Monohybrid Cross (Leaving Cert Biology): Revision Notes

Monohybrid Cross

Key Terms:

• F1 Generation: The first generation offspring of two parents.
• F2 Generation: The second generation offspring of two parents.
• Genetic Cross: A diagram or table showing how characteristics are inherited.
• Progeny: The offspring produced from a genetic cross.

Step-by-Step: Performing a Monohybrid Cross

1. Assign symbols to the alleles
2. Determine the genotype of the parents
3. Set up a punnett square

1. Assign Symbols to the Alleles:

• Use a capital letter for the dominant allele and a lowercase letter for the recessive allele.
• Example:
  • B = brown eyes (dominant).
  • b = blue eyes (recessive).

2. Determine the Genotype of the Parents:

• Write the genotype of both parents involved in the cross.
• Example:
  • One parent is homozygous dominant (BB)
  • The other is homozygous recessive (bb).

3. Set Up a Punnett Square:

A Punnett Square is a grid used to predict the genotypes and phenotypes of the offspring.

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Example (a): BB (brown) x bb (blue)

1. Parent Genotypes:

• Parent 1: BB (brown eyes).
• Parent 2: bb (blue eyes).

2. Gametes Produced by Each Parent:

• Parent 1 (BB) produces B gametes.
• Parent 2 (bb) produces b gametes.

3. Set Up the Punnett Square: | | B | B | |---|---|---| | b | Bb | Bb | | b | Bb | Bb |

4. Genotype of Offspring (F1 Generation):

• All offspring are Bb.

5. Phenotype of Offspring (F1 Generation):

• All offspring have brown eyes because the dominant allele (B) masks the recessive allele (b).

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Example (b): Crossing Two Heterozygous (Bb) Individuals

1. Parent Genotypes:

• Both parents are Bb (brown eyes).

2. Gametes Produced by Each Parent:

• Each parent produces B and b gametes.

3. Set Up the Punnett Square: | | B | b | |---|---|---| | B | BB | Bb | | b | Bb | bb |

4. Genotype of Offspring (F2 Generation):

• 1 BB (homozygous dominant).
• 2 Bb (heterozygous).
• 1 bb (homozygous recessive).

5. Phenotype of Offspring (F2 Generation):

• 3 brown-eyed (1 BB + 2 Bb).
• 1 blue-eyed (1 bb).

6. Genotypic Ratio:

• 1 BB : 2 Bb : 1 bb

7. Phenotypic Ratio:

• 3 brown eyes : 1 blue eyes

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Key Points:

1. Dominance: In a monohybrid cross, the dominant allele always masks the recessive allele in heterozygous individuals.
2. Ratios: A cross between two heterozygous parents (Bb x Bb) typically produces offspring with a 3:1 phenotypic ratio (dominant: recessive) and a 1:2:1 genotypic ratio (homozygous dominant : heterozygous : homozygous recessive).
3. Punnett Squares: Always use a Punnett Square to visualise and calculate the potential offspring genotypes and phenotypes.