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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 10a

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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Step 1: Determine the inheritance pattern. The data suggests that the crested trait is dominant because when two crested birds are crossed, all offspring are crested. This indicates that the crested phenotype (C) is dominant over the non-crested phenotype (c).
Step 2: Analyze the first cross (22 non-crested, 7 crested). Since both parents are non-crested and produce both crested and non-crested offspring, they must be heterozygous (Cc) to allow for the appearance of the dominant crested trait in their offspring.
Step 3: Assign genotypes to the offspring from the first cross. The ratio of non-crested to crested (approximately 3:1) suggests a Mendelian inheritance of a single gene. Therefore, the genotypes of the offspring are likely 1 CC (crested), 2 Cc (crested), and 1 cc (non-crested).
Step 4: Examine the second cross (two crested birds producing all crested offspring). Both parents are likely CC, as crossing two CC would result in 100% crested offspring (CC).
Step 5: Evaluate the third cross (7 non-crested, 6 crested from a non-crested and a crested parent). The non-crested parent must be cc, and the crested parent must be Cc, as this cross produces about a 1:1 ratio of non-crested to crested, consistent with a cross of cc x Cc.

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Key Concepts

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

Genetics and Inheritance Patterns

Genetics is the study of heredity and the variation of inherited characteristics. In this context, understanding inheritance patterns, such as dominant and recessive traits, is crucial. The crested appearance in pigeons likely follows Mendelian inheritance, where one allele may dominate over another, influencing the phenotype of the offspring.
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Phenotype and Genotype

The phenotype refers to the observable traits of an organism, such as feather appearance in pigeons, while the genotype is the genetic makeup that determines these traits. In this scenario, the non-crested and crested phenotypes suggest different genotypes, which can be represented using symbols to denote dominant and recessive alleles.
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Punnett Squares and Genetic Crosses

Punnett squares are a tool used to predict the genotypes of offspring from genetic crosses. By analyzing the crosses described, one can set up Punnett squares to visualize the potential combinations of alleles from the parents, helping to determine the expected ratios of phenotypes in the offspring based on the genotypes of the parents.
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Related Practice
Textbook Question

In garden peas, yellow seeds (Y) are dominant to green seeds (y), and inflated pods (I) are dominant to constricted pods (i). Suppose you have crossed YYII parents with yyii parents. List the genotype(s) of gametes produced by F1 individuals.

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Textbook Question

In garden peas, yellow seeds (Y) are dominant to green seeds (y), and inflated pods (I) are dominant to constricted pods (i). Suppose you have crossed YYII parents with yyii parents. Draw the F2 Punnett square. Based on this Punnett square, predict the expected phenotype(s) in the F2 generation and the expected frequency of each phenotype.

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Textbook Question

In parakeets, two autosomal genes that are located on different chromosomes control the production of feather pigment. Gene B codes for an enzyme that is required for the synthesis of a blue pigment, and gene Y codes for an enzyme required for the synthesis of a yellow pigment. Green results from a mixture of yellow and blue pigments, and recessive mutations that prevent production of either pigment are known for both genes. Suppose that a breeder has two green parakeets and mates them. The offspring are green, blue, yellow, and albino (unpigmented).

Based on this observation, what are the genotypes of the green parents?

What genotypes produce each color in the offspring? What fraction of the progeny should exhibit each type of color?

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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. Label the stages of meiosis, the homologous chromosomes, sister chromatids, nonhomologous chromosomes, genes, and alleles.

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