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Chapter 4: Problem 3

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 are obtained from various crosses: red \(\times\) red \(\longrightarrow\) all red white \(\times\) white \(\longrightarrow\) all white red \(\times\) white \(\longrightarrow\) all roan \\[\text { roan } \times \operatorname{roan} \longrightarrow 1 / 4 \text { red: } 1 / 2\\] roan: \(1 / 4\) white (a) How is coat color inherited? What are the genotypes of parents and offspring for each cross? (b) Does the roan phenotype illustrate a case of incomplete dominance or a case of codominance? Explain.

Short Answer

Expert verified
Based on the data from the Shorthorn cattle crosses, we can conclude that the coat colors are inherited following Mendelian inheritance principles, with both red and white alleles being dominant. The genotypes for the coat colors are: Red (RR), White (WW), and Roan (RW). The Roan phenotype illustrates a case of codominance, as it is a clear mixture of both Red and White phenotypes rather than a blend.

Step by step solution

01

Analyze the data from the crosses

Looking at the data, we can see the following results: 1. Red x Red: all Red 2. White x White: all White 3. Red x White: all Roan 4. Roan x Roan: 1/4 Red, 1/2 Roan, 1/4 White From this information, we can see that both red and white coat colors breed true and when crossed, an intermediate phenotype (Roan) is produced.
02

Determine the genotypes for each color

To understand the inheritance pattern, we need to assign a genotype for each coat color. Let's use R for red allele and W for white allele. Both alleles are dominant and seen together as Roan. So, we can assign the following genotypes: 1. Red: RR 2. White: WW 3. Roan: RW
03

Discuss the inheritance pattern based on analysis

Using the genotypes we assigned, let's write down the outcomes for each cross: 1. Red x Red: RR x RR -> all RR 2. White x White: WW x WW -> all WW 3. Red x White: RR x WW -> all RW (Roan) 4. Roan x Roan: RW x RW -> 1:2:1 ratio of RR (Red): RW (Roan): WW (White) The inheritance pattern seems to be in line with Mendelian inheritance principles, with both alleles showing dominance but together producing a third phenotype (Roan).
04

Incomplete dominance or codominance?

In the case of incomplete dominance, the heterozygous individuals show a phenotype that is intermediate between both homozygous phenotypes (a blend of both). In the case of codominance, the heterozygous individuals show a phenotype with both homozygous traits fully expressed and distinguishable. Here, the roan coat color is a mixture of red and white hairs. Given that the Roan phenotype is a clear mixture of both Red and White phenotypes rather than a blend, we can conclude that the roan phenotype illustrates a case of codominance.

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