveals the expected ratios of offspring: 1 PP (lethal), 2 Pp (platinum), and 1 pp (silver), resulting in a 2:1 ratio of platinum to silver foxes. This analysis helps clarify the inheritance patterns of the alleles involved.

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Multiple Choice

Which of the following parents could produce offspring with an AB blood type?

Blood types are an example of what type of dominance?

How were early geneticists able to ascertain inheritance patterns that did not fit typical Mendelian ratios?

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Textbook Question

In shorthorn cattle, coat color may be red, white, or roan. Roan is an intermediate phenotype expressed as a mixture of red and white hairs. The following data were obtained from various crosses:

How is coat color inherited? What are the genotypes of parents and offspring for each cross?

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Textbook Question

In mice, a short-tailed mutant was discovered. When it was crossed to a normal long-tailed mouse, 4 offspring were short-tailed and 3 were long-tailed. Two short-tailed mice from the F1 generation were selected and crossed. They produced 6 short-tailed and 3 long-tailed mice. These genetic experiments were repeated three times with approximately the same results. What genetic ratios are illustrated? Hypothesize the mode of inheritance and diagram the crosses.

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Textbook Question

The ABO and MN blood groups are shown for four sets of parents (1 to 4) and four children (a to d). Recall that the ABO blood group has three alleles: I^A, I^B and i. The MN blood group has two codominant alleles, M and N. Using your knowledge of these genetic systems, match each child with every set of parents who might have conceived the child, and exclude any parental set that could not have conceived the child.

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Textbook Question

List all possible genotypes for the A, B, AB, and O phenotypes. Is the mode of inheritance of the ABO blood types representative of dominance, recessiveness, or codominance?

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Textbook Question

The wild-type color of horned beetles is black, although other colors are known. A black horned beetle from a pure-breeding strain is crossed to a pure-breeding green female beetle. All of their F₁ progeny are black. These F₁ are allowed to mate at random with one another, and 320 F₂ beetles are produced. The F₂ consists of 179 black, 81 green, and 60 brown. Use these data to explain the genetics of horned beetle color.

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