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

Penetrance and Expressivity

Genetics

3. Extensions to Mendelian Inheritance

Penetrance and Expressivity

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1:35 minutes

Problem 31

Textbook Question

In goats, the development of the beard is due to a recessive gene. The following cross involving true-breeding goats was made and carried to the F₂ generation:

Offer an explanation for the inheritance and expression of this trait, diagramming the cross. Propose one or more crosses to test your hypothesis.

Verified step by step guidance

Start by identifying the key genetic information: The beard trait in goats is controlled by a recessive gene. This means that for a goat to express the beard trait, it must inherit two copies of the recessive allele (homozygous recessive). Denote the dominant allele as 'B' (no beard) and the recessive allele as 'b' (beard).

Diagram the initial cross (P generation): Since the goats are true-breeding, one parent is homozygous dominant (BB, no beard) and the other is homozygous recessive (bb, beard). Perform a Punnett square for this cross to determine the genotype and phenotype of the F₁ generation. All offspring in the F₁ generation will inherit one 'B' allele and one 'b' allele, making them heterozygous (Bb) and phenotypically without a beard.

Carry the cross to the F₂ generation: Cross two F₁ heterozygous goats (Bb × Bb). Use a Punnett square to determine the genotypic and phenotypic ratios in the F₂ generation. The possible genotypes are BB, Bb, and bb, with a phenotypic ratio of 3:1 (3 no beard: 1 beard).

Propose a test cross to confirm the hypothesis: To test the inheritance pattern, cross an F₁ heterozygous goat (Bb) with a homozygous recessive goat (bb). Perform a Punnett square for this cross. The expected offspring genotypes are 50% Bb (no beard) and 50% bb (beard), confirming the recessive inheritance pattern.

Summarize the findings: The inheritance of the beard trait in goats follows Mendelian principles of recessive inheritance. The F₂ generation phenotypic ratio (3:1) and the test cross results (1:1 ratio) support the hypothesis that the beard trait is controlled by a single recessive gene.

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Key Concepts

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

Recessive and Dominant Alleles

In genetics, alleles are different forms of a gene. A recessive allele only expresses its trait when two copies are present (homozygous), while a dominant allele can mask the presence of a recessive allele (heterozygous). In the case of beard development in goats, the trait is determined by a recessive gene, meaning that only goats with two recessive alleles will exhibit the beard phenotype.

Punnett Square

A Punnett square is a diagram used to predict the genetic outcomes of a cross between two organisms. It displays the possible combinations of alleles from each parent, allowing for the visualization of genotype and phenotype ratios in the offspring. This tool is essential for understanding inheritance patterns, especially when analyzing traits governed by dominant and recessive alleles.

Test Cross

A test cross is a breeding experiment used to determine the genotype of an individual exhibiting a dominant phenotype. By crossing the individual with a homozygous recessive individual, one can observe the phenotypes of the offspring to infer whether the dominant individual is homozygous or heterozygous. This method is crucial for confirming hypotheses about inheritance patterns in traits like beard development in goats.

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Related Practice

Multiple Choice

Penetrance measures which of the following?

Define and distinguish incomplete penetrance and variable expressivity.

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Textbook Question

Brachydactyly type D is a human autosomal dominant condition in which the thumbs are abnormally short and broad. In most cases, both thumbs are affected, but occasionally just one thumb is involved. The accompanying pedigree shows a family in which brachydactyly type D is segregating. Filled circles and squares represent females and males who have involvement of both thumbs. Half-filled symbols represent family members with just one thumb affected.

Is there any evidence of variable expressivity in this family? Explain.

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Textbook Question

Is there evidence of incomplete penetrance in this family? Explain.

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Textbook Question

Discuss the topic of phenotypic expression and the many factors that impinge on it.

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Textbook Question