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Ch. 4 - Modification of Mendelian Ratios
Klug - Essentials of Genetics 10th Edition
Klug10th EditionEssentials of GeneticsISBN: 9780135588789Not the one you use?Change textbook
Chapter 4, Problem 24b

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.
Probabilities of offspring coat colors from matings: cremello × palomino, chestnut × palomino, and palomino × palomino.
Predict the F1 and F2 results of many initial matings between cremello and chestnut horses.

Verified step by step guidance
1
Step 1: Identify the genotypes corresponding to each horse color phenotype. Cremello is homozygous for the cream allele (let's denote it as CC), chestnut is homozygous recessive (cc), and palomino is heterozygous (Cc).
Step 2: Determine the F1 generation from a cross between cremello (CC) and chestnut (cc). Use a Punnett square to combine the alleles from each parent: all offspring will be heterozygous (Cc), which corresponds to the palomino phenotype.
Step 3: For the F2 generation, cross two F1 palomino horses (Cc × Cc). Use a Punnett square to find the genotypic ratios: 1 CC (cremello), 2 Cc (palomino), and 1 cc (chestnut).
Step 4: Translate the genotypic ratios into phenotypic ratios for the F2 generation: 1/4 cremello, 1/2 palomino, and 1/4 chestnut.
Step 5: Summarize the results: F1 offspring from cremello × chestnut matings are all palomino, while the F2 generation shows a 1:2:1 phenotypic ratio of cremello, palomino, and chestnut, respectively.

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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 neither allele is completely dominant, resulting in a heterozygous phenotype that is a blend of the two homozygous phenotypes. In horses, the palomino coat color is an example, where crossing a chestnut and a cremello produces a palomino, a mix of the two colors.
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Variations on Dominance

Genotype and Phenotype Ratios in Crosses

Predicting offspring involves understanding genotype combinations and their corresponding phenotypes. For example, crossing cremello and chestnut horses produces palomino offspring, and analyzing F1 and F2 generations requires calculating expected ratios based on Mendelian inheritance patterns.
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Gamete Genotypes

Breeding True vs. Segregation of Alleles

Breeding true means offspring consistently show the parent's phenotype, indicating homozygosity. Palominos do not breed true because they are heterozygous, producing a mix of cremello, chestnut, and palomino offspring in subsequent generations due to allele segregation.
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Related Practice
Textbook Question

Labrador retrievers may be black, brown (chocolate), or golden (yellow) in color (see chapter-opening photo). While each color may breed true, many different outcomes are seen when numerous litters are examined from a variety of matings where the parents are not necessarily true breeding. Following are just some of the many possibilities.

(a) black x brown → all black

(b) black x brown → 1/2 black, 1/2 brown

(c) black x brown → 3/4 black, 1/4 golden

(d) black x golden → all black

(e) black x golden → 4/8 golden 3/8 black 1/8 brown

(f) black x golden → 2/4 golden 1/4 black 1/4 brown

(g) brown x brown → 3/4 brown 1/4 golden

(h) black x black → 9/16 black 4/16 golden, 3/16 brown

Propose a mode of inheritance that is consistent with these data, and indicate the corresponding genotypes of the parents in each mating. Indicate as well the genotypes of dogs that breed true for each color.

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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

Pigment in mouse fur is only produced when the C allele is present. Individuals of the cc genotype are white. If color is present, it may be determined by the A, a alleles. AA or Aa results in agouti color, while aa results in black coats. What F₁ and F₂ genotypic and phenotypic ratios are obtained from a cross between AACC and aacc mice?

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Textbook Question

Pigment in mouse fur is only produced when the C allele is present. Individuals of the cc genotype are white. If color is present, it may be determined by the A, a alleles. AA or Aa results in agouti color, while aa results in black coats. In three crosses between agouti females whose genotypes were unknown and males of the aacc genotype, the following phenotypic ratios were obtained:

(1) 8 agouti

(2) 9 agouti

(3) 4 agouti, 8 white 10 black, 5 black, 10 white

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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:

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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