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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 39a
Chapter 2, Problem 39a

A cross between a spicy variety of Capsicum annum pepper and a sweet (nonspicy) variety produces F1 progeny plants that all have spicy peppers. The F1 are crossed, and among the F2 plants are 56 that produce spicy peppers and 20 that produce sweet peppers. Dr. Ara B. Dopsis, an expert on pepper plants, discovers a gene he designates Pun1 that he believes is responsible for spicy versus sweet flavor of peppers. Dr. Dopsis proposes that a dominant allele P produces spicy peppers and that a recessive mutant allele p results in sweet peppers.
Are the data on the parental cross and the F1 and F2 consistent with the proposal made by Dr. Dopsis? Explain why or why not, using P and p to indicate probable genotypes of pepper plants.

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Step 1: Understand the problem and the genetic hypothesis. Dr. Dopsis proposes that the spicy flavor in peppers is controlled by a single gene, Pun1, with a dominant allele (P) for spicy and a recessive allele (p) for sweet. This means that plants with genotypes PP or Pp will produce spicy peppers, while plants with genotype pp will produce sweet peppers.
Step 2: Analyze the parental cross. The problem states that a cross between a spicy variety (likely PP or Pp) and a sweet variety (pp) produces F₁ progeny that are all spicy. This suggests that the spicy parent is homozygous dominant (PP), as crossing PP with pp would result in all F₁ offspring being heterozygous (Pp) and therefore spicy.
Step 3: Analyze the F₂ generation. The F₁ plants (all Pp) are crossed to produce the F₂ generation. A Pp × Pp cross follows Mendel's laws of inheritance, specifically the monohybrid cross. The expected genotypic ratio is 1:2:1 (PP:Pp:pp), and the corresponding phenotypic ratio is 3:1 (spicy:sweet).
Step 4: Compare the observed F₂ data to the expected ratio. The problem states that there are 56 spicy plants and 20 sweet plants in the F₂ generation. Calculate the total number of F₂ plants (56 + 20 = 76) and determine the expected numbers based on the 3:1 phenotypic ratio. Use the formula: Expected spicy = (3/4) × total plants, and Expected sweet = (1/4) × total plants.
Step 5: Evaluate consistency with the hypothesis. Compare the observed numbers (56 spicy, 20 sweet) to the expected numbers calculated in Step 4. Use a chi-square test to determine if the observed data significantly deviates from the expected 3:1 ratio. If the chi-square value is not significant, the data are consistent with Dr. Dopsis's hypothesis. If it is significant, the hypothesis may need to be revised.

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Key Concepts

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

Dominant and Recessive Alleles

In genetics, alleles are different forms of a gene that can exist at a specific locus on a chromosome. A dominant allele, represented by a capital letter (e.g., P), masks the effect of a recessive allele (e.g., p) when both are present in a heterozygous individual. This means that if an organism has at least one dominant allele, the dominant trait will be expressed, while the recessive trait will only be expressed in the absence of the dominant allele.
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Variations on Dominance

Punnett Square and Mendelian Inheritance

A Punnett square is a diagram used to predict the genotypes of offspring from a genetic cross. It illustrates the possible combinations of alleles from the parents, allowing for the calculation of expected phenotypic ratios. According to Mendelian inheritance, the offspring of a cross between a homozygous dominant and a homozygous recessive parent will all be heterozygous, while a cross between two heterozygous parents will yield a 3:1 ratio of dominant to recessive phenotypes in the F₂ generation.
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Chi Square Analysis

Phenotypic Ratios in F₂ Generation

The phenotypic ratio refers to the relative number of offspring expressing different traits. In the context of a monohybrid cross involving one trait, such as spicy versus sweet peppers, the expected phenotypic ratio in the F₂ generation from two heterozygous parents (Pp x Pp) is typically 3:1, where three offspring exhibit the dominant trait and one exhibits the recessive trait. This ratio can be used to assess whether the observed data from the cross aligns with Mendelian predictions.
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Mutations and Phenotypes
Related Practice
Textbook Question

Galactosemia is an autosomal recessive disorder caused by the inability to metabolize galactose, a component of the lactose found in mammalian milk. Galactosemia can be partially managed by eliminating dietary intake of lactose and galactose. Amanda is healthy, as are her parents, but her brother Alonzo has galactosemia. Brice has a similar family history. He and his parents are healthy, but his sister Brianna has galactosemia. Amanda and Brice are planning a family and seek genetic counseling. Based on the information provided, complete the following activities and answer the questions.


