Ch. 3 - Mendelian Genetics

Klug - Concepts of Genetics 12th Edition

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Klug 12th Edition

Ch. 3 - Mendelian Genetics

Problem 16

Chapter 3, Problem 16

Mendel crossed peas having round green seeds with peas having wrinkled yellow seeds. All F₁ plants had seeds that were round and yellow. Predict the results of testcrossing these F₁ plants.

Verified step by step guidance

Identify the traits and their dominance relationships: Round (R) is dominant over wrinkled (r), and yellow (Y) is dominant over green (y).

Determine the genotype of the F₁ plants. Since the parents were round green (R_yy) and wrinkled yellow (rrY_), and all F₁ were round yellow, the F₁ genotype must be heterozygous for both traits: RrYy.

Understand that a testcross involves crossing the F₁ heterozygote (RrYy) with a homozygous recessive individual for both traits (rryy).

Set up a Punnett square for the testcross, where the F₁ parent can produce four types of gametes: RY, Ry, rY, and ry, and the testcross parent produces only ry gametes.

Predict the phenotypic ratios of the offspring by combining each F₁ gamete with the testcross gamete, resulting in four phenotypic classes: round yellow, round green, wrinkled yellow, and wrinkled green, each with equal probability.

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Key Concepts

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

Mendelian Inheritance and Dominance

Mendelian inheritance describes how traits are passed from parents to offspring through dominant and recessive alleles. In this case, round seed shape and yellow seed color are dominant traits, while wrinkled and green are recessive. The F₁ generation shows dominant phenotypes because they inherit one dominant allele from each parent.

Testcross

A testcross involves crossing an individual with a dominant phenotype but unknown genotype with a homozygous recessive individual. This helps reveal the genotype of the dominant individual by analyzing the phenotypes of the offspring, distinguishing between homozygous dominant and heterozygous genotypes.

Independent Assortment and Dihybrid Cross

Independent assortment states that alleles of different genes segregate independently during gamete formation. Crossing round yellow (heterozygous for both traits) peas with wrinkled green (homozygous recessive) peas results in a 1:1:1:1 phenotypic ratio in offspring, reflecting all combinations of seed shape and color.

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Related Practice

Textbook Question

Mendel crossed peas having green seeds with peas having yellow seeds. The F₁ generation produced only yellow seeds. In the F₂, the progeny consisted of 6022 plants with yellow seeds and 2001 plants with green seeds. Of the F₂ yellow-seeded plants, 519 were self-fertilized with the following results: 166 bred true for yellow and 353 produced an F₃ ratio of 3/4 yellow: 1/4 green. Explain these results by diagramming the crosses.

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Textbook Question

In assessing data that fell into two phenotypic classes, a geneticist observed values of 250:150. She decided to perform a $χ$ ² analysis by using the following two different null hypotheses:

(a) the data fit a 3:1 ratio, and

(b) the data fit a 1:1 ratio.

Calculate the $χ$ ² values for each hypothesis. What can be concluded about each hypothesis?

Textbook Question

Thalassemia is an inherited anemic disorder in humans. Affected individuals exhibit either a minor anemia or a major anemia. Assuming that only a single gene pair and two alleles are involved in the inheritance of these conditions, is thalassemia a dominant or recessive disorder?

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Textbook Question

The following are F₂ results of two of Mendel's monohybrid crosses.

For each cross, state a null hypothesis to be tested using x² analysis. Calculate the x² value and determine the p value for both. Interpret the p-values. Can the deviation in each case be attributed to chance or not? Which of the two crosses shows a greater amount of deviation?

Textbook Question

How many different types of gametes can be formed by individuals of the following genotypes:

(a) AaBb

(b) AaBB

(d) AaBBcc

(e) AaBbcc

(f) AaBbCcDdEe

What are the gametes in each case?

Textbook Question

In a study of black guinea pigs and white guinea pigs, 100 black animals were crossed with 100 white animals, and each cross was carried to an F₂ generation. In 94 of the crosses, all the F₁ offspring were black and an F₂ ratio of 3 black:1 white was obtained. In the other 6 cases, half of the F₁ animals were black and the other half were white. Why? Predict the results of crossing the black and white F₁ guinea pigs from the 6 exceptional cases.