interpret and construct genetic diagrams, including Punnett squares, to explain and predict the results of test crosses

Objective

Interpret and construct genetic diagrams, including Punnett squares, to explain and predict the results of test crosses.

1. Key Concepts

1.1 Genes, Alleles and Chromosomes

• A gene is a segment of DNA that codes for a specific trait.
• Different versions of a gene are called alleles.
• Genes are located on chromosomes; each organism has two copies of each chromosome (diploid).

1.2 Genotype vs. Phenotype

• Genotype: the genetic makeup of an individual (e.g., AA, Aa, aa).
• Phenotype: the observable trait resulting from the genotype and environment (e.g., tall plant, white flower).

1.3 Dominance Relationships

• Complete dominance: one allele masks the effect of the other (dominant allele = A, recessive = a).
• Incomplete dominance: heterozygote shows an intermediate phenotype (A + a → phenotype = blend).
• Codominance: both alleles are expressed in the heterozygote (e.g., blood type AB).

2. Constructing Punnett Squares

2.1 Basic Steps

1. Identify the parental genotypes.
2. Write the possible gametes for each parent across the top and side of the square.
3. Combine gametes to fill the squares, giving the possible genotypes of the offspring.
4. Interpret the results in terms of genotype and phenotype ratios.

2.2 Example: Monohybrid Cross (Complete Dominance)

Parental cross: AA (homozygous dominant) × aa (homozygous recessive).

Result: 100 % Aa (heterozygous). Phenotype = dominant trait.

2.3 Example: Dihybrid Cross

Cross: RrYy × RrYy (where R = round seed, r = wrinkled; Y = yellow, y = green).

Gametes from each parent: RY, Ry, rY, ry.

Genotypic ratio: 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1 (simplified to 9:3:3:1 phenotypic ratio).

3. Test Crosses

3.1 Purpose

A test cross determines the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive individual.

3.2 Procedure

1. Identify the individual with the dominant phenotype (unknown genotype).
2. Cross it with an individual that is homozygous recessive (aa).
3. Analyse the offspring phenotypes:

   • If all offspring display the dominant phenotype, the unknown parent is homozygous dominant (AA).
   • If both dominant and recessive phenotypes appear, the unknown parent is heterozygous (Aa).

3.3 Example

Parent with dominant phenotype (unknown) × recessive parent (aa).

4. Predicting Probabilities

For a single gene with two alleles, the Hardy–Weinberg principle gives the genotype frequencies:

\$\$

p^{2} + 2pq + q^{2} = 1

\$\$

• \$p^{2}\$ = frequency of homozygous dominant (AA)
• \$2pq\$ = frequency of heterozygotes (Aa)
• \$q^{2}\$ = frequency of homozygous recessive (aa)

5. Common Pitfalls

• Confusing genotype with phenotype – always state both.
• For dihybrid crosses, forgetting that gametes are formed by independent assortment (Mendel’s 2nd law).
• Assuming complete dominance when incomplete dominance or codominance is present.
• Neglecting the possibility of linked genes, which alter expected ratios.

6. Summary Checklist

1. Identify alleles and their dominance relationships.
2. Write all possible gametes for each parent.
3. Construct the Punnett square correctly.
4. Convert genotype ratios to phenotype ratios.
5. Use a test cross to determine unknown genotypes.
6. Apply Hardy–Weinberg if population frequencies are required.

7. Practice Questions

1. Two pea plants, both heterozygous for flower colour (purple = P, white = p), are crossed. Construct the Punnett square and give the phenotypic ratio.
2. A pea plant with a dominant tall phenotype is crossed with a short plant (tt) and produces 12 tall and 8 short offspring. What is the genotype of the tall parent?
3. Perform a test cross for a plant that is heterozygous for seed shape (Rr) and homozygous recessive for seed colour (yy). Show the expected offspring genotypes and phenotypes.