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Chapter 6: Problem 34

The frizzle fowl is much admired by poultry fanciers. It gets its name from the unusual way that its feathers curl up, giving the impression that it has been (in the memorable words of animal geneticist \(\mathrm{F}\) B. Hutt) "pulled backwards through a knothole." Unfortunately, frizzle fowl do not breed true: when two frizzles are intercrossed, they always produce 50 percent frizzles, 25 percent normal, and 25 percent with peculiar woolly feathers that soon fall out, leaving the birds naked. a. Give a genetic explanation for these results, showing genotypes of all phenotypes, and provide a statement of how your explanation works. b. If you wanted to mass-produce frizzle fowl for sale, which types would be best to use as a breeding pair?

Short Answer

Expert verified
The genetic explanation involves incomplete dominance: frizzle (\(Ff\)) and normal (\(ff\)), with woolly (\(FF\)) as a homozygous dominant phenotype. Breed \(Ff\) with \(ff\) for optimal frizzle production.

Step by step solution

01

Identify the Traits and Their Dominance

In this scenario, there are three observable traits: frizzle, normal, and "woolly" (which results in naked birds). Our goal is to determine the genetic basis of these traits. Given the inheritance pattern, we suspect that frizzle is incompletely dominant over normal, and the woolly trait results from homozygosity. Let's use the notation: \(F\) for frizzle allele and \(f\) for normal feather allele.
02

Determine the Genotypes of the Phenotypes

Based on the inheritance pattern (50% frizzle, 25% normal, 25% "woolly"), and using the \(F\) and \(f\) alleles, we can assign genotypes to each phenotype: \(Ff\) produces frizzle feathers (heterozygous), \(ff\) produces normal feathers (homozygous recessive), and \(FF\) produces the woolly feathers that lead to being bald.
03

Check the Cross Results

When two frizzle birds (\(Ff\)) are crossed, the Punnett square would look like this: each parent contributes either \(F\) or \(f\). The resulting genotypic ratio is 1 \(FF\) : 2 \(Ff\) : 1 \(ff\), leading to phenotypic ratios of 25% woolly (\(FF\)), 50% frizzle (\(Ff\)), and 25% normal (\(ff\)), which matches the observed outcomes.
04

Determine Ideal Breeding Pair for Frizzle Fowls

To mass-produce frizzle fowls, you should aim for a breeding pair that yields the highest percentage of frizzle offspring. The best pairing for this is a frizzle (\(Ff\)) with a normal (\(ff\)), as this yields a 50% offspring with \(Ff\) (frizzle) and 50% with \(ff\) (normal), avoiding the woolly phenotype (\(FF\)).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Incomplete Dominance
Incomplete dominance occurs when a dominant allele does not completely mask the effects of a recessive allele in a genotype. This results in a new, intermediate phenotype that is not identical to either parent. In the case of the frizzle fowl, the frizzle trait is incompletely dominant over the normal trait.

When we say incomplete dominance, we mean that neither trait is truly dominant over the other. Instead, the heterozygous genotype (\(Ff\) in this case) produces a phenotype that appears midway between the two parental traits. In our example:
  • Homozygous (\(FF\)) results in woolly feathers that eventually lead the bird to become bald.
  • Heterozygous (\(Ff\)) produces the characteristic frizzle feathers.
  • Homozygous recessive (\(ff\)) exhibits the normal feather trait.
Understanding incomplete dominance is key to predicting outcomes in these genetic crosses and ensuring we can explain why certain traits appear in the breeding process.

While not as straightforward as simple dominance/recessive relationships, incomplete dominance offers a fascinating example of the complexity of genetics.
Punnett Square
The Punnett square is a helpful tool used to predict the possible genotypes of offspring from two parent organisms. It allows us to visualize how alleles are passed from parents to their offspring, as seen with frizzle fowls.

To use a Punnett square for the frizzle fowl, we start by writing down the genotypes of the parents, both heterozygous frizzle (\(Ff\)). We then draw a grid, placing one parent's alleles across the top and the other parent's alleles along the side:

\[\begin{array}{c|c|c} & F & f \\hlineF & FF & Ff \\hlinef & Ff & ff \end{array}\]

In each box of the grid, we combine the alleles from the top and side. The outcomes are:
  • 25% will have the genotype \(FF\)
  • 50% will have the genotype \(Ff\)
  • 25% will have the genotype \(ff\)
These genotypes correlate with the phenotypes we observe, with 25% woolly, 50% frizzle, and 25% normal feathers. Punnett squares make it easier to visually identify potential outcomes and their respective probabilities.
Phenotypic Ratios
Phenotypic ratios are the observable characteristics resulting from specific genetic combinations. In the frizzle fowl example, we see a clear pattern: 50% of the offspring display frizzle feathers (\(Ff\)), 25% display normal feathers (\(ff\)), and 25% develop woolly feathers that lead to baldness (\(FF\)).

