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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
All textbooksKlug 10th EditionCh. 4 - Modification of Mendelian RatiosProblem 3a
Chapter 4, Problem 3a

In shorthorn cattle, coat color may be red, white, or roan. Roan is an intermediate phenotype expressed as a mixture of red and white hairs. The following data were obtained from various crosses:
Table showing coat color crosses in cattle with outcomes: red x red all red, white x white all white, red x white all roan, roan x roan mixed ratios.
How is coat color inherited? What are the genotypes of parents and offspring for each cross?

Verified step by step guidance
1
Step 1: Identify the inheritance pattern. The data shows that crossing red with red produces all red offspring, white with white produces all white offspring, and red with white produces all roan offspring. Roan crossed with roan produces a 1:2:1 ratio of red, roan, and white. This suggests incomplete dominance, where neither allele is completely dominant, and the heterozygote shows an intermediate phenotype.
Step 2: Assign genotypes to the coat colors. Let the allele for red be \( R \) and the allele for white be \( W \). Since red crossed with white produces all roan, roan must be the heterozygous genotype \( RW \). Thus, red is \( RR \), white is \( WW \), and roan is \( RW \).
Step 3: Analyze each cross using genotypes. For red x red (\( RR \times RR \)), all offspring are \( RR \) (red). For white x white (\( WW \times WW \)), all offspring are \( WW \) (white). For red x white (\( RR \times WW \)), all offspring are \( RW \) (roan).
Step 4: For roan x roan (\( RW \times RW \)), use a Punnett square to determine offspring genotypes. The possible genotypes are \( RR \), \( RW \), and \( WW \) in a 1:2:1 ratio, corresponding to red, roan, and white phenotypes respectively.
Step 5: Summarize the inheritance: coat color in shorthorn cattle is controlled by incomplete dominance with two alleles, \( R \) and \( W \). The genotypes and phenotypes are: \( RR \) = red, \( WW \) = white, and \( RW \) = roan.

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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 over the other, resulting in a heterozygous phenotype that is intermediate between the two homozygous phenotypes. In shorthorn cattle, red and white coat colors show incomplete dominance, producing roan offspring with a mix of red and white hairs.
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Genotype and Phenotype Relationships

Genotypes represent the genetic makeup of an organism, while phenotypes are the observable traits. Understanding how different allele combinations (e.g., RR, RW, WW) correspond to coat colors (red, roan, white) is essential to predict offspring outcomes from various crosses.
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Punnett Square and Mendelian Ratios

Punnett squares are tools used to predict the genotypic and phenotypic ratios of offspring from parental crosses. The observed ratios in roan x roan crosses (1/4 red, 1/2 roan, 1/4 white) reflect Mendelian inheritance patterns consistent with incomplete dominance.
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Related Practice
Textbook Question

The discussion centers on extensions and modifications of Mendelian principles and ratios. In the process, we encountered many opportunities to consider how this information was acquired. On the basis of these discussions, what answers would you propose to the following fundamental questions?

How do we know that specific genes are located on the sex-determining chromosomes rather than on autosomes?

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Textbook Question

The discussion centers on extensions and modifications of Mendelian principles and ratios. In the process, we encountered many opportunities to consider how this information was acquired. On the basis of these discussions, what answers would you propose to the following fundamental questions?

For genes whose expression seems to be tied to the sex of individuals, how do we know whether a gene is X-linked in contrast to exhibiting sex-limited or sex-influenced inheritance?

505
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Textbook Question

With regard to the ABO blood types in humans, determine the genotype of the male parent and female parent shown here:

Male parent: Blood type B; mother type O

Female parent: Blood type A; father type B

Predict the blood types of the offspring that this couple may have and the expected proportion of each.

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Textbook Question

In foxes, two alleles of a single gene, P and p, may result in lethality (PP), platinum coat (Pp), or silver coat (pp). What ratio is obtained when platinum foxes are interbred? Is the P allele behaving dominantly or recessively in causing (a) lethality; (b) platinum coat color?

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