Question

Two distinct phenotypes are found in the salamander Plethodon cinereus: a red form and a black form. Some biologists have speculated that the red phenotype is due to an allele that is dominant to an allele for black. Unfortunately, these salamanders will not mate in captivity, so the hypothesis that red is dominant to black has never been tested. One day, a genetics student is hiking through the forest and finds 30 female salamanders, some red and some black, laying eggs. The student places each female with her eggs (about 20 to 30 eggs per female) in a separate plastic bag and takes them back to the lab. There, the student successfully raises the eggs until they hatch. After the eggs have hatched, the student records the phenotypes of the juvenile salamanders, along with the phenotypes of their mothers. Thus, the student has the phenotypes for 30 females and their progeny, but no information is available about the phenotypes of the fathers. Explain how the student can determine whether red is dominant to black with this information on the phenotypes of the females and their offspring.

          Two distinct phenotypes are found in the salamander Plethodon cinereus: a red form and a black form. Some biologists have speculated that the red phenotype is due to an allele that is dominant to an allele for black. Unfortunately, these salamanders will not mate in captivity, so the hypothesis that red is dominant to black has never been tested.
One day, a genetics student is hiking through the forest and finds 30 female salamanders, some red and some black, laying eggs. The student places each female with her eggs (about 20 to 30 eggs per female) in a separate plastic bag and takes them back to the lab. There, the student successfully raises the eggs until they hatch. After the eggs have hatched, the student records the phenotypes of the juvenile salamanders, along with the phenotypes of their mothers. Thus, the student has the phenotypes for 30 females and their progeny, but no information is available about the phenotypes of the fathers.
Explain how the student can determine whether red is dominant to black with this information on the phenotypes of the females and their offspring.

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Added by Kristy K.

Biology for AP Courses

Julianne Zedalis, John Eggebrecht

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Solved by Expert Kerissa Bures

08/31/2022

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Step 1: Determine the phenotypes of the female salamanders (red or black). Show more…

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Two distinct phenotypes are found in the salamander Plethodon cinereus: a red form and a black form. Some biologists have speculated that the red phenotype is due to an allele that is dominant to an allele for black. Unfortunately, these salamanders will not mate in captivity, so the hypothesis that red is dominant to black has never been tested. One day, a genetics student is hiking through the forest and finds 30 female salamanders, some red and some black, laying eggs. The student places each female with her eggs (about 20 to 30 eggs per female) in a separate plastic bag and takes them back to the lab. There, the student successfully raises the eggs until they hatch. After the eggs have hatched, the student records the phenotypes of the juvenile salamanders, along with the phenotypes of their mothers. Thus, the student has the phenotypes for 30 females and their progeny, but no information is available about the phenotypes of the fathers. Explain how the student can determine whether red is dominant to black with this information on the phenotypes of the females and their offspring.

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Transcript

00:01 So in this question, we're looking at two different types of salamanders, red and black.

00:06 And we think that the red is dominant to the black.

00:09 So i'm going to put a big b for red, for the red allele, and a little b for the black allele.

00:15 This is what we think, right? now, we have 30 females and offspring.

00:29 But we have no dad, no fathers, no males.

00:33 Not no males, no fathers.

00:38 How do we prove that? well, if a female is red, so let's take a look at this.

00:50 If the female is red, they can be big b, big b in a punnet square, and they're going to give a big b, no matter what, and 100 % will be red.

01:09 Right? it doesn't matter what this one is because they all have that dominant gene.

01:17 Or you could see a big b, little b, as a.

01:21 That female is still red, right? and now it would depend at most.

01:39 You would have 50 % right here, black.

01:50 If it was that, then you're going to have a 50 -50, right? if the father is this on the other hand, heterozygous, then you're going to have 25 %.

02:14 Black and if the father was the homozygous dominant red then you're going to have zero percent black now if if the mother is black it really depends on the father right if the father is well let's put you're going to have 100 percent black if the father was big b, little b, 50 % black.

03:44 And if your father was this, father was homozygous, you would have 0 % black, right? they'd all be red.

04:00 Basically, if you have a red female, let's change this over here, 0 % black.

04:20 If you have a red female at most with a red female, with a red mothered, red mother, you will have, at most a 50 % black.

04:46 If you were to see a 100 % black, then you would know that black was not recessive.

05:20 So this is going to support if you see at most 50%, if you saw more than 50%, if you saw 100%, if you saw 100%, then you know you're disproving that...

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