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Ch. 14 - Mendel and the Gene
Chapter 14, Problem 12c

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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1
Understand the concept of independent assortment: The alleles of different genes segregate independently of each other during the formation of gametes. This is due to the genes being located on different chromosomes.
Identify the alleles for each gene: You have alleles A and a for the first gene, and alleles B and b for the second gene.
Determine possible combinations of alleles in gametes: Since the genes assort independently, each allele combination is equally likely. The possible gametes are AB, Ab, aB, and ab.
Calculate the frequency of each gamete type: Each gamete type has an equal probability of forming because each allele combination is equally likely. Therefore, each gamete (AB, Ab, aB, ab) has a frequency of 1/4 or 25%.
Summarize the gamete types and their frequencies: The genetically different gametes that could form are AB, Ab, aB, and ab, each occurring with a frequency of 25%.

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Key Concepts

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

Mendelian Genetics

Mendelian genetics is the study of how traits are inherited through generations based on the principles established by Gregor Mendel. It involves understanding dominant and recessive alleles, as well as the segregation and independent assortment of alleles during gamete formation. This framework is essential for predicting the genetic makeup of offspring from parental genotypes.
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Gamete Formation

Gamete formation occurs through the process of meiosis, where diploid cells divide to produce haploid gametes. In the case of heterozygous individuals for two genes on different chromosomes, each gamete can receive one allele from each gene, leading to a combination of alleles. The number of genetically different gametes can be calculated using the formula 2^n, where n is the number of heterozygous gene pairs.
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Independent Assortment

Independent assortment is a principle of genetics stating that alleles for different genes segregate independently of one another during gamete formation. This means that the inheritance of one trait will not affect the inheritance of another trait located on a different chromosome. In the scenario provided, this principle allows for the combination of alleles A, a, B, and b in various ways, leading to four possible gametes: AB, Ab, aB, and ab.
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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. 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. 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. 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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Textbook Question

Two mothers give birth to sons at the same time in a busy hospital. The son of couple 1 is afflicted with hemophilia A, which is a recessive X-linked disease. Neither parent has the disease. Couple 2 has a normal son even though the father has hemophilia A. The two couples sue the hospital in court, claiming that a careless staff member swapped their babies at birth. You appear in court as an expert witness. What do you tell the jury? Make a diagram that you can submit to the jury.

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