Mendelian Genetics: Dihybrid Crosses and Pea Plant Traits

Mendelian Genetics: Dihybrid Crosses in Pea Plants

Introduction to Mendelian Genetics

Gregor Mendel's experiments with pea plants established the foundational principles of inheritance. He studied traits such as seed shape and color, which are controlled by pairs of alleles. In this exercise, we analyze the inheritance of two traits: seed shape (round vs. wrinkled) and seed color (yellow vs. green), using a dihybrid cross.

• Allele: A variant form of a gene. For each trait, pea plants have two alleles.

• Dominant allele: Expressed in the phenotype when present (e.g., R for round, Y for yellow).

• Recessive allele: Masked by the dominant allele (e.g., r for wrinkled, y for green).

Key Principle: In a dihybrid cross, two traits are considered simultaneously, and their inheritance follows Mendel's law of independent assortment.

Genotypes and Phenotypes of Pea Plants

Allele Combinations and Their Effects

Each pea plant has two alleles for each trait, resulting in the following possible genotypes and associated phenotypes:

• Seed Shape: R (round, dominant), r (wrinkled, recessive)

• Seed Color: Y (yellow, dominant), y (green, recessive)

Possible genotype combinations:

• RRYY: Homozygous dominant for both traits (round, yellow)

• RRYy: Homozygous dominant for shape, heterozygous for color (round, yellow)

• RRyy: Homozygous dominant for shape, homozygous recessive for color (round, green)

• RrYY: Heterozygous for shape, homozygous dominant for color (round, yellow)

• RrYy: Heterozygous for both traits (round, yellow)

• Rryy: Heterozygous for shape, homozygous recessive for color (round, green)

• rrYY: Homozygous recessive for shape, homozygous dominant for color (wrinkled, yellow)

• rrYy: Homozygous recessive for shape, heterozygous for color (wrinkled, yellow)

• rryy: Homozygous recessive for both traits (wrinkled, green)

Deducing Parent Genotypes and Phenotypes from F1 Progeny

Selfing and F1 Progeny Analysis

Selfing refers to self-fertilization, where a plant's pollen fertilizes its own ovules. The resulting F1 progeny display phenotypes that reveal the parent's genotype.

• Phenotype: Observable traits (e.g., round yellow, round green, wrinkled yellow, wrinkled green).

• Genotype: Genetic makeup (e.g., RrYy, RRYY).

Table: Parent Genotypes, Phenotypes, and F1 Progeny

Classification and Comparison

The table below summarizes the relationship between parent genotype, parent phenotype, and the observed F1 progeny phenotypes after selfing four different pea plants.

Plant	Parent Phenotype	Parent Genotype	F1 Progeny Phenotypes from Selfed Parent
Plant 1	Round, Green	RRyy	All round, green
Plant 2	Round, Yellow	RRYy	Round, yellow and round, green
Plant 3	Round, Yellow	RrYy	Round, yellow, round, green, wrinkled yellow, wrinkled green
Plant 4	Round, Yellow	RrYY	Round, yellow and wrinkled yellow

Additional info: The table entries are inferred based on standard Mendelian ratios and the phenotypes shown in the image. The actual F1 progeny phenotypes for each genotype are determined by performing Punnett square analysis for each cross.

Punnett Square and Expected Ratios

Predicting Offspring Genotypes and Phenotypes

Punnett squares are used to predict the outcome of genetic crosses. For a dihybrid cross (e.g., RrYy x RrYy), the expected phenotypic ratio is:

• 9 round, yellow

• 3 round, green

• 3 wrinkled, yellow

• 1 wrinkled, green

General formula for the number of possible genotypes in a dihybrid cross:

$(2^n)$

where n is the number of gene pairs (for two traits, n = 2, so 4 possible phenotypes).

Summary Table: Genotype-Phenotype Relationships

Genotype	Phenotype
RRYY	Round, Yellow
RRYy	Round, Yellow
RRyy	Round, Green
RrYY	Round, Yellow
RrYy	Round, Yellow
Rryy	Round, Green
rrYY	Wrinkled, Yellow
rrYy	Wrinkled, Yellow
rryy	Wrinkled, Green

Key Concepts and Applications

• Dihybrid cross: A genetic cross involving two traits, each with two alleles.

• Law of Independent Assortment: Alleles for different traits segregate independently during gamete formation.

• Application: Understanding dihybrid crosses is essential for predicting inheritance patterns in plants and animals.