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Ch. 4 - Extensions of Mendelian Genetics
Klug - Concepts of Genetics 12th Edition
Klug12th EditionConcepts of Genetics ISBN: 9780135564776Not the one you use?Change textbook
All textbooksKlug 12th EditionCh. 4 - Extensions of Mendelian GeneticsProblem 39a
Chapter 4, Problem 39a

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
How many total gene pairs are involved in the inheritance of both traits? Support your answer.

Verified step by step guidance
1
Step 1: Identify the phenotypes and their ratios in the F₂ generation and recognize that the traits are likely controlled by multiple gene pairs. The traits are eye color (purple, blue, green) and croaking sound ('rib-it' vs. 'knee-deep'). The F₁ generation is uniform with blue eyes and 'rib-it' croaking, indicating dominance patterns.
Step 2: Analyze the F₂ phenotypic ratios given: 27/64, 12/64, 9/64, 9/64, 4/64, and 3/64. Notice that the denominators are all 64, which is a clue that the total number of gene pairs involved is related to the number of possible genotype combinations, since \$2^n = 64\( implies \)n=6$ gene pairs if each gene is independently assorting with two alleles.
Step 3: Consider that each gene pair segregates independently and contributes to the phenotypic ratios. The total number of genotypic combinations for \(n\) gene pairs is \$2^{2n}\(, but since each gene pair has two alleles, the number of phenotypic classes in a dihybrid cross is \)4^n\(. Here, the denominator 64 suggests \)4^n = 64\(, so solve for \)n\( by setting \)4^n = 64$.
Step 4: Calculate \(n\) by taking logarithms: \(n = \log_4 64\). Since \$4^3 = 64\(, this means \)n=3$. Therefore, three gene pairs are involved in the inheritance of both traits combined.
Step 5: Support this conclusion by explaining that the observed phenotypic ratios correspond to a trihybrid cross (three gene pairs), where each gene pair contributes to the traits of eye color and croaking sound, and the F₂ ratios reflect the independent assortment and combination of these gene pairs.

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Key Concepts

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

Dihybrid and Multihybrid Crosses

Dihybrid and multihybrid crosses involve tracking inheritance of two or more gene pairs simultaneously. The phenotypic ratios in offspring reflect the number of gene pairs and their independent assortment. For example, a typical dihybrid cross yields a 9:3:3:1 ratio, while more gene pairs produce more complex ratios based on 4^n total phenotypic classes.
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Mendelian Ratios and Probability

Mendelian genetics predicts offspring ratios based on dominant and recessive alleles segregating independently. The total number of phenotypic classes is 4^n for n gene pairs with two alleles each. Observed ratios can be compared to expected ratios to deduce the number of gene pairs controlling traits.
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Phenotypic Classification and Gene Interaction

Phenotypes result from combinations of alleles at different loci. Each gene pair can influence distinct traits, and their combinations produce unique phenotypes. Understanding how to classify phenotypes by traits and their ratios helps infer the number of gene pairs involved in inheritance.
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Related Practice
Textbook Question

Labrador retrievers may be black, brown (chocolate), or golden (yellow) in color (see chapter-opening photo). While each color may breed true, many different outcomes are seen when numerous litters are examined from a variety of matings where the parents are not necessarily true breeding. Following are just some of the many possibilities.

(a) black x brown → all black

(b) black x brown → 1/2 black, 1/2 brown

(c) black x brown → 3/4 black, 1/4 golden

(d) black x golden → all black

(e) black x golden → 4/8 golden 3/8 black 1/8 brown

(f) black x golden → 2/4 golden 1/4 black 1/4 brown

(g) brown x brown → 3/4 brown 1/4 golden

(h) black x black → 9/16 black 4/16 golden, 3/16 brown

Propose a mode of inheritance that is consistent with these data, and indicate the corresponding genotypes of the parents in each mating. Indicate as well the genotypes of dogs that breed true for each color.

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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.

575
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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.

727
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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

Of these, how many are controlling eye color? How can you tell? How many are controlling croaking?

576
views
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

Assign gene symbols for all phenotypes and indicate the genotypes of the P₁ and F₁ frogs.

508
views
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

Indicate the genotypes of the six F₂ phenotypes.

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