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Chapter 3: Problem 29

In German cockroaches, curved wings \((c v)\) are recessive to normal wings \(\left(c v^{+}\right) .\) A homozygous cockroach having normal wings is crossed with a homozygous cockroach having curved wings. The \(\mathrm{F}_{1}\) are intercrossed to produce the \(\mathrm{F}_{2}\). Assume that the pair of chromosomes containing the locus for wing shape is metacentric. Draw this pair of chromosomes as it would appear in the parents, the \(\mathrm{F}_{1}\), and each class of \(\mathrm{F}_{2}\) progeny at metaphase I of meiosis. Assume that no crossing over takes place. At each stage, label a location for the alleles for wing shape \((c v\) and \(\mathrm{cv}^{+}\) ) on the chromosomes.

Short Answer

Expert verified
F2 genotypes: 1 \( cv^{+}/cv^{+} \), 2 \( cv^{+}/cv \), 1 \( cv/cv \); Draw chromosomes for each genotype.

Step by step solution

01

Identify Parental Genotypes

Determine the genetic makeup of the parent cockroaches. The cockroach with normal wings is homozygous, written as \( cv^{+}/cv^{+} \), and the cockroach with curved wings is also homozygous, written as \( cv/cv \).
02

Draw Parental Chromosomes

Draw the pair of chromosomes for each parent at metaphase I. For the normal wing parent \( (cv^{+}/cv^{+}) \), show two metacentric chromosomes with the \( cv^{+} \) allele on each chromosome. For the curved wing parent \( (cv/cv) \), show two metacentric chromosomes with the \( cv \) allele on each chromosome.
03

Determine F1 Genotype

Cross the parental genotypes to produce the \( F_1 \) generation. A \( cv^{+} \) allele from each parent combines to form a heterozygous \( cv^{+}/cv \) genotype in F1.
04

Draw F1 Chromosomes

Draw the pair of chromosomes for an \( F_1 \) individual at metaphase I. One chromosome will have the \( cv^{+} \) allele, and the other will have the \( cv \) allele, reflecting their heterozygous genotype.
05

Determine F2 Genotypes

Intercross \( F_1 \) individuals \( (cv^{+}/cv) \) to produce the \( F_2 \) generation. The possible genotypes are \( cv^{+}/cv^{+} \), \( cv^{+}/cv \), and \( cv/cv \) with probabilities of \( 1:2:1 \).
06

Draw F2 Chromosomes

Draw three diagrams for the \( F_2 \) progeny at metaphase I. One should show two chromosomes with \( cv^{+}/cv^{+} \) alleles, another with \( cv^{+}/cv \) alleles (heterozygous), and the last one with \( cv/cv \) alleles, corresponding to homozygous recessive individuals.

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Key Concepts

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

Metacentric Chromosomes
Metacentric chromosomes are a type of chromosome where the centromere is located approximately in the middle, making the two arms of the chromosome roughly equal in length. This symmetrical centromere position allows for balanced tension across the chromosome during cell division.
  • In the exercise, the cockroach chromosomes that contain the gene for wing shape are described as metacentric.
  • During metaphase I of meiosis, the metacentric chromosomes align at the cell's equator, with centromeres facilitating the attachment to spindle fibers on each side.
This ensures that when the chromosomes are pulled apart during anaphase, each new cell receives an equal complement, preserving genetic information through the generations.
Alleles
An allele is a variant form of a gene at a particular locus on a chromosome. In the given exercise about German cockroaches:
  • The alleles for wing shape are mentioned: Normal wings are represented as the dominant allele, denoted by \( cv^{+} \).
  • Curved wings possess the recessive allele, represented by \( cv \).
The presence of different alleles at a locus is what creates genetic diversity and allows for phenotypic variations, such as the distinction between normal and curved wings in the cockroaches.
When both alleles at a locus are the same, as in the homozygous cockroaches, either all normal wings \( (cv^{+}/cv^{+}) \) or all curved wings \( (cv/cv) \) are showcased. In the F1 generation, where one \( cv^{+} \) and one \( cv \) allele combine, genetic variation is introduced.
Meiosis
Meiosis is a type of cell division responsible for producing gametes, such as sperm and eggs, which possess half the number of chromosomes as the parent cell. It is crucial for sexual reproduction and genetic diversity.
In this educational problem, meiosis is the process that occurs in both the parental cockroaches and their offspring.
  • During meiosis, homologous chromosomes line up in pairs at the metaphase I stage. Both metacentric chromosomes of the parental and F1 progeny align in preparation for division.
  • This alignment ensures that alleles from both parents are mixed through the gametes.
The process reinforces genetic variation, ensuring that offspring, like the F2 generation in the exercise, receive a mix of genotypes.
Homozygous and Heterozygous Genotypes
Genotypes reflect the genetic constitution of an organism regarding a specific trait. Understanding homozygous and heterozygous genotypes is essential to grasping the inheritance patterns showcased in the exercise.
  • A homozygous genotype consists of two identical alleles at a locus. For example, parents in the exercise are homozygous, with one having normal wings \( (cv^{+}/cv^{+}) \) and the other having curved wings \( (cv/cv) \).
  • Heterozygous genotypes, as seen in the F1 offspring, have two different alleles, one dominant \( (cv^{+}) \) and one recessive \( (cv) \). This variation allows for a mix of traits and is crucial for passing along dominant and recessive characteristics.
In the F2 generation, breeding heterozygous individuals creates possibilities for recombination of alleles, resulting in genetic ratios, such as the 1:2:1 described in the solution.

