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Ch. 2 - Transmission Genetics
Sanders - Genetic Analysis: An Integrated Approach 3rd Edition
Sanders3rd EditionGenetic Analysis: An Integrated ApproachISBN: 9780135564172Not the one you use?Change textbook
All textbooksSanders 3rd EditionCh. 2 - Transmission GeneticsProblem 30a
Chapter 2, Problem 30a

A male and a female are each heterozygous for both cystic fibrosis (CF) and phenylketonuria (PKU). Both conditions are autosomal recessive, and they assort independently.
What proportion of the children of this couple will have neither condition?

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1
Determine the genotypes of the parents for both traits. Since both parents are heterozygous for cystic fibrosis (CF) and phenylketonuria (PKU), their genotypes are CcPp, where 'C' and 'P' represent the dominant alleles, and 'c' and 'p' represent the recessive alleles.
Use the principle of independent assortment to analyze each trait separately. For cystic fibrosis, the cross is Cc x Cc. For phenylketonuria, the cross is Pp x Pp.
Construct a Punnett square for each trait. For the CF trait (Cc x Cc), the possible offspring genotypes are CC, Cc, Cc, and cc. For the PKU trait (Pp x Pp), the possible offspring genotypes are PP, Pp, Pp, and pp.
Identify the offspring genotypes that result in neither condition. For a child to have neither condition, they must inherit at least one dominant allele for each trait. This means the child must have a genotype that is not 'cc' for CF and not 'pp' for PKU. Combine the probabilities of these outcomes using the rule of multiplication.
Calculate the proportion of offspring with neither condition by multiplying the probabilities of having at least one dominant allele for CF (from the CF Punnett square) and at least one dominant allele for PKU (from the PKU Punnett square). This gives the final proportion of children with neither condition.

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Key Concepts

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

Autosomal Recessive Inheritance

Autosomal recessive inheritance means that a trait or condition is expressed only when an individual has two copies of the recessive allele, one from each parent. In the case of cystic fibrosis (CF) and phenylketonuria (PKU), both conditions require the individual to inherit two recessive alleles (cf/cf and pku/pku) to express the disease. If an individual has at least one dominant allele (CF or PKU), they will not exhibit the condition.
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Autosomal Pedigrees

Heterozygous Genotype

A heterozygous genotype consists of two different alleles for a particular gene, one inherited from each parent. In this scenario, both parents are heterozygous for CF and PKU, meaning they each carry one normal allele (C and P) and one recessive allele (cf and pku). This genetic makeup is crucial for determining the potential genotypes of their offspring through Punnett squares.
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Gamete Genotypes

Independent Assortment

Independent assortment is a principle of genetics stating that alleles for different traits segregate independently during gamete formation. In this case, the alleles for CF and PKU assort independently, allowing for various combinations of alleles in the offspring. This principle is essential for calculating the probabilities of the children inheriting neither condition, as it allows for the separate consideration of each trait's inheritance.
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Related Practice
Textbook Question

A variety of pea plant called Blue Persian produces a tall plant with blue seeds. A second variety of pea plant called Spanish Dwarf produces a short plant with white seed. The two varieties are crossed, and the resulting seeds are collected. All of the seeds are white; and when planted, they produce all tall plants. These tall F₁ plants are allowed to self-fertilize. The results for seed color and plant stature in the F₂ generation are as follows:

   F₂ Plant Phenotype    Number

   Blue seed, tall plant.     97

   White seed, tall plant    270

   Blue seed, short plant    33

   White seed, short plant  100

  TOTAL                500


Examine the data in the table by the chi-square test and determine whether they conform to expectations of the hypothesis.

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Textbook Question

In tomato plants, the production of red fruit color is under the control of an allele R. Yellow tomatoes are rr. The dominant phenotype for fruit shape is under the control of an allele T, which produces two lobes. Multilobed fruit, the recessive phenotype, has the genotype tt. Two different crosses are made between parental plants of unknown genotype and phenotype. Use the progeny phenotype ratios to determine the genotypes and phenotypes of each parent.

 Cross 1 progeny:

3/8 two-lobed, Red

                    3/8 two-lobed, yellow

                    1/8 multilobed, Red

                    1/8 multilobed, Yellow

 Cross 2 progeny:

1/4 two-lobed, Red

                     1/4 two-lobed, yellow

                     1/4 multilobed, Red

                     1/4 multilobed, yellow

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Textbook Question

During your work as a laboratory assistant in the research facilities of Dr. O. Sophila, a world-famous geneticist, you come across an unusual bottle of fruit flies. All the flies in the bottle appear normal when they are in an incubator set at 22°C. When they are moved to a 30°C incubator, however, a few of the flies slowly become paralyzed; and after about 20 to 30 minutes, they are unable to move. Returning the flies to 22°C restores their ability to move after about 30 to 45 minutes.

With Dr. Sophila's encouragement, you set up 10 individual crosses between single male and female flies that exhibit the unusual behavior. Among 812 progeny, 598 exhibit the unusual behavior and 214 do not. When you leave one of the test bottles in the 30°C incubator too long, you discover that more than 2 hours at high temperature kills the paralyzed flies. When you tell this to Dr. Sophila, he says, 'Aha! I know how to explain this condition.' What is his explanation?

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Textbook Question

A male and a female are each heterozygous for both cystic fibrosis (CF) and phenylketonuria (PKU). Both conditions are autosomal recessive, and they assort independently.

What proportion of the children will have either PKU or CF but not both?

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Textbook Question

A male and a female are each heterozygous for both cystic fibrosis (CF) and phenylketonuria (PKU). Both conditions are autosomal recessive, and they assort independently.

What proportion of the children will be carriers of one or both conditions?

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Textbook Question

Dr. Ara B. Dopsis and Dr. C. Ellie Gans are performing genetic crosses on daisy plants. They self-fertilize a blue-flowered daisy and grow 100 progeny plants that consist of 55 blue-flowered plants, 22 purple-flowered plants, and 23 white-flowered plants. Dr. Dopsis believes this is the result of segregation of two alleles at one locus and that the progeny ratio is 1:2:1. Dr. Gans thinks the progeny phenotypes are the result of two epistatic genes and that the ratio is 9:3:4.

The two scientists ask you to resolve their conflict by performing chi-square analysis on the data for both proposed genetic mechanisms. For each proposed mechanism, fill in the values requested on the form the researchers have provided for your analysis.


Using any of the 100 progeny plants, propose a cross that will verify the conclusion you proposed in part (c). Plants may be self-fertilized, or one plant can be crossed to another. What result will be consistent with the 1:2:1 hypothesis? What result will be consistent with the 9:3:4 hypothesis?

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