Problem 16 - Campbell Biology

Question

SCIENTIFIC INQUIRY You are handed a mystery pea plant with tall stems and axial flowers and asked to determine its genotype as quickly as possible. You know that the allele for tall stems \((T)\) is dominant to that for dwarf stems \((t)\) and that the allele for axial flowers \((A)\) is dominant to that for terminal flowers (a). (a) Identify all the possible genotypes for your mystery plant. (b) Describe the one cross you would do, out in your garden, to determine the exact genotype of your mystery plant. (c) While waiting for the results of your cross, you predict the results for each possible genotype listed in part a. Explain how you do this and why this is not called "performing a cross." a Explain how the results of your cross and your predictions will help you learn the genotype of your mystery plant.

Step-by-Step Solution

Answer

Possible genotypes: TTAA, TTAa, TtAA, TtAa. Perform a test cross with a ttaa plant. Analyze offspring phenotypes to determine the mystery plant's genotype.

Step 1 - Identify Phenotype Characteristics

Determine the phenotype of the mystery pea plant. The plant exhibits tall stems and axial flowers.

Step 2 - Identify Possible Genotypes

Given that tall stems \((T)\) are dominant to dwarf stems \((t)\) and axial flowers \((A)\) are dominant to terminal flowers \((a)\), the possible genotypes for tall stems could be \((TT)\) or \((Tt)\); for axial flowers, the possible genotypes could be \((AA)\) or \((Aa)\). Combining these, the possible genotypes of the mystery plant are TTAA, TTAa, TtAA, and TtAa.

Step 3 - Describe the Test Cross

To determine the exact genotype of the mystery plant, conduct a test cross. Cross the mystery plant with a plant that is homozygous recessive for both traits (ttaa).

Step 4 - Predict the Possible Results

Depending on the genotype of the mystery plant, the following offspring ratios are expected: \((1)\) TTAA \times\ ttaa: all offspring tall with axial flowers, \((2)\) TTAa \times\ ttaa: 1:1 ratio of tall with axial flowers to tall with terminal flowers, \((3)\) TtAA \times\ ttaa: 1:1 ratio of tall with axial flowers to dwarf with axial flowers, \((4)\) TtAa \times\ ttaa: 1:1:1:1 ratio of all four phenotypes.

Step 5 - Explain Predictions vs. Cross

These are predictions, not an actual cross, because predictions are based on Mendelian genetics and probability, whereas performing a cross involves the actual breeding of plants and observing the offspring.

Step 6 - Connect Results to Determining Genotype

Analyze the offspring from the test cross. For example, if any offspring show dwarf stems or terminal flowers, the mystery plant must have a recessive allele for that trait. The exact ratios of the phenotypes will reveal whether the mystery plant is homozygous or heterozygous for each trait.

Key Concepts

Mendelian genetics

Mendelian genetics is the study of how traits are inherited through the actions of alleles, based on the work of Gregor Mendel. He is known as the father of genetics. Mendel experimented with pea plants and discovered how traits are passed from one generation to the next.
He determined there are dominant and recessive traits. A dominant trait hides a recessive trait when they are together. This is now known as the Law of Dominance.
Mendel also discovered that traits are inherited independently of each other. This is the Law of Independent Assortment. For example, the color of a pea does not affect the shape.
Understanding Mendelian genetics helps us predict the outcome of crosses, like determining the genotype of a pea plant by looking at the phenotypes of its offspring.

dominant and recessive traits

Dominant and recessive traits are key to understanding genetics. A dominant trait is one that appears in the offspring if just one of the parents contributes it. A recessive trait only appears if both parents contribute it.
For example, in pea plants, tall stems \(T\) are dominant over dwarf stems \(t\), and axial flowers \(A\) are dominant over terminal flowers \(a\). This means that a plant with the genotype \(TtAA\) will appear tall with axial flowers.
When trying to determine the genotype of a plant, we look at the physical appearance, or phenotype. If a plant shows a dominant trait, like tall stems, it could be \(TT\) or \(Tt\). Only a test cross can reveal the exact genotype by examining the traits of the offspring.

test cross

A test cross is a way to determine the genotype of an organism. To perform a test cross, you breed the organism with another that has homozygous recessive traits.
For example, with our mystery pea plant showing tall stems and axial flowers, you would cross it with a pea plant having dwarf stems and terminal flowers (genotype \(ttaa\)).
The offspring phenotypes can then be analyzed:

• All tall with axial flowers? The mystery plant genotype is \(TTAA\).
• Half tall with axial and half tall with terminal flowers? The genotype is \(TTAa\).
• Half tall with axial and half dwarf with axial flowers? The genotype is \(TtAA\).
• Equal mix of all four phenotypes? The genotype is \(TtAa\).

A test cross provides a simple way to reveal hidden recessive alleles and confirm the genotype of the mystery plant.