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Chapter 3: Problem 12

Assume independent assortment and start with a plant that is dihybrid \(A / a ; B / b\) a. What phenotypic ratio is produced from selfing it? b. What genotypic ratio is produced from selfing it? c. What phenotypic ratio is produced from testcrossing it? d. What genotypic ratio is produced from testcrossing it?

Short Answer

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a. 9:3:3:1 b. 1:2:1:2:4:2:1:2:1 c. 1:1:1:1 d. 1:1:1:1

Step by step solution

01

Understanding the Dihybrid Plant

A dihybrid plant is heterozygous for two traits. In this case, the genotype is \(A / a ; B / b\). This plant has two alleles for each trait: one dominant (A or B) and one recessive (a or b). We'll analyze the outcomes from selfing and testcrossing this dihybrid plant.
02

Step for Phenotypic Ratio from Selfing

When the dihybrid plant \(A/a ; B/b\) is selfed (crossed with itself), each pair of alleles segregates independently. The phenotypic ratio typically follows 9:3:3:1, resulting in: 9 plants with both dominant traits (A_B_), 3 plants with the first dominant and second recessive trait (A_bb), 3 plants with the first recessive and second dominant trait (aaB_), and 1 plant with both recessive traits (aabb).
03

Step for Genotypic Ratio from Selfing

To get the genotypic ratio, consider each pair of alleles and how they combine independently. The genotypic ratio results in: 1 (AABB): 2 (AABb): 1 (AAbb): 2 (AaBB): 4 (AaBb): 2 (Aabb): 1 (aaBB): 2 (aaBb): 1 (aabb).
04

Step for Phenotypic Ratio from Testcrossing

Testcrossing involves crossing the dihybrid plant \(A/a ; B/b\) with a homozygous recessive plant \(a/a ; b/b\). This results in a 1:1:1:1 phenotypic ratio as follows: 1 plant with both dominant traits (A_B_), 1 plant with the first dominant and second recessive trait (A_bb), 1 plant with the first recessive and second dominant trait (aaB_), and 1 plant with both recessive traits (aabb).
05

Step for Genotypic Ratio from Testcrossing

Testcrossing \(A/a ; B/b\) with \(a/a ; b/b\) results in a genotypic ratio of: 1 (AaBb): 1 (Aabb): 1 (aaBb): 1 (aabb). Each phenotype corresponds directly to the genotype in this case.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Dihybrid Cross
A dihybrid cross is a genetic cross involving two different traits, where each trait has two varied alleles. In the exercise, the plant has a genotype characterized as \(A / a ; B / b\). This means it carries two alleles for each trait, which consist of one dominant (\(A\) or \(B\)) and one recessive (\(a\) or \(b\)). During reproduction, these traits segregate independently, leading to multiple possible combinations.
  • The parental generation is heterozygous for both traits.
  • The offspring from a dihybrid cross show diverse genotypic and phenotypic variation.
Understanding how these traits are passed on involves considering both independent assortment and the resulting phenotypic or genotypic ratios.
Independent Assortment
Independent assortment is a key principle of Mendelian genetics. It describes how alleles of different genes separate into gametes independently of each other. This is crucial in understanding dihybrid crosses because each trait's alleles segregate without influencing others. In a dihybrid cross:
  • Each pair of alleles segregates independently during gamete formation.
  • The concepts result in varied combinations of traits in offspring.
For the plant described in the exercise, this principle results in independent distributions of the \(A/a\) and \(B/b\) alleles. These combinations give rise to diverse genotypes and phenotypes, reinforcing the genetic variety seen in offspring.
Phenotypic Ratio
The phenotypic ratio is a depiction of the observed traits in the offspring. It is derived by analyzing offspring appearances rather than the specific genetic makeup. For the dihybrid plant \(A/a ; B/b\) during selfing, the classic phenotypic ratio emerges as 9:3:3:1:
  • 9 have both dominant traits (\(A_B_\)).
  • 3 express the first dominant, second recessive trait (\(A_bb\)).
  • 3 show the first recessive, second dominant trait (\(aaB_\)).
  • 1 exhibits both recessive traits (\(aabb\)).
This describes how frequently each combination of traits appears in the progeny.
Genotypic Ratio
The genotypic ratio reflects the specific genetic combination of alleles that represent each phenotype. For the described dihybrid cross \(A/a ; B/b\), the genotypic ratio resulting from selfing is more complex because it specifies each possible genotype combination. This ratio is:
  • 1 (AABB)
  • 2 (AABb)
  • 1 (AAbb)
  • 2 (AaBB)
  • 4 (AaBb)
  • 2 (Aabb)
  • 1 (aaBB)
  • 2 (aaBb)
  • 1 (aabb)
The genotypic ratio underscores the underlying genetic diversity present in the offspring, beyond their simple appearances.

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Most popular questions from this chapter

Assume that diploid plant A has a cytoplasm genetically different from that of plant B. To study nuclear-cytoplasmic relations, you wish to obtain a plant with the cytoplasm of plant A and the nuclear genome predominantly of plant B. How would you go about producing such a plant?

You have three jars containing marbles, as follows: jar \(1 \quad 600\) red and 400 white jar \(2 \quad 900\) blue and 100 white jar \(3 \quad 10\) green and 990 white a. If you blindly select one marble from each jar, calculate the probability of obtaining (1) a red, a blue, and a green. (2) three whites. (3) a red, a green, and a white. (4) a red and two whites. (5) a color and two whites. (6) at least one white. b. In a certain plant, \(R=\operatorname{red}\) and \(r=\) white. You self a red \(R / r\) heterozygote with the express purpose of obtaining a white plant for an experiment. What minimum number of seeds do you have to grow to be at least 95 percent certain of obtaining at least one white individual? c. When a woman is injected with an egg fertilized in vitro, the probability of its implanting successfully is 20 percent. If a woman is injected with five eggs simultaneously, what is the probability that she will become pregnant? (Part \(c\) is from Margaret Holm.)

Suppose you discover two interesting rare cytological abnormalities in the karyotype of a human male. karyotype is the total visible chromosome complement.) There is an extra piece (satellite) on one of the chromosomes of pair \(4,\) and there is an abnormal pattern of staining on one of the chromosomes of pair \(7 .\) With the assumption that all the gametes of this male are equally viable, what proportion of his children will have the same karyotype that he has?

In mice, dwarfism is caused by an X-linked recessive allele, and pink coat is caused by an autosomal dominant allele (coats are normally brownish). If a dwarf female from a pure line is crossed with a pink male from a pure line, what will be the phenotypic ratios in the \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2}\) in each sex? (Invent and define your own gene symbols.)

The plant Haplopappus gracilis has a \(2 n\) of \(4 .\) A diploid cell culture was established and, at premitotic \(\mathrm{S}\) phase, a radioactive nucleotide was added and was incorporated into newly synthesized DNA. The cells were then removed from the radioactivity, washed, and allowed to proceed through mitosis. Radioactive chromosomes or chromatids can be detected by placing photographic emulsion on the cells; radioactive chromosomes or chromatids appeared covered with spots of silver from the emulsion. (The chromosomes "take their own photograph.") Draw the chromosomes at prophase and telophase of the first and second mitotic divisions after the radioactive treatment. If they are radioactive, show it in your diagram. If there are several possibilities, show them, too.
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