Dihybrid Cross (Leaving Cert Biology): Revision Notes

Dihybrid Cross

• A dihybrid cross is a genetic cross where two separate genes are studied simultaneously.
• This allows us to examine how traits controlled by two different genes are inherited together.

Key Concepts for Dihybrid Crosses:

1. Each gene has two alleles (dominant and recessive).
2. Gametes carry only one allele for each gene.
3. Use a Punnett square to predict the combinations of alleles and resulting offspring.

Step-by-Step Guide with Examples

Step 1: Determine the Parent Genotypes

Step 2: Identify Possible Gametes

Step 3: Perform the Cross

Step 4: F1 Generation

---

Example (a): Homozygous Dominant x Homozygous Recessive

Cross: A pea plant homozygous dominant for both traits (TTPP) is crossed with a pea plant homozygous recessive for both traits (ttpp).

Traits:

• T = Tall
• t = Short.
• P = Purple flowers
• p = White flowers.

---

Step 1: Determine the Parent Genotypes

• Parent 1: TTPP
• Parent 2: ttpp

---

Step 2: Identify Possible Gametes

• Parent 1 (TTPP): Can only produce TP gametes.
• Parent 2 (ttpp): Can only produce tp gametes.

---

Step 3: Perform the Cross

	TP
tp	TtPp

---

Step 4: F1 Generation

• Genotype of Offspring: 100% TtPp.
• Phenotype of Offspring: All offspring will be tall with purple flowers (dominant traits).

---

Example (b): Homozygous Recessive x Heterozygous

Cross: A pea plant homozygous recessive for both traits (ttpp) is crossed with a plant heterozygous for both traits (TtPp).

---

Step 1: Determine the Parent Genotypes

• Parent 1: ttpp
• Parent 2: TtPp

---

Step 2: Identify Possible Gametes

• Parent 1 (ttpp): Can only produce tp gametes.
• Parent 2 (TtPp): Can produce TP, Tp, tP, and tp gametes.

---

Step 3: Perform the Cross

	TP	Tp	tP	tp
tp	TtPp	Ttpp	ttPp	ttpp

---

Step 4: F1 Generation

• Genotypes:
  • TtPp, Ttpp, ttPp, ttpp (all equally likely).
• Phenotypes:
  • Tall and purple.
  • Tall and white.
  • Short and purple.
  • Short and white.
• Phenotypic Ratio: 1:1:1:1.

---

Example (c): Heterozygous x Heterozygous

Cross: Two plants, both heterozygous for both traits (TtPp x TtPp).

---

Step 1: Determine the Parent Genotypes

• Both parents: TtPp.

---

Step 2: Identify Possible Gametes

• Each parent can produce four gametes: TP, Tp, tP, tp.

---

Step 3: Perform the Cross (Using a 4x4 Punnett Square)

	TP	Tp	tP	tp
TP	TTPP	TTPp	TtPP	TtPp
Tp	TTPp	TTpp	TtPp	Ttpp
tP	TtPP	TtPp	ttPP	ttPp
tp	TtPp	Ttpp	ttPp	ttpp

---

Step 4: F1 Generation

• Genotypes: Refer to punnett square

• Phenotypes (Phenotypic Ratio):

  • 9 Tall and Purple.
  • 3 Tall and White.
  • 3 Short and Purple.
  • 1 Short and White.
  • Ratio: 9:3:3:1.