What is the probability that the first child of Amanda and Brice will have galactosemia? Show your work.

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Textbook Question

Galactosemia is an autosomal recessive disorder caused by the inability to metabolize galactose, a component of the lactose found in mammalian milk. Galactosemia can be partially managed by eliminating dietary intake of lactose and galactose. Amanda is healthy, as are her parents, but her brother Alonzo has galactosemia. Brice has a similar family history. He and his parents are healthy, but his sister Brianna has galactosemia. Amanda and Brice are planning a family and seek genetic counseling. Based on the information provided, complete the following activities and answer the questions.


If the first child has galactosemia, what is the probability that the second child will have galactosemia? Explain the reasoning for your answer.

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Textbook Question

Sweet yellow tomatoes with a pear shape bring a high price per basket to growers. Pear shape, yellow color, and terminal flower position are recessive traits produced by alleles f, r, and t, respectively. The dominant phenotypes for each trait—full shape, red color, and axial flower position—are the product of dominant alleles F, R, and T. A farmer has two pure-breeding tomato lines. One is full, yellow, terminal and the other is pear, red, axial. Design a breeding experiment that will produce a line of tomato that is pure-breeding for pear shape, yellow color, and axial flower position.

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Textbook Question

A cross between a spicy variety of Capsicum annum pepper and a sweet (nonspicy) variety produces F1 progeny plants that all have spicy peppers. The F1 are crossed, and among the F2 plants are 56 that produce spicy peppers and 20 that produce sweet peppers. Dr. Ara B. Dopsis, an expert on pepper plants, discovers a gene he designates Pun1 that he believes is responsible for spicy versus sweet flavor of peppers. Dr. Dopsis proposes that a dominant allele P produces spicy peppers and that a recessive mutant allele p results in sweet peppers.

Assuming the proposal is correct, what proportion of the spicy F2 pepper plants do you expect will be pure-breeding? Explain your answer.

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Textbook Question

Alkaptonuria is an infrequent autosomal recessive condition. It is first noticed in newborns when the urine in their diapers turns black upon exposure to air. The condition is caused by the defective transport of the amino acid phenylalanine through the intestinal walls during digestion. About 4 people per 1000 are carriers of alkaptonuria.

Sara and James had never heard of alkaptonuria and were shocked to discover that their first child had the condition. Sara's sister Mary and her husband, Frank, are planning to have a family and are concerned about the possibility of alkaptonuria in one of their children.

The four adults (Sara, James, Mary, and Frank) seek information from a neighbor who is a retired physician. After discussing their family histories, the neighbor says, "I never took genetics, but I know from my many years in practice that Sara and James are both carriers of this recessive condition. Since their first child had the condition, there is a very low chance that the next child will also have it, because the odds of having two children with a recessive condition are very low. Mary and Frank have no chance of having a child with alkaptonuria because Frank has no family history of the condition." The two couples each have babies and both babies have alkaptonuria.


What are the genotypes of the four adults?

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Textbook Question

Alkaptonuria is an infrequent autosomal recessive condition. It is first noticed in newborns when the urine in their diapers turns black upon exposure to air. The condition is caused by the defective transport of the amino acid phenylalanine through the intestinal walls during digestion. About 4 people per 1000 are carriers of alkaptonuria.

Sara and James had never heard of alkaptonuria and were shocked to discover that their first child had the condition. Sara's sister Mary and her husband, Frank, are planning to have a family and are concerned about the possibility of alkaptonuria in one of their children.

The four adults (Sara, James, Mary, and Frank) seek information from a neighbor who is a retired physician. After discussing their family histories, the neighbor says, 'I never took genetics, but I know from my many years in practice that Sara and James are both carriers of this recessive condition. Since their first child had the condition, there is a very low chance that the next child will also have it, because the odds of having two children with a recessive condition are very low. Mary and Frank have no chance of having a child with alkaptonuria because Frank has no family history of the condition.' The two couples each have babies and both babies have alkaptonuria.


What was incorrect about the information given to Sara and James? What is incorrect about the information given to Mary and Frank?

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