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Ch. 14 - Mendel and the Gene
Freeman - Biological Science 8th Edition
Freeman8th EditionBiological ScienceISBN: 9780138276263Not the one you use?Change textbook
All textbooksFreeman 8th EditionCh. 14 - Mendel and the GeneProblem 12b
Chapter 14, Problem 12b

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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Begin with the cell in the interphase stage, where the chromosomes are not yet visible. During this stage, DNA replication occurs, resulting in each chromosome consisting of two sister chromatids. You have alleles A and a on one pair of homologous chromosomes and alleles B and b on another pair of homologous chromosomes.
Proceed to prophase I of meiosis. Here, homologous chromosomes pair up in a process called synapsis, forming tetrads. Each tetrad consists of four chromatids: two from each homologous chromosome. Label the homologous chromosomes carrying alleles A and a, and the homologous chromosomes carrying alleles B and b.
In metaphase I, the tetrads align at the metaphase plate. The homologous chromosomes are still paired, and the spindle fibers attach to the centromeres. Label the sister chromatids within each homologous chromosome, noting that they are identical copies of each chromosome.
During anaphase I, the homologous chromosomes are pulled apart to opposite poles of the cell. This separates the alleles A from a and B from b, but the sister chromatids remain together. Label the nonhomologous chromosomes, which are the chromosomes carrying different genes (A/a and B/b).
In telophase I and cytokinesis, the cell divides into two cells, each with a haploid set of chromosomes. Each chromosome still consists of two sister chromatids. In meiosis II, these sister chromatids will be separated, resulting in four haploid cells, each carrying one allele for each gene (either A or a, and either B or b).

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Key Concepts

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

Meiosis Stages

Meiosis is a type of cell division that reduces the chromosome number by half, creating four haploid cells. It consists of two stages: Meiosis I and Meiosis II. Meiosis I separates homologous chromosomes, while Meiosis II separates sister chromatids. Understanding these stages is crucial for labeling the process correctly.
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Homologous and Nonhomologous Chromosomes

Homologous chromosomes are pairs of chromosomes containing the same genes but possibly different alleles, one inherited from each parent. Nonhomologous chromosomes are those that do not pair during meiosis and contain different sets of genes. Distinguishing between these types is essential for understanding genetic inheritance and chromosome behavior during meiosis.
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Genes and Alleles

Genes are segments of DNA that code for proteins, determining specific traits. Alleles are different versions of a gene that can lead to variations in the trait. In this scenario, alleles A and a, and B and b represent different forms of two genes located on separate chromosomes, highlighting the concept of genetic variation and inheritance.
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Related Practice
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. Draw a diagram illustrating what happens to these genes and alleles when meiosis occurs in your reproductive tissues.

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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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Textbook Question

The blending inheritance hypothesis proposed that the genetic material from parents is mixed in the offspring. As a result, traits of offspring and later descendants should lie between the phenotypes of parents. Mendel, in contrast, proposed that genes are discrete and that their integrity is maintained in the offspring and in subsequent generations. Suppose the year is 1890. You are a horse breeder who has just read Mendel's paper. You don't believe his results, however, because you often work with cremello (very light-colored) and chestnut (reddish-brown) horses. You know that when you breed a cremello individual from a pure-breeding line with a chestnut individual from a pure-breeding line, the offspring are palomino—meaning they have an intermediate (golden-yellow) body color. What additional cross would you do to test whether Mendel's model is valid in the case of genes for horse color? According to his model, what offspring phenotype frequencies would you get from your experimental cross? Explain why your cross would test Mendel's model versus blending inheritance.

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