Question

In peas, tall (D) is dominant to dwarf (d) and yellow cotyledons (G) is dominant to green (g). If a tall, homozygous, yellow pea plant is crossed with a dwarf, green pea plant, what will be the phenotypic results in the \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2} ?\)

Short answer

The phenotypic results in the F1 generation will be all tall, yellow pea plants. In the F2 generation, the phenotypic ratio will be 9 (tall, yellow) : 3 (tall, green) : 3 (dwarf, yellow) : 1 (dwarf, green).

Step-by-step solution

Identify the genotypes of the parental plants

Given that tall (D) is dominant to dwarf (d), and yellow cotyledons (G) are dominant to green (g), we have the following: - Tall, homozygous, yellow pea plant: \( DDGG \) - Dwarf, green pea plant: \( ddgg \)

Determine the possible gametes of the parental plants

Each gamete contains one allele of each gene, so we have: - For the tall, homozygous, yellow pea plant: \( DG \) - For the dwarf, green pea plant: \( dg \)

Perform the cross using the Punnett square to find F1 genotypes

The Punnett square for the given genotypes is: ``` | DG ---+----- dg | DdGg ``` The resulting genotype of the F1 generation plants is \( DdGg \), which is heterozygous for both traits.

Determine the phenotype of the F1 generation

The F1 genotype \( DdGg \) corresponds to the following phenotypes: - Tall (D is present, so the plant is tall) - Yellow cotyledons (G is present, so the cotyledons are yellow) The F1 phenotypic result is all tall, yellow pea plants.

Determine the possible gametes of the F1 plants

The possible gametes of the F1 plants (\( DdGg \)) are: - \( DG \) - \( Dg \) - \( dG \) - \( dg \)

Perform the cross using the Punnett square to find F2 genotypes

Next, we perform a cross between F1 plants with the genotype \( DdGg \). The Punnett square for this cross is: ``` | DG | Dg | dG | dg ---+------+------+------+------+ DG | DDGG | DDGg | DdGG | DdGg ---+------+------+------+------+ Dg | DDGg | DDgg | DdGg | Ddgg ---+------+------+------+------+ dG | DdGG | DdGg | ddGG | ddGg ---+------+------+------+------+ dg | DdGg | Ddgg | ddGg | ddgg ```

Determine the phenotypes of the F2 generation

The phenotypes for each genotype in the F2 generation are: - DDGG: Tall, Yellow - DDGg: Tall, Yellow - DdGG: Tall, Yellow - DdGg: Tall, Yellow - DDgg: Tall, Green - DdGg: Tall, Yellow - Ddgg: Tall, Green - ddGG: Dwarf, Yellow - ddGg: Dwarf, Yellow - ddGg: Dwarf, Yellow - ddgg: Dwarf, Green The phenotypic ratio in the F2 generation is 9 (tall, yellow) : 3 (tall, green) : 3 (dwarf, yellow) : 1 (dwarf, green).

Key concepts

Punnett Square Analysis

Punnett square analysis is a tool used in genetics to predict the possible genotypes of offspring from a particular cross. It’s a simple graphical representation that helps illustrate how alleles from parents can combine to give rise to different genotypes.

Using a Punnett square involves drawing a grid and placing the gametes of one parent on the top and those of the other parent on the side. Each cell within the grid represents a possible fertilization event. By filling in the grid, you create a visual map of all the possible genotypes of the offspring. It’s an essential method for understanding Mendelian inheritance patterns.

Phenotypic Ratios

Phenotypic ratios refer to the relative number of offspring with different traits observed in a particular generation. These traits are the outward, physical manifestations of the genetic information (phenotype). In the example given, when F1 heterozygous plants (DdGg) are crossed, the F2 generation exhibits a phenotypic ratio of 9:3:3:1. This ratio is a classic Mendelian ratio for dihybrid crosses, indicating 9 offspring with both dominant traits, 3 with one dominant and one recessive trait, another 3 with the other dominant and recessive trait, and 1 with both recessive traits.

Understanding these ratios helps predict the likelihood of particular traits appearing in the offspring and is a fundamental concept in the study of genetics.

Dominant and Recessive Alleles

Dominant and recessive alleles are two types of alleles that differ in their ability to express a trait. A dominant allele is one that expresses its phenotype even when only one copy is present, while a recessive allele requires two copies (homozygous state) to express its phenotype.

In our example, allele 'D' for tall plants is dominant over allele 'd' for dwarf plants. Similarly, 'G' for yellow cotyledons is dominant over 'g' for green cotyledons. Consequently, any genotype containing at least one 'D' and one 'G' allele will result in a tall, yellow pea plant, which demonstrates the principle of dominance in genetic inheritance.

Genotypic Ratios

Genotypic ratios describe the proportion of different gene combinations among the offspring. Unlike phenotypic ratios, genotypic ratios consider the genetic makeup (genotype) rather than the observable traits. In the F2 generation of our pea plant example, the genotypic ratio is more complex due to the dihybrid cross involving two traits. There are different combinations possible: 1 DDGG, 2 DDGg, 2 DdGG, 4 DdGg, and so forth.

The genotypic ratio helps us understand how alleles segregate and recombine to form new genetic combinations in the offspring population and is a key concept in understanding the transmission of genetic traits.