2. Mendel's Laws of Inheritance

Probability and Genetics

2. Mendel's Laws of Inheritance

Probability and Genetics

2:34 minutes

Problem 39e

Textbook Question

A geneticist from an alien planet that prohibits genetic research brought with him to Earth two pure-breeding lines of frogs. One line croaks by uttering 'rib-it rib-it' and has purple eyes. The other line croaks more softly by muttering 'knee-deep knee-deep' and has green eyes. With a newfound freedom of inquiry, the geneticist mated the two types of frogs, producing F₁ frogs that were all utterers and had blue eyes. A large F₂ generation then yielded the following ratios:
27/64 blue-eyed, 'rib-it' utterer
12/64 green-eyed, 'rib-it' utterer
9/64 blue-eyed, 'knee-deep' mutterer
9/64 purple-eyed, 'rib-it' utterer
4/64 green-eyed, 'knee-deep' mutterer
3/64 purple-eyed, 'knee-deep' mutterer
After years of experiments, the geneticist isolated pure-breeding strains of all six F₂ phenotypes. Indicate the F₁ and F₂ phenotypic ratios of the following cross using these pure-breeding strains: blue-eyed, 'knee-deep' mutterer x purpled-eyed, 'rib-it' utterer.

Verified step by step guidance

Step 1: Begin by analyzing the F₁ generation. The F₁ frogs are all 'rib-it' utterers and have blue eyes. This suggests that the traits for 'rib-it' utterance and blue eyes are dominant over 'knee-deep' muttering and other eye colors (green and purple). Determine the genotypes of the F₁ frogs based on this dominance pattern.

Step 2: Examine the F₂ generation phenotypic ratios. The ratios (27/64, 12/64, etc.) suggest a dihybrid cross involving two genes, each with two alleles. Use the phenotypic ratios to deduce the inheritance pattern of the traits (e.g., whether they follow Mendelian inheritance and whether there is any epistasis or interaction between the genes).

Step 3: Identify the genotypes of the pure-breeding strains isolated by the geneticist. Pure-breeding strains are homozygous for their respective traits. For example, a pure-breeding blue-eyed, 'knee-deep' mutterer would be homozygous recessive for the 'knee-deep' trait and homozygous for the blue-eye trait. Write out the genotypes for all six phenotypes.

Step 4: Set up the cross between the pure-breeding blue-eyed, 'knee-deep' mutterer and the pure-breeding 'rib-it' utterer. Determine the genotypes of the parents and predict the genotypes and phenotypes of the F₁ generation using a Punnett square. Consider the dominance relationships established earlier.

Step 5: Use the F₁ generation genotypes to predict the F₂ generation phenotypic ratios. Perform a dihybrid cross (if applicable) and calculate the expected phenotypic ratios based on Mendelian inheritance. Ensure that the ratios align with the observed F₂ generation data provided in the problem.

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Key Concepts

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

Mendelian Genetics

Mendelian genetics is the study of how traits are inherited through generations based on the principles established by Gregor Mendel. It involves understanding dominant and recessive alleles, genotype and phenotype distinctions, and the laws of segregation and independent assortment. These principles help predict the outcomes of genetic crosses, such as the ratios of phenotypes in offspring.

Phenotypic Ratios

Phenotypic ratios represent the relative frequencies of different observable traits (phenotypes) in the offspring resulting from a genetic cross. In the context of the question, the F₂ generation's phenotypic ratios provide insight into the inheritance patterns of traits like eye color and croaking sounds. Analyzing these ratios is crucial for understanding the genetic makeup of the offspring.

Pure-Breeding Strains

Pure-breeding strains are genetically uniform lines that consistently produce offspring with the same phenotype when self-crossed. These strains are essential for genetic studies as they provide a stable genetic background for experiments. In the context of the question, isolating pure-breeding strains of the F₂ phenotypes allows the geneticist to perform controlled crosses and predict the phenotypic ratios of the resulting offspring.

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Textbook Question

A true-breeding purple-leafed plant isolated from one side of El Yunque, the rain forest in Puerto Rico, was crossed to a true-breeding white variety found on the other side. The F₁ offspring were all purple. A large number of F₁ x F₁ crosses produced the following results:

purple: 4219, white: 5781 (Total = 10,000)

Propose an explanation for the inheritance of leaf color. As a geneticist, how might you go about testing your hypothesis? Describe the genetic experiments that you would conduct.

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Textbook Question

In Dexter and Kerry cattle, animals may be polled (hornless) or horned. The Dexter animals have short legs, whereas the Kerry animals have long legs. When many offspring were obtained from matings between polled Kerrys and horned Dexters, half were found to be polled Dexters and half polled Kerrys. When these two types of F₁ cattle were mated to one another, the following F₂ data were obtained:

3/8 polled Dexters

3/8 polled Kerrys

1/8 horned Dexters

1/8 horned Kerrys

A geneticist was puzzled by these data and interviewed farmers who had bred these cattle for decades. She learned that Kerrys were true breeding. Dexters, on the other hand, were not true breeding and never produced as many offspring as Kerrys. Provide a genetic explanation for these observations.

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Textbook Question

A geneticist from an alien planet that prohibits genetic research brought with him to Earth two pure-breeding lines of frogs. One line croaks by uttering 'rib-it rib-it' and has purple eyes. The other line croaks more softly by muttering 'knee-deep knee-deep' and has green eyes. With a newfound freedom of inquiry, the geneticist mated the two types of frogs, producing F₁ frogs that were all utterers and had blue eyes. A large F₂ generation then yielded the following ratios:

27/64 blue-eyed, 'rib-it' utterer

12/64 green-eyed, 'rib-it' utterer

9/64 blue-eyed, 'knee-deep' mutterer

9/64 purple-eyed, 'rib-it' utterer

4/64 green-eyed, 'knee-deep' mutterer

3/64 purple-eyed, 'knee-deep' mutterer

In another experiment, the geneticist crossed two purple-eyed, 'rib-it' utterers together with the results shown here:

9/16 purple-eyed, 'rib-it' utterer

3/16 purple-eyed, 'knee-deep' mutterer

3/16 green-eyed, 'rib-it' utterer

1/16 green-eyed, 'knee-deep' mutterer

What were the genotypes of the two parents?

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Textbook Question

27/64 blue-eyed, 'rib-it' utterer

12/64 green-eyed, 'rib-it' utterer

9/64 blue-eyed, 'knee-deep' mutterer

9/64 purple-eyed, 'rib-it' utterer

4/64 green-eyed, 'knee-deep' mutterer

3/64 purple-eyed, 'knee-deep' mutterer

One set of crosses with his true-breeding lines initially caused the geneticist some confusion. When he crossed true-breeding purple-eyed, 'knee-deep' mutterers with true-breeding green-eyed, 'knee-deep' mutterers, he often got different results. In some matings, all offspring were blue-eyed, 'knee-deep' mutterers, but in other matings all offspring were purple-eyed, 'knee-deep' mutterers. In still a third mating, 1/2 blue-eyed, 'knee-deep' mutterers and 1/2 purple-eyed, 'knee-deep' mutterers were observed. Explain why the results differed.

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Textbook Question

27/64 blue-eyed, 'rib-it' utterer

12/64 green-eyed, 'rib-it' utterer

9/64 blue-eyed, 'knee-deep' mutterer

9/64 purple-eyed, 'rib-it' utterer

4/64 green-eyed, 'knee-deep' mutterer

3/64 purple-eyed, 'knee-deep' mutterer

Indicate the genotypes of the six F₂ phenotypes.

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