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Ch. 4 - Extensions of Mendelian Genetics
Chapter 4, Problem 14

Horses can be cremello (a light cream color), chestnut (a brownish color), or palomino (a golden color with white in the horse's tail and mane). Of these phenotypes, only palominos never breed true. Predict the F₁ and F₂ results of many initial matings between cremello and chestnut horses.

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Identify the genotypes associated with each phenotype: Cremello (CC), Chestnut (cc), and Palomino (Cc).
Determine the genotypes of the parent horses: Cremello (CC) and Chestnut (cc).
Perform a Punnett square for the F₁ generation using the genotypes of the parents: CC (Cremello) and cc (Chestnut).
Analyze the F₁ generation results: All offspring will have the genotype Cc, which corresponds to the Palomino phenotype.
For the F₂ generation, perform a Punnett square using the F₁ generation's genotype (Cc) crossed with itself (Cc x Cc) to predict the phenotypic ratios.

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Key Concepts

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

Incomplete Dominance

Incomplete dominance occurs when the phenotype of heterozygotes is intermediate between the phenotypes of the two homozygotes. In the case of horse coat colors, a cremello (homozygous for a cream allele) crossed with a chestnut (homozygous for a brown allele) may produce offspring that exhibit a blend of these traits, leading to a distinct phenotype in the F₁ generation.
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Variations on Dominance

Punnett Square

A Punnett square is a diagram used to predict the genotypes of offspring from a genetic cross. By setting up a Punnett square for the cremello and chestnut horses, one can visualize the potential combinations of alleles in the F₁ generation, helping to determine the expected ratios of phenotypes and genotypes in the offspring.
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F₁ and F₂ Generations

The F₁ generation refers to the first filial generation of offspring resulting from a cross between two parental (P) genotypes. The F₂ generation is produced by interbreeding the F₁ individuals. Analyzing the F₁ and F₂ generations allows for understanding inheritance patterns, including the ratios of different phenotypes and the potential for traits to segregate in subsequent generations.
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Related Practice
Textbook Question
In rabbits, a series of multiple alleles controls coat color in the following way: C is dominant to all other alleles and causes full color. The chinchilla phenotype is due to the c^ch allele, which is dominant to all alleles other than C. The c^h allele, dominant only to (albino), results in the Himalayan coat color. Thus, the order of dominance is C > c^ch > c^h > c^a. For each of the following three cases, the phenotypes of the P₁ generations of two crosses are shown, as well as the phenotype of one member of the F₁ generation. For each case, determine the genotypes of the P₁ generation and the F₁ offspring, and predict the results of making each indicated cross between F₁ individuals.
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Textbook Question

In an earlier Problems and Discussion section (see Chapter 7, Problem 27), we described CC, the cat created by nuclear transfer cloning, whereby a diploid nucleus from one cell is injected into an enucleated egg cell to create an embryo. Cattle, sheep, rats, dogs, and several other species have been cloned using nuclei from somatic cells. Embryos and adults produced by this approach often show a number of different mitochondrial defects. Explain possible reasons for the prevalence of mitochondrial defects in embryos created by nuclear transfer cloning.

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Textbook Question
Horses can be cremello (a light cream color), chestnut (a brownish color), or palomino (a golden color with white in the horse's tail and mane). Of these phenotypes, only palominos never breed true. From the results given above, determine the mode of inheritance by assigning gene symbols and indicating which genotypes yield which phenotypes.
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Textbook Question
If you knew that a devastating late-onset inherited disease runs in your family (in other words, a disease that does not appear until later in life) and you could be tested for it at the age of 20, would you want to know whether you are a carrier? Would your answer be likely to change when you reach age 40?
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Textbook Question
The genes encoding the red- and green-color-detecting proteins of the human eye are located next to one another on the X chromosome and probably evolved from a common ancestral pigment gene. The two proteins demonstrate 76 percent homology in their amino acid sequences. A normal-visioned woman (with both genes present on each of her two X chromosomes) has a red-color-blind son who was shown to have one copy of the green-detecting gene and no copies of the red-detecting gene. Devise an explanation for these observations at the chromosomal level (involving meiosis).
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Textbook Question
Five human matings (1–5), identified by both maternal and paternal phenotypes for ABO and MN blood-group antigen status, are shown on the left side of the following table: Parental Phenotypes Offspring (1) A, M x A, N (a) A, N (2) B, M x B, M (b) O, N (3) O, N x B, N (c) O, MN (4) AB, M x O, N (d) B, M (5) AB, MN x AB, MN (e) B, MN Each mating resulted in one of the five offspring shown in the right-hand column (a–e). Match each offspring with one correct set of parents, using each parental set only once. Is there more than one set of correct answers?
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