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Chapter 3: Problem 16

Mendel crossed peas having round green seeds with peas having wrinkled yellow seeds. All \(\mathrm{F}_{1}\) plants had seeds that were round and yellow. Predict the results of testcrossing these \(\mathrm{F}_{1}\) plants.

Short Answer

Expert verified
The predicted results of the testcross are that 25% of the offspring will have round yellow seeds, 25% will have round green seeds, 25% will have wrinkled yellow seeds, and 25% will have wrinkled green seeds.

Step by step solution

01

Identify traits and their alleles in the parent plants

Mendel crossed peas with round green seeds (parent 1) and peas with wrinkled yellow seeds (parent 2). The traits involved are: 1. Seed shape: round (dominant, R) vs wrinkled (recessive, r) 2. Seed color: green (dominant, G) vs yellow (recessive, g) Parent 1 has round and green seeds, so its genotype can be represented as RRGG. Parent 2 has wrinkled and yellow seeds, so its genotype can be represented as rrgg.
02

Determine the genotype of the F1 generation

When Parent 1 (RRGG) and Parent 2 (rrgg) are crossed, their offspring (F1 generation) will inherit one allele for shape and one allele for color from each parent. - From Parent 1, offspring will inherit allele R and allele G - From Parent 2, offspring will inherit allele r and allele g Therefore, the genotype of the F1 plants will be RrGg (round and yellow seeds).
03

Perform the testcross

A testcross involves crossing an organism with an unknown genotype with an organism with a homozygous recessive genotype. In this case, we cross the F1 plants (RrGg) with plants having wrinkled yellow seeds (rrgg).
04

Determine the offspring genotypes from the testcross

We can use a Punnett square to predict the results of the testcross between F1 plants (RrGg) and plants with wrinkled yellow seeds (rrgg). The Punnett square will involve one R or r allele and one G or g allele from the F1 plants, and one r and one g allele from the testcross plants. The possible gametes for the F1 plants (RrGg) are RG, Rg, rG, rg. The possible gametes for the testcross plants (rrgg) are rg. Now, we construct the Punnett square: | | RG | Rg | rG | rg | |----|----|----|----|----| | rg |RrGg|Rrgg|rrGg|rrgg|
05

Interpret the results of the testcross

From the resulting Punnett square, we can observe that the offspring will have the following genotypes: 1. RrGg (round, yellow) - 25% 2. Rrgg (round, green) - 25% 3. rrGg (wrinkled, yellow) - 25% 4. rrgg (wrinkled, green) - 25% So, the predicted results of the testcross are that 25% of the offspring will have round yellow seeds, 25% will have round green seeds, 25% will have wrinkled yellow seeds, and 25% will have wrinkled green seeds.

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Most popular questions from this chapter

Two organisms, \(A A B B C C D D E E\) and aabbccddee, are mated to produce an \(\mathrm{F}_{1}\) that is self-fertilized. If the capital letters represent dominant, independently assorting alleles: (a) How many different genotypes will occur in the \(\mathrm{F}_{2}\) ? (b) What proportion of the \(\mathrm{F}_{2}\) genotypes will be recessive for all five loci? (c) Would you change your answers to (a) and/or (b) if the initial cross occurred between \(A A b b C C\)ddee\(\times\)aaBBccDDEE parents? (d) Would you change your answers to (a) and/or (b) if the initial cross occurred between \(A A B B C C D D E E \times\) aabbccddEE parents?

Discuss how Mendel's monohybrid results served as the basis for all but one of his postulates. Which postulate was not based on these results? Why?

In this chapter, we focused on the Mendelian postulates, probability, and pedigree analysis. We also considered some of the methods and reasoning by which these ideas, concepts, and techniques were developed. On the basis of these discussions, what answers would you propose to the following questions: (a) How was Mendel able to derive postulates concerning the behavior of "unit factors" during gamete formation, when he could not directly observe them? (b) How do we know whether an organism expressing a dominant trait is homozygous or heterozygous? (c) In analyzing genetic data, how do we know whether deviation from the expected ratio is due to chance rather than to another, independent factor? (d) since experimental crosses are not performed in humans, how do we know how traits are inherited?

Mendel crossed peas having round seeds and yellow cotyledons (seed leaves) with peas having wrinkled seeds and green cotyledons. All the \(F_{1}\) plants had round seeds with yellow cotyledons. Diagram this cross through the \(\mathrm{F}_{2}\) generation, using both the Punnett square and forked-line, or branch diagram, methods.

In a family of eight children, what is the probability that (a) the third child is a girl? (b) six of the children are boys? (c) all the children are girls? (d) there are four boys and four girls? Assume that the probability of having a boy is equal to the probability of having a girl \((p=1 / 2)\).
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