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Chapter 6: Problem 65

The production of pigment in the outer layer of seeds of corn requires each of the three independently assorting genes \(A, C,\) and \(R\) to be represented by at least one dominant allele, as specified in Problem \(64 .\) The dominant allele \(P r\) of a fourth independently assorting gene is required to convert the biochemical precursor into a purple pigment, and its recessive allele \(p r\) makes the pigment red. Plants that do not produce pigment have yellow seeds. Consider a cross of a strain of genotype \(A / A ; C / C ; R / R ; p r / p r\) with a strain of genotype \(a / a ; c / c ; r / r ; \operatorname{Pr} / \operatorname{Pr}\) a. What are the phenotypes of the parents? b. What will be the phenotype of the \(\mathrm{F}_{1}\) ? c. What phenotypes, and in what proportions, will appear in the progeny of a selfed \(\mathrm{F}_{1}\) ? d. What progeny proportions do you predict from the testcross of an \(\mathrm{F}_{1}\) ?

Short Answer

Expert verified
a. Red (first parent), Yellow (second parent). b. Purple. c. 9/16 Purple, 3/16 Red, 4/16 Yellow. d. 1/4 Purple, 1/4 Red, 1/2 Yellow.

Step by step solution

01

Determine Parental Phenotypes

The first parent has genotype \( A/A; C/C; R/R; pr/pr \), which means all the pigment genes are present, but due to \( pr/pr \), the seeds produce red pigment, resulting in red seeds. The second parent has genotype \( a/a; c/c; r/r; Pr/Pr \), which does not have all the pigment genes, thus producing no pigment and resulting in yellow seeds.
02

Determine F1 Phenotype

To find the phenotype of the \( F_1 \) generation, we look at the cross between the two parents' genotypes. All offspring will be \( A/a; C/c; R/r; Pr/pr \), possessing at least one dominant allele in each gene responsible for pigment. The presence of \( Pr \) ensures pigment formation, so the \( F_1 \) plants will produce purple seeds.
03

Predict F2 Phenotypes and Proportions

For a self-crossed \( F_1 \), we use a Punnett square. Plants must have at least one dominant allele in each of \( A, C, \) and \( R \) to produce pigment, and a dominant \( Pr \) to be purple. Possibilities are: 9/16 will be purple (\( A/-; C/-; R/-; Pr/- \)), 3/16 will be red (\( A/-; C/-; R/-; pr/pr \)), and 4/16 will be yellow (all others).
04

Predict Proportions from F1 Testcross

The testcross is done with the \( F_1 \) and a homozygous recessive for all genes: \( a/a; c/c; r/r; pr/pr \). The offspring will partition into four equal parts: 1/4 purple (\( A/a; C/c; R/r; Pr/pr \)), 1/4 red (\( A/a; C/c; R/r, pr/pr \)), 1/4 yellow (\( a/a; any \)), and 1/4 yellow without pigment due to presence of trios of recessive alleles \( a/a, c/c, \) or \( r/r \).

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Key Concepts

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

Phenotype Analysis
Phenotype analysis plays a crucial role in understanding how different traits manifest in living organisms. When it comes to genetic inheritance, the phenotype is essentially the observable physical properties, such as color or shape, influenced by genetic and environmental factors. In our corn seed example, phenotypes of seed color range from red, yellow, to purple due to the influence of various genes.

To effectively analyze these phenotypes, consider:
  • Presence of Dominant Alleles: Dominant alleles like those in the parents' genotypes can determine whether a trait like color is expressed.
  • Gene Interactions: Certain genes need to work together for a phenotype to manifest, as with the corn, where multiple genes contribute to pigment production.
  • Environment: While not detailed in this exercise, phenotypes can also be influenced by environmental conditions that interact with genetic instructions.
When studying phenotypes, it's essential to note the genotypic combinations and understand their impact. Here, both parental and offspring genotypes helped predict the visible traits seen as purple, red or yellow corn seeds.
Mendelian Genetics
Mendelian genetics is at the heart of understanding heredity. It involves the study of how traits are passed from parents to offspring through the principles laid out by Gregor Mendel. Mendel鈥檚 laws of inheritance鈥擫aw of Segregation and Law of Independent Assortment鈥攈elp explain the genetic outcomes seen in breeding patterns.

In our exercise, the following key principles apply:
  • Law of Segregation: This principle explains that alleles segregate during gamete formation, meaning offspring inherit one allele for each gene from each parent. This helps predict possible genotypes of the offspring.
  • Independent Assortment: This law states that genes for different traits can segregate independently during gamete formation. In our corn example, each of the three pigment genes ( A, C, and R ) independently sort into gametes, impacting the resulting seed color.
Mendelian genetics provides a framework for understanding how different combinations of alleles impact the inheritance of traits like seed color in corn, helping us predict outcomes in genetic crosses.
Punnett Square
The Punnett square is a handy tool in genetics for predicting the outcome of genetic crosses. It's essentially a grid that allows you to visualize allele combinations from parental genotypes to see the possible genotypic and phenotypic ratios in offspring.