This ratio reflects the probabilities derived from their genetic makeup. The interplay of alleles (\(F\) and \(f\)) in frizzle fowls determines these phenotypes directly.

It's important to remember:
  • Phenotypic ratios highlight how frequently we can expect certain phenotypes in offspring, given particular parental genotypes.
  • Ratios are crucial for breeders who aim to predict and select for desirable traits.
  • Understanding these ratios empowers efficient breeding practices, especially when aiming to minimize undesirable traits, such as the woolly feather phenotype.
Being attuned to these ratios aids not just in planning breeding programs, but also in advancing our broader understanding of genetic inheritance patterns.

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Most popular questions from this chapter

A dominant allele H reduces the number of body bristles that Drosophila flies have, giving rise to a "hairless" phenotype. In the homozygous condition, \(H\) is lethal. An independently assorting dominant allele Shas no effect on bristle number except in the presence of \(H,\) in which case a single dose of \(S\) suppresses the hairless phenotype, thus restoring the hairy phenotype. However, \(S\) also is lethal in the homozygous (S/S) condition. a. What ratio of hairy to hairless flies would you find in the live progeny of a cross between two hairy flies both carrying \(H\) in the suppressed condition? b. When the hairless progeny are backcrossed with a parental hairy fly, what phenotypic ratio would you expect to find among their live progeny?

For several years, Hans Nachtsheim investigated an inherited anomaly of the white blood cells of rabbits. This anomaly, termed the Pelger anomaly, is the arrest of the segmentation of the nuclei of certain white cells. This anomaly does not appear to seriously burden the rabbits. a. When rabbits showing the Pelger anomaly were mated with rabbits from a true-breeding normal stock, Nachtsheim counted 217 offspring showing the Pelger anomaly and 237 normal progeny. What is the genetic basis of the Pelger anomaly? b. When rabbits with the Pelger anomaly were mated with each other, Nachtsheim found 223 normal progeny, 439 with the Pelger anomaly, and 39 extremely abnormal progeny. These very abnormal progeny not only had defective white blood cells, but also showed severe deformities of the skeletal system; almost all of them died soon after birth. In genetic terms, what do you suppose these extremely defective rabbits represented? Why were there only 39 of them? c. What additional experimental evidence might you collect to test your hypothesis in part \(b\) ? d. In Berlin, about 1 human in 1000 shows a Pelger anomaly of white blood cells very similar to that described for rabbits. The anomaly is inherited as a simple dominant, but the homozygous type has not been observed in humans. Based on the condition in rabbits, why do you suppose the human homozygous has not been observed? e. Again by analogy with rabbits, what phenotypes and genotypes would you expect among the children of a man and woman who both show the Pelger anomaly? (Data from A. M. Srb, R. D. Owen, and R. S. Edgar, General Genetics, 2 nd ed. W. H. Freeman and Company, \(1965 .)\)

On a fox ranch in Wisconsin, a mutation arose that gave a "platinum" coat color. The platinum color proved very popular with buyers of fox coats, but the breeders could not develop a pure-breeding platinum strain. Every time two platinums were crossed, some normal foxes appeared in the progeny. For example, the repeated matings of the same pair of platinums produced 82 platinum and 38 normal progeny. All other such matings gave similar progeny ratios. State a concise genetic hypothesis that accounts for these results.

In Drosophila, an autosomal gene determines the shape of the hair, with \(B\) giving straight and \(b\) giving bent hairs. On another autosome, there is a gene of which a dominant allele \(I\) inhibits hair formation so that the fly is hairless ( \(i\) has no known phenotypic effect). a. If a straight-haired fly from a pure line is crossed with a fly from a pure-breeding hairless line known to be an inhibited bent genotype, what will the genotypes and phenotypes of the \(\mathrm{F}_{1}\) and the \(\mathrm{F}_{2}\) be? b. What cross would give the ratio 4 hairless: 3 straight: 1 bent?

Two normal-looking fruit flies were crossed, and, in the progeny, there were 202 females and 98 males. a. What is unusual about this result? b. Provide a genetic explanation for this anomaly. c. Provide a test of your hypothesis.
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