interpret and construct genetic diagrams, including Punnett squares, to explain and predict the results of monohybrid crosses and dihybrid crosses that involve dominance, codominance, multiple alleles and sex linkage

Objective

Interpret and construct genetic diagrams, including Punnett squares, to explain and predict the results of monohybrid and dihybrid crosses that involve:

• Complete dominance
• Codominance
• Incomplete (partial) dominance
• Multiple alleles
• Sex‑linked inheritance

1. Genes, Alleles and Phenotype

A gene is a segment of DNA that encodes a particular trait. Different versions of a gene are called alleles. The combination of alleles an individual possesses (its genotype) determines the observable characteristics (phenotype).

Key points:

• Organisms are diploid for autosomal chromosomes – they carry two alleles per gene.
• Alleles can be dominant or recessive. A dominant allele masks the effect of a recessive allele in a heterozygote.
• Other patterns of interaction include codominance, incomplete dominance, multiple alleles and sex‑linkage.

1.1 Types of Allelic Interaction

• Complete dominance: \$A\$ (dominant) masks \$a\$ (recessive). \$AA\$ and \$Aa\$ give the same phenotype.
• Incomplete dominance: Heterozygote phenotype is intermediate. Example: \$R\$ (red) + \$W\$ (white) → \$RW\$ (pink).
• Codominance: Both alleles are expressed fully. Example: \$I^A\$ and \$I^B\$ in human blood type → \$I^AI^B\$ (AB).
• Multiple alleles: More than two alleles exist in the population (e.g., \$I^A\$, \$I^B\$, \$i\$).
• Sex‑linked inheritance: Genes located on sex chromosomes (usually X‑linked). Males are hemizygous for X‑linked genes.

2. Constructing Punnett Squares

Punnett squares are a visual tool for predicting genotype and phenotype ratios of offspring from a given cross.

2.1 Monohybrid Cross – Complete Dominance

Cross: \$Aa \times Aa\$ (heterozygous parents).

Genotype ratio: \$1\:AA : 2\:Aa : 1\:aa\$

Phenotype ratio (dominant : recessive): \$3\:dominant : 1\:recessive\$

2.2 Monohybrid Cross – Incomplete Dominance

Cross: \$Rr \times Rr\$ (red × white snapdragon).

Genotype ratio: \$1\:RR : 2\:Rr : 1\:rr\$

Phenotype ratio: \$1\:red : 2\:pink : 1\:white\$

2.3 Monohybrid Cross – Codominance

Cross: \$I^AI^B \times i i\$ (AB blood type × O type).

Genotype ratio: \$2\:I^Ai : 2\:I^Bi\$ → Phenotype ratio: \$1\:A : 1\:B\$

2.4 Dihybrid Cross – Independent Assortment

Cross: \$AaBb \times AaBb\$ (heterozygous for two traits).

Each parent can produce \$2^2 = 4\$ types of gametes: \$AB\$, \$Ab\$, \$aB\$, \$ab\$.

Genotype ratio (simplified): \$9\:A\B\ : 3\:A\bb : 3\:aaB\ : 1\:aabb\$

Phenotype ratio (dominant for both traits): \$9\:both\ dominant : 3\:dominant\ trait\ 1\ only : 3\:dominant\ trait\ 2\ only : 1\:recessive\ for\ both\$

2.5 Sex‑Linked Monohybrid Cross

Example: Red‑green colour blindness (X‑linked recessive, allele \$c\$). Cross a carrier female (\$X^CX^c\$) with a normal male (\$X^CY\$).

Result:

• Females: 50 % normal, 50 % carriers.
• Males: 50 % normal, 50 % colour‑blind.

3. Interpreting Genetic Diagrams

When analysing a Punnett square, follow these steps:

1. Identify the genotype of each parent.
2. List all possible gametes each parent can produce (considering segregation and independent assortment).
3. Combine gametes in a grid to obtain offspring genotypes.
4. Convert genotypes to phenotypes using the appropriate dominance relationship.
5. Count the number of each genotype/phenotype to obtain ratios or percentages.

4. Common Pitfalls

• Assuming all traits assort independently – linkage can alter ratios.
• Confusing genotype with phenotype, especially in codominance or multiple‑allele systems.
• For sex‑linked traits, remembering that males have only one X chromosome (hemizygous).
• Neglecting the possibility of lethal genotypes that do not survive to birth.

5. Practice Problems

1. Construct a Punnett square for a dihybrid cross \$AaBb \times aaBB\$ and state the phenotypic ratio.
2. In a population where \$I^A\$, \$I^B\$, and \$i\$ are present, predict the offspring blood types from a cross \$I^AI^B \times I^Ai\$.
3. Show the expected results of a cross between a colour‑blind male (\$X^cY\$) and a carrier female (\$X^CX^c\$).
4. Explain why a cross between two heterozygous pea plants for flower colour (\$Rr \times Rr\$) can produce a 1:2:1 phenotypic ratio when incomplete dominance is involved.