Monohybrid and Dihybrid Crosses — Revision Notes

⚡ 30-Second Revision

Monohybrid Cross: — Tracks 1 trait. F1: all dominant phenotype, heterozygous. F2 (F1 x F1): Phenotypic ratio $3:1$, Genotypic ratio $1:2:1$. Law of Segregation.
Dihybrid Cross: — Tracks 2 traits. F1: all dominant phenotypes, double heterozygous. F2 (F1 x F1): Phenotypic ratio $9:3:3:1$. Law of Independent Assortment.
Test Cross: — Unknown dominant phenotype x Homozygous recessive. Used to determine unknown genotype.
Gamete Formation (RrYy): — RY, Ry, rY, ry (equal proportions).
Probability Rule: — For independent events, $P(A \text{ and } B) = P(A) \times P(B)$.

2-Minute Revision

Monohybrid and dihybrid crosses are fundamental to understanding heredity. A monohybrid cross focuses on one trait, starting with pure-breeding parents (e.g., TT x tt). The F1 generation is always heterozygous (Tt) and expresses the dominant phenotype.

When F1 individuals are self-crossed (Tt x Tt), the F2 generation exhibits a 3:1 phenotypic ratio (dominant:recessive) and a 1:2:1 genotypic ratio (homozygous dominant:heterozygous:homozygous recessive).

This illustrates Mendel's Law of Segregation, where alleles for a single gene separate during gamete formation. A dihybrid cross tracks two traits simultaneously, beginning with pure-breeding parents (e.

g., RRYY x rryy). The F1 generation is double heterozygous (RrYy). Self-crossing F1 individuals (RrYy x RrYy) yields an F2 generation with a characteristic 9:3:3:1 phenotypic ratio. This demonstrates Mendel's Law of Independent Assortment, stating that alleles for different genes assort independently.

Punnett squares are essential tools for predicting offspring ratios, and a test cross (crossing an unknown dominant individual with a homozygous recessive) helps determine the unknown genotype.

5-Minute Revision

Mastering monohybrid and dihybrid crosses is crucial for NEET. Start with the monohybrid cross, which analyzes the inheritance of a single character. If you cross pure-breeding tall (TT) pea plants with pure-breeding dwarf (tt) plants (P generation), all F1 offspring will be tall and heterozygous (Tt).

This demonstrates complete dominance. When these F1 plants self-pollinate (Tt x Tt), the F2 generation will show a 3:1 phenotypic ratio (3 tall : 1 dwarf) and a 1:2:1 genotypic ratio (1 TT : 2 Tt : 1 tt).

This is the essence of Mendel's Law of Segregation, where the two alleles for a trait separate during gamete formation, ensuring each gamete receives only one allele.

Next, consider the dihybrid cross, which tracks two traits simultaneously. For example, crossing pure-breeding round, yellow-seeded plants (RRYY) with wrinkled, green-seeded plants (rryy). The F1 generation will all be heterozygous for both traits (RrYy) and display both dominant phenotypes (round, yellow).

When these F1 plants self-pollinate (RrYy x RrYy), they produce four types of gametes (RY, Ry, rY, ry) in equal proportions. The F2 generation, derived using a 4x4 Punnett square, will exhibit a distinct 9:3:3:1 phenotypic ratio (9 Round-Yellow : 3 Round-green : 3 Wrinkled-Yellow : 1 Wrinkled-green).

This illustrates Mendel's Law of Independent Assortment, which states that alleles for different genes assort independently during gamete formation, provided they are on different chromosomes or far apart on the same chromosome.

Key tools include the Punnett square for predicting outcomes and the test cross (crossing an individual with a dominant phenotype but unknown genotype with a homozygous recessive individual) to determine the unknown genotype.

For instance, if a round-seeded plant (R_) is test-crossed with a wrinkled-seeded plant (rr) and produces 50% round and 50% wrinkled offspring, the unknown parent must be heterozygous (Rr). Remember to apply the product rule of probability for dihybrid problems: $P(\text{A and B}) = P(A) \times P(B)$.

Prelims Revision Notes

Mendel's Laws:

- Law of Dominance: In a heterozygote, one allele (dominant) masks the expression of the other (recessive). - Law of Segregation: Alleles for a single gene separate during gamete formation, so each gamete gets only one allele. - Law of Independent Assortment: Alleles for different genes assort independently during gamete formation (for unlinked genes).

Monohybrid Cross (1 trait):

- P Generation: Homozygous dominant (e.g., TT) x Homozygous recessive (e.g., tt). - F1 Generation: All heterozygous (Tt), showing dominant phenotype. - F1 x F1 Cross (Tt x Tt): - F2 Genotypic Ratio: $1:2:1$ (1 TT : 2 Tt : 1 tt) - F2 Phenotypic Ratio: $3:1$ (3 Dominant : 1 Recessive)

Dihybrid Cross (2 traits):

- P Generation: Homozygous dominant for both (e.g., RRYY) x Homozygous recessive for both (e.g., rryy). - F1 Generation: All double heterozygous (RrYy), showing both dominant phenotypes. - Gametes from F1 (RrYy): RY, Ry, rY, ry (in equal proportions, $1/4$ each).

- F1 x F1 Cross (RrYy x RrYy): - F2 Phenotypic Ratio: $9:3:3:1$ - 9: Both dominant (e.g., Round, Yellow) - 3: One dominant, one recessive (e.g., Round, green) - 3: Other dominant, other recessive (e.

g., Wrinkled, Yellow) - 1: Both recessive (e.g.

Test Cross:

- Purpose: To determine the unknown genotype of an individual showing a dominant phenotype. - Method: Cross the unknown individual with a homozygous recessive individual. - **Monohybrid Test Cross (e.

g., T_ x tt):** - If offspring are all dominant phenotype: Unknown parent is TT. - If offspring are $1:1$ dominant:recessive phenotype: Unknown parent is Tt. - Dihybrid Test Cross (e.g., R_Y_ x rryy): - If unknown is RrYy, offspring ratio is $1:1:1:1$ (Round-Yellow : Round-green : Wrinkled-Yellow : Wrinkled-green).

Probability in Genetics:

- Product Rule: The probability of two or more independent events occurring together is the product of their individual probabilities. $P(A \text{ and } B) = P(A) \times P(B)$. - Sum Rule: The probability of any one of two or more mutually exclusive events occurring is the sum of their individual probabilities. $P(A \text{ or } B) = P(A) + P(B)$.

Deviations from Mendelian Ratios:

- Incomplete Dominance: Heterozygote shows an intermediate phenotype (e.g., red x white $\rightarrow$ pink). F2 phenotypic ratio $1:2:1$. - Codominance: Both alleles expressed distinctly in heterozygote (e.g., AB blood group). F2 phenotypic ratio $1:2:1$. - Linkage: Genes on the same chromosome, especially if close, do not assort independently, violating Mendel's Law of Independent Assortment.

Vyyuha Quick Recall

My Dear Genetics Problem: Monohybrid: 3:1 (Pheno), 1:2:1 (Geno) Dihybrid: 9:3:3:1 (Pheno) Gametes (RrYy): Really Yummy, Really yummy, really Yummy, really yummy (RY, Ry, rY, ry) Punnett Square: Predicts Possibilities