Monohybrid Inheritance & Co-dominance

Chromosomes & Alleles

• Human cells are diploid (2n) – two copies of each chromosome (one from each parent).

• Chromosomes exist in homologous pairs (e.g., two chromosome 1s).

• Each gene exists at a fixed position called a locus on both chromosomes in a pair.

• Versions of a gene are called alleles.

Genotype & Phenotype Terms

Term	Meaning
Allele	A version of a gene
Locus	The fixed position of a gene on a chromosome
Genotype	The combination of alleles an organism has (e.g., Aa)
Phenotype	The physical expression of the genotype (e.g., brown eyes)
Homozygous dominant	Two dominant alleles (e.g., AA)
Homozygous recessive	Two recessive alleles (e.g., aa)
Heterozygous	One dominant and one recessive allele (e.g., Aa)
Dominant allele	Always expressed in the phenotype if present
Recessive allele	Only expressed in the phenotype if both alleles are recessive

Monohybrid Inheritance

What is it?

• Mono = one, hybrid = cross.

• Involves inheritance of one gene with two alleles.

• Also known as Mendelian inheritance.

Generations

• F₁ Generation: First generation of offspring

• F₂ Generation: Offspring of F₁ crossed with each other

Genetic Crosses & Predictable Ratios

Cross: Homozygous dominant × Homozygous recessive

Cross: Heterozygous × Heterozygous (F₁ × F₁)

Cross: Homozygous dominant × Homozygous recessive

Example: Round (RR) × Wrinkled (rr) peas

• All offspring: Rr (heterozygous)

• Phenotype: 100% round

• Ratio: 100% dominant phenotype

Cross: Heterozygous × Heterozygous (F₁ × F₁)

Example: Rr × Rr

• Genotypes: RR, Rr, Rr, rr

• Phenotypic ratio: 3:1

  • 3 round : 1 wrinkled

• Genotypic ratio: 1 RR : 2 Rr : 1 rr

Why Might Actual Ratios Differ from Expected

1. Random fertilisation – gamete fusion is chance-based

2. Small sample size – low offspring number skews ratios

3. Crossing over & independent assortment – adds genetic variation

4. Co-dominance – alters typical dominant/recessive patterns

5. Sex linkage – inheritance affected by sex chromosomes (covered separately)

What is co-dominance?

• Both alleles are expressed in the phenotype.

• Neither allele is recessive.

• Can result in:

  • Blended phenotype 

  • Patchwork phenotype 

Co-dominance with Multiple Alleles: Blood Groups

Gene: I (Immunoglobulin)

• Iᴬ: A antigen

• Iᴮ: B antigen

• Iᴼ: No antigen (recessive)

Blood Group Combinations

Genotype	Blood Type	Phenotype
IᴬIᴬ or IᴬIᴼ	A	A antigens
IᴮIᴮ or IᴮIᴼ	B	B antigens
IᴬIᴮ	AB	A & B antigens (co-dominant)
IᴼIᴼ	O	No antigens

Key Points:

• A and B are co-dominant

• O is recessive

• O individuals can donate to anyone (universal donor)

• AB individuals can receive from anyone (universal recipient)