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Ch. 14 - Mendel and the Gene
All textbooksFreeman 8th EditionCh. 14 - Mendel and the GeneProblem 12a
Chapter 14, Problem 12a

Suppose you are heterozygous for two genes that are located on different chromosomes. You carry alleles A and a for one gene and alleles B and b for the other. Draw a diagram illustrating what happens to these genes and alleles when meiosis occurs in your reproductive tissues.

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Start with a diploid cell containing homologous chromosomes for Gene 1 (A and a) and Gene 2 (B and b).
During Meiosis I, homologous chromosomes are separated into two different cells. Each cell will have one chromosome from each pair (e.g., one cell will have A and B, the other will have a and b).
In Meiosis II, the sister chromatids of each chromosome are separated into different cells.
This results in four haploid cells, each with one allele from each gene (e.g., AB, Ab, aB, ab).
These haploid cells can then participate in fertilization, combining with another haploid cell to form a diploid organism.

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Key Concepts

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

Meiosis

Meiosis is a specialized type of cell division that reduces the chromosome number by half, resulting in four haploid gametes. It consists of two sequential divisions: meiosis I and meiosis II. During meiosis I, homologous chromosomes are separated, while meiosis II separates sister chromatids. This process is crucial for sexual reproduction, ensuring genetic diversity through recombination and independent assortment.
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Independent Assortment

Independent assortment is a principle of genetics that states that alleles for different genes segregate independently of one another during gamete formation. This occurs during metaphase I of meiosis when homologous chromosome pairs align randomly at the cell's equator. As a result, the combination of alleles that end up in gametes is a mix of maternal and paternal genes, contributing to genetic variation in offspring.
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Heterozygosity

Heterozygosity refers to the presence of two different alleles at a specific gene locus on homologous chromosomes. In the context of the question, being heterozygous for genes A/a and B/b means that the individual carries one dominant and one recessive allele for each gene. This genetic variation is important for the expression of traits and can influence the phenotype of the organism, especially when considering dominant and recessive interactions.
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Related Practice
Textbook Question

Imagine repeating the experiment on epigenetic inheritance that is shown in Figure 19.6. You measure the amount of radioactive uridine (U) incorporated into Hnf4a mRNA in counts per minute (cpm) to determine the level of Hnf4a gene transcription in rats born to mothers fed either a normal diet or a low-protein diet. The results are 11,478 cpm for the normal diet and 7368 cpm for the low-protein diet. For this problem, your task is to prepare a graph similar to the one at the bottom of Figure 19.6 that shows the normalized results for the low-protein diet relative to the normal diet. Normalizing values means that the value obtained from one condition is expressed as 1.0 (the norm; the normal diet in this case) and the values obtained from any other conditions (low-protein diet in this case) are expressed as decimal values relative to the norm.

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Textbook Question

The smooth feathers on the back of the neck in pigeons can be reversed by a mutation to produce a 'crested' appearance in which feathers form a distinctive spike at the back of the head. A pigeon breeder examined offspring produced by a single pair of non-crested birds and recorded the following: 22 non-crested and 7 crested. She then made a series of crosses using offspring from the first cross. When she crossed two of the crested birds, all 20 of the offspring were crested. When she crossed a non-crested bird with a crested bird, 7 offspring were non-crested and 6 were crested. For these three crosses, provide genotypes for parents and offspring that are consistent with these results.

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Textbook Question

The smooth feathers on the back of the neck in pigeons can be reversed by a mutation to produce a 'crested' appearance in which feathers form a distinctive spike at the back of the head. A pigeon breeder examined offspring produced by a single pair of non-crested birds and recorded the following: 22 non-crested and 7 crested. She then made a series of crosses using offspring from the first cross. When she crossed two of the crested birds, all 20 of the offspring were crested. When she crossed a non-crested bird with a crested bird, 7 offspring were non-crested and 6 were crested. Which allele is dominant?

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Textbook Question

Suppose you are heterozygous for two genes that are located on different chromosomes. You carry alleles A and a for one gene and alleles B and b for the other. Label the stages of meiosis, the homologous chromosomes, sister chromatids, nonhomologous chromosomes, genes, and alleles.


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Textbook Question

Suppose you are heterozygous for two genes that are located on different chromosomes. You carry alleles A and a for one gene and alleles B and b for the other. Be sure to list all the genetically different gametes that could form and indicate how frequently each type should be observed.

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Textbook Question

Suppose you are heterozygous for two genes that are located on different chromosomes. You carry alleles A and a for one gene and alleles B and b for the other. On the diagram, identify the events responsible for the principle of segregation and the principle of independent assortment.

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