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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
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 that since the genes are located on different chromosomes, they will assort independently according to Mendel's law of independent assortment.
Identify the alleles you carry: A and a for one gene, B and b for the other gene. Being heterozygous means you have one dominant and one recessive allele for each gene.
Determine the possible combinations of alleles in the gametes. Since the genes assort independently, each gamete will receive one allele from each gene pair.
List the possible gametes: AB, Ab, aB, and ab. These combinations arise from the independent assortment of the alleles.
Calculate the frequency of each gamete type. Since the genes assort independently, each combination is equally likely, so each gamete type should be observed with a frequency of 25% or 1/4.

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Key Concepts

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

Mendelian Inheritance

Mendelian inheritance refers to the principles of genetic inheritance discovered by Gregor Mendel. It involves the segregation and independent assortment of alleles during gamete formation. In this context, each parent contributes one allele for each gene, and the alleles segregate independently if located on different chromosomes, leading to various combinations in gametes.
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Polygenic Inheritance

Independent Assortment

Independent assortment is a key principle of genetics stating that alleles of different genes are distributed independently of one another during meiosis. This occurs because genes located on different chromosomes are assorted into gametes independently, resulting in a variety of genetic combinations. For heterozygous genes A/a and B/b, this means gametes can be AB, Ab, aB, or ab.
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Probability of Gamete Formation

The probability of gamete formation is determined by the independent assortment of alleles. For two heterozygous genes on different chromosomes, each allele combination (AB, Ab, aB, ab) is equally likely, with a probability of 25% for each. This equal distribution arises because each allele pair segregates independently during meiosis, ensuring all combinations are equally probable.
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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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