74. Genetic Diagrams and Punnett Squares

Genetic diagrams are used to predict the possible genotypes and phenotypes of offspring based on the alleles of the parents.

1. Key Concepts: Alleles and Representation

• Allele: A different version of a gene.

• Dominant Allele: Represented by an uppercase letter (e.g., A). It is always expressed if present.

• Recessive Allele: Represented by a lowercase letter (e.g., a). It is only expressed if two copies are present.

• Homozygous: Having two of the same allele (AA or aa).

• Heterozygous: Having two different alleles (Aa).

2. Drawing a Genetic Diagram

To create a complete genetic diagram, follow these five steps:

1. Parent Phenotypes: Identify the physical traits of the parents (e.g., Normal vs. Muscular).

2. Parent Genotypes: Identify the genetic code (e.g., AA and aa).

3. Gametes: Split the parent genotypes into single letters to represent the cells after meiosis.

4. Offspring Genotypes: Combine the gametes in all possible combinations.

5. Offspring Phenotypes: Determine the physical traits based on the resulting genotypes.

3. Punnett Squares

A Punnett Square is a grid-based shortcut to find offspring genotypes.

• Place one parent's gametes across the top and the other parent's down the left side.

• Fill in the four squares by combining the corresponding top and side letters.

• Benefit: It is simpler and faster than a full genetic diagram.

• Limitation: It typically doesn't show the phenotypes or the step-by-step process of meiosis.

4. Example: Crossing Two Heterozygous Parents (Aa x Aa)

When two heterozygous "Normal" mice mate, the Punnett Square reveals:

• 1 AA (Homozygous Dominant - Normal)

• 2 Aa (Heterozygous - Normal)

• 1 aa (Homozygous Recessive - Muscular)

Statistical Outcomes:

• Phenotype Ratio: 3 Normal : 1 Muscular.

• Probability: There is a 25% (1 in 4) chance of the offspring being muscular.

5. Genetic Interaction and Environment

While these diagrams focus on single genes, most characteristics (like height) are determined by the interaction of multiple genes and environmental factors (such as diet and sleep).

Summary Table: Genotype to Phenotype