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Most popular questions from this chapter

J. A. Moore investigated the inheritance of spotting patterns in leopard frogs (J. A. Moore. 1943. Journal of Heredity 34:3-7). The pipiens phenotype had the normal spots that give leopard frogs their name. In contrast, the burnsi phenotype lacked spots on its back. Moore carried out the following crosses, producing the progeny indicated. $$\begin{array}{ll}\text { Parent phenotypes } & \text { Progeny phenotypes } \\\\\hline \text { burnsi } \times \text { burnsi } & 39 \text { burnsi, } 6 \text { pipiens } \\\\\text { burnsi } \times \text { pipiens } & 23 \text { burnsi, } 33 \text { pipiens } \\\\\text { burnsi } \times \text { pipiens } & 196 \text { burnsi, } 210 \text { pipiens }\end{array}$$ a. On the basis of these results, is the burnsi phenotype most likely inherited as a dominant trait or as a recessive trait? b. Give the most likely genotypes of the parent in each cross (use \(B\) for the burnsi allele and \(B^{+}\) for the pipiens allele). c. Use a chi-square test to evaluate the fit of the observed numbers of progeny to the number expected on the basis of your proposed genotypes.

In guinea pigs, the allele for black fur \((B)\) is dominant to the allele for brown \((b)\) fur. A black guinea pig is crossed with a brown guinea pig, producing five \(\mathrm{F}_{1}\) black guinea pigs and six \(\mathrm{F}_{1}\) brown guinea pigs. a. How many copies of the black allele \((B)\) will be present in each cell of an \(\mathrm{F}_{1}\) black guinea pig at the following stages: \(G_{1}, G_{2},\) metaphase of mitosis, metaphase I of meiosis, metaphase II of meiosis, and after the second cytokinesis following meiosis? Assume that no crossing over takes place. b. How many copies of the brown allele \((b)\) will be present in each cell of an \(\mathrm{F}_{1}\) brown guinea pig at the same stages as those listed in part \(a\) ? Assume that no crossing over takes place.

In which stages of mitosis and meiosis are the principles of segregation and independent assortment at work?

In cucumbers, dull fruit \((D)\) is dominant to glossy fruit \((d)\), orange fruit \((R)\) is dominant to cream fruit \((r),\) and bitter cotyledons \((B)\) are dominant to non-bitter cotyledons \((b) .\) The three characters are encoded by genes located on different pairs of chromosomes. A plant homozygous for dull, orange fruit and bitter cotyledons is crossed with a plant that has glossy, cream fruit and non-bitter cotyledons. The \(\mathrm{F}_{1}\) are intercrossed to produce the \(\mathrm{F}_{2}\). a. Give the phenotypes and their expected proportions in the \(\mathrm{F}_{2}\). b. An \(\mathrm{F}_{1}\) plant is crossed with a plant that has glossy, cream fruit and non-bitter cotyledons. Give the phenotypes and expected proportions among the progeny of this cross.

In cucumbers, orange fruit color \((R)\) is dominant to cream fruit color \((r) .\) A cucumber plant homozygous for orange fruit is crossed with a plant homozygous for cream fruit. The \(\mathrm{F}_{1}\) are intercrossed to produce the \(\mathrm{F}_{2}\). a. Give the genotypes and phenotypes of the parents, the \(\mathrm{F}_{1}\), and the \(\mathrm{F}_{2}\) b. Give the genotypes and phenotypes of the offspring of a backcross between the \(\mathrm{F}_{1}\) and the orange-fruited parent. c. Give the genotypes and phenotypes of a backcross between the \(\mathrm{F}_{1}\) and the cream-fruited parent.
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