In our corn seed color example, the Punnett square helps calculate:
  • Genotypic Ratios: By setting up the grid with different alleles from each parent, you can determine the likelihood of offspring inheriting specific allele combinations.
  • Phenotypic Ratios: From these genotypic ratios, you can then deduce the probabilities of different phenotypes occurring, such as purple, red, or yellow seeds.
For instance, in a complex scenario like this, with multiple genes involved, the Punnett square breaks it down: 9/16 purple, 3/16 red, and 4/16 yellow. It simplifies predictions about which phenotypes will appear and in what proportions, showing the powerful role of dominant and recessive alleles in shaping genetic traits.

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Most popular questions from this chapter

In one species of Drosophila, the wings are normally round in shape, but you have obtained two pure lines, one of which has oval wings and the other sickle-shaped wings. Crosses between pure lines reveal the following results: $$\begin{array}{llll} {3}{c} {\text {Parents}} & {2}{c} {F 1} \\ \hline \text { Female } & \text { Male } & \text { Female } & \text { Male } \\\ \hline \text { sickle } & \text { round } & \text { sickle } & \text { sickle } \\ \text { round } & \text { sickle } & \text { sickle } & \text { round } \\ \text { sickle } & \text { oval } & \text { oval } & \text { sickle } \\ \hline \end{array}$$ a. Provide a genetic explanation of these results, defining all allele symbols. b. If the \(F_{1}\) oval females from cross 3 are crossed with the \(\mathrm{F}_{1}\) round males from cross \(2,\) what phenotypic proportions are expected for each sex in the progeny?

Because snapdragons (Antirrhinum) possess the pigment anthocyanin, they have reddish purple petals. Two pure anthocyaninless lines of Antirrhinum were developed, one in California and one in Holland. They looked identical in having no red pigment at all, manifested as white (albino) flowers. However, when petals from the two lines were ground up together in buffer in the same test tube, the solution, which appeared colorless at first, gradually turned red. a. What control experiments should an investigator conduct before proceeding with further analysis? b. What could account for the production of the red color in the test tube? c. According to your explanation for part \(b\), what would be the genotypes of the two lines? d. If the two white lines were crossed, what would you predict the phenotypes of the \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2}\) to be?

On a fox ranch in Wisconsin, a mutation arose that gave a "platinum" coat color. The platinum color proved very popular with buyers of fox coats, but the breeders could not develop a pure-breeding platinum strain. Every time two platinums were crossed, some normal foxes appeared in the progeny. For example, the repeated matings of the same pair of platinums produced 82 platinum and 38 normal progeny. All other such matings gave similar progeny ratios. State a concise genetic hypothesis that accounts for these results.

The petals of the plant Collinsia parviflora are normally blue, giving the species its common name, blue-eyed Mary. Two pure-breeding lines were obtained from color variants found in nature; the first line had pink petals, and the second line had white petals. The following crosses were made between pure lines, with the results shown: $$\begin{array}{ccl} \text { Parents } & \mathrm{F}_{1} & \mathrm{F}_{2} \\ \hline \text { blue } \times \text { white } & \text { blue } & 101 \text { blue, } 33 \text { white } \\ \text { blue } \times \text { pink } & \text { blue } & 192 \text { blue, } 63 \text { pink } \\ \text { pink } \times \text { white } & \text { blue } & 272 \text { blue, } 121 \text { white }, 89 \text { pink } \\ \hline \end{array}$$ a. Explain these results genetically. Define the allele symbols that you use, and show the genetic constitution of the parents, the \(\mathrm{F}_{1},\) and the \(\mathrm{F}_{2}\) in each cross. b. \(A\) cross between a certain blue \(F_{2}\) plant and a certain white \(\mathrm{F}_{2}\) plant gave progeny of which \(\frac{3}{8}\) were blue, \(\frac{1}{8}\) were pink, and \(\frac{1}{2}\) were white. What must the genotypes of these two \(\mathrm{F}_{2}\) plants have been?

A pure-breeding strain of squash that produced diskshaped fruits (see the accompanying illustration) was crossed with a pure-breeding strain having long fruits. The \(\mathrm{F}_{1}\) had disk fruits, but the \(\mathrm{F}_{2}\) showed a new phenotype, sphere, and was composed of the following proportions: long 32 sphere 178 disk 270 Propose an explanation for these results, and show the genotypes of the \(\mathrm{P}, \mathrm{F}_{1},\) and \(\mathrm{F}_{2}\) generations.
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