Question

Three independently assorting genes are known to control the biochemical pathway below that provides the basis for flower color in a hypothetical plant colorless \(\stackrel{A-}{\longrightarrow}\) yellow \(\stackrel{B-}{\longrightarrow}\) green \(\stackrel{C-}{\longrightarrow}\) speckled Homozygous recessive mutations, which disrupt enzyme function controlling each step, are known. Determine the phenotypic results in the \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2}\) generations resulting from the \(\mathrm{P}_{1}\) crosses involving true-breeding plants given here. (a) speckled \(\quad(A A B B C C) \quad \times \quad\) yellow \(\quad(A A b b C C)\) (b) yellow \(\quad(A A b b C C) \quad \times \quad\) green \((A A B B C C)\) (c) colorless \((a a B B C C) \quad \times \quad\) green \(\quad(A A B B C c)\)

Short answer

Answer: (a) All plants will have a speckled phenotype, (b) All plants will have a speckled phenotype, and (c) 50% of the plants will have a speckled phenotype and 50% will have a green phenotype.

Step-by-step solution

Identify the parental genotypes

The parental genotypes are as follows: Parent 1: Speckled (AABBCC) Parent 2: Yellow (AAbbCC)

Create Punnett squares for each gene

For each gene, a simple Punnett square represents the possible allele combinations between the parents. For gene A: ``` A A A A A A A A ``` For gene B: ``` B b B B B b b b ``` For gene C: ``` C C C C C C C C ```

Calculate the expected phenotypes

Combine the possible allele combinations from each gene and analyze the outcomes: In the F1 generation, all plants will have the genotype AABBCC (speckled) due to dominance. (b) yellow \((A A b b C C) \times\) green \((A A B B C C)\)

Identify the parental genotypes

The parental genotypes are as follows: Parent 1: Yellow (AAbbCC) Parent 2: Green (AABBCC)

Create Punnett squares for each gene

For each gene, a simple Punnett square represents the possible allele combinations between the parents. For gene A: ``` A A A A A A A A ``` For gene B: ``` B b B B B b b b ``` For gene C: ``` C C C C C C C C ```

Calculate the expected phenotypes

Combine the possible allele combinations from each gene and analyze the phenotypic outcomes in the F1 generation: All plants with genotype AABBCC (speckled) (c) colorless \((a a B B C C) \times\) green \((A A B B C c)\)

Identify the parental genotypes

The parental genotypes are as follows: Parent 1: Colorless (aaBBCC) Parent 2: Green (AABBCC)

Create Punnett squares for each gene

For each gene, a simple Punnett square represents the possible allele combinations between the parents. For gene A: ``` a A a a A A A A ``` For gene B: ``` B B B B B B B B ``` For gene C: ``` C c C C C C C c ```

Calculate the expected phenotypes

Combine the possible allele combinations from each gene and analyze the phenotypic outcomes in the F1 generation: 50% speckled (AaBBCC) and 50% green (AaBBCC)

Key concepts

Independent Assortment

Genetic inheritance is governed by several fundamental principles, one core concept being independent assortment. This principle is crucial to understanding how traits are inherited from one generation to the next. Independent assortment refers to the way that different genes independently separate from one another when reproductive cells develop.

It plays a significant role during meiosis, particularly in Metaphase I, where homologous chromosomes are distributed randomly. This randomness contributes to genetic variation among offspring. In the case of our hypothetical plant exercise, the alleles for flower color genes (A, B, C) sort independently during gamete formation.

What this means practically is that each gene’s inheritance doesn’t depend on the other gene's alleles. For example, the sorting of alleles for gene A doesn’t affect the sorting for gene B. This independence results in a wide array of possible genetic combinations in the offspring, contributing to genetic diversity. Understanding this concept is essential for predicting phenotypic outcomes of genetic crosses.

Phenotypic Ratios

Phenotypic ratios are a way to express the relative frequency of different outcomes in the offspring generation. They provide a quantitative measure of how often we expect each phenotype to appear, based on the underlying genetic makeup from a cross.

In the provided textbook exercise, analyzing F1 phenotypes involves observing combinations of dominant and recessive alleles. In these specific hypothetical plant experiments, the gene sequences determine different flower colors: speckled, yellow, green, and colorless.

For instance, in scenarios where heterozygous plants are involved in a cross (e.g., AaBbCc x AABBCC), using Punnett squares or Mendelian inheritance rules, students can predict the proportion of offspring with each trait. With independent assortment, these ratios can be exactly 9:3:3:1 or similar, depending on the gene combinations. By mastering the calculation of these ratios, students can effectively predict how different traits will express in offspring.

Punnett Squares

Punnett squares are a graphical representation used to predict the allele composition of offspring from a particular genetic cross. They are immensely useful for visualizing how genetic information is transmitted from one generation to the next.

They work by laying out all possible combinations of maternal and paternal alleles. For each gene, a Punnett square can show potential outcomes, which can be either homozygous or heterozygous combinations. This provides an easy method for predicting offspring genotypes and their corresponding phenotypes.

In this exercise, for example, the Punnett square helps visualize how different flower color traits (controlled by genes A, B, and C) could manifest in the F1 generation, given the P1 crosses. With three genes each possibly having two different alleles in a 2x2 format, individuals can deduce the likeliness of each genotype and ultimately predict the phenotypic outcome for these plants. Mastering Punnett squares is crucial for efficiently determining phenotypic ratios and understanding the outcomes of genetic crosses.