Genotype, Phenotype and Dominance Types

• Learning objectives include explaining the relationship between genotypes and phenotypes, and the concept and types of dominance

Relationship Between Genotype and Phenotype

• Genotype refers to the genetic makeup of an organism (e.g., AA, Aa, or aa).
• Phenotype includes the observable traits or characteristics of an organism (e.g., purple flowers, tall height).
• Traits are determined by pairs of alleles, one from each parent.
• Alleles can be dominant (expressed in phenotype when present) or recessive (expressed only when both alleles are recessive).

Determining Dominance

• A dominant allele expresses its trait even if only one copy is present (e.g., Aa or AA).
• A recessive allele expresses its trait only when two copies are present (e.g., aa).
• In a heterozygous individual (e.g., Aa), the dominant trait is observed in the phenotype.
• Prediction of genotypes and phenotypes of offspring based to find dominant allele, based on observed outcomes

Dominance Types

• In Complete Dominance, heterozygous and homozygous dominant phenotypes are indistinguishable, purple flowers (P) dominate over white flowers (p)
• In Incomplete Dominance, heterozygote phenotype is intermediate between homozygous dominant and recessive, pink snapdragon flowers are intermediate between red and white
• In Co-Dominance both alleles in a heterozygote are fully expressed, and blood type AB shows both A and B antigens.

Is Dominance Always Common?

• Dominance pertains to allele interaction in phenotype production, not allele frequency in a population.
• Dominant alleles are not always common, but Polydactyly (extra fingers/toes) is caused by a dominant allele, and rare in human populations.
• Recessive alleles can be common; the recessive allele for normal hemoglobin (HbA) is more common than the dominant allele for sickle-cell disease (HbS) in many populations.
• Achondroplasia (dwarfism) and Huntington's disease are dominant but rare.
• Cystic fibrosis a recessive, but common in certain populations.
• Dominance is related to phenotypic expression, not allele widespread.

Complete Dominance

• Complete dominance exists when one allele completely masks the effect of the other allele in the heterozygous state.
• The heterozygote phenotype is identical to the homozygous dominant phenotype.
• The recessive trait only appears in the homozygous recessive genotype.
• These patterns simplifies inheritance, following Mendel's laws of segregation.
• Complete dominance is the basic inheritance form, where the dominant allele determines the phenotype of the heterozygote.

Mendel's Pea Plants

• Parental: Pure tall plants (TT) crossed with pure short plants (tt).
• F1: All offspring are tall (Tt); the dominant allele (T) masks the recessive allele (t).
• F2 (Tt x Tt): Genotypic ratio is 1 TT : 2 Tt : 1 tt, phenotypic ratio is 3 tall : 1 short.

Flower Color in Pea Plants

• Dominant Trait: Purple flowers (P).
• Recessive Trait: White flowers (p).
• F1 (PP x pp): All plants have purple flowers (Pp).

Incomplete Dominance

• Incomplete dominance occurs when neither allele is completely dominant over the other.
• The heterozygous genotype results in a blended or intermediate phenotype and is not inherited in a simple dominant-recessive manner.

Flower Color in Snapdragon Example

• Parental (P): Red flowers (RR) crossed with white flowers (WW).
• F1: All offspring have pink flowers (RW), showing the intermediate phenotype.
• F2 (RW x RW): Phenotypic ratio: 1 red : 2 pink : 1 white; Genotypic ratio: 1 RR : 2 RW : 1 WW.

Coat Color in Andalusian Chickens

• Parental: Black chickens (BB) crossed with white chickens (WW).
• F1: All offspring are gray (BW), showing incomplete dominance.
• F2: Phenotypic ratio: 1 black: 2 gray : 1 white; Genotypic ratio: 1 BB : 2 BW : 1 WW.

Co-Dominance

• Co-dominance occurs when both alleles are fully expressed in the phenotype of a heterozygous individual.
• Neither allele is dominant or recessive; instead, both traits appear simultaneously, and the phenotype is not blended; both traits are distinctly visible.
• Co-dominance differentiates form incomplete dominance, phenotype is intermediate.
• Co-dominance illustrates how multiple alleles can influence a phenotype without one overshadowing the other.
• It broadens the understanding of inheritance patterns beyond simple dominant-recessive relationships.

ABO Blood Groups

• IA and IB alleles are co-dominant; i (O) is recessive.
• IAIA or IAi is blood type A (A antigen present).
• IBIB or IBi is blood type B (B antigen present).
• IAIB is blood type AB (both A and B antigens fully expressed).
• ii (O) have no antigens present.

Coat Color in Roan Cattle

• A cross between red-coated cattle (RR) and white-coated cattle (WW) produces roan cattle (RW).
• Roan phenotype: Both red and white hairs are present, appearing mottled.
• The F2 generation (e.g., RW x RW): Genotypic ratio: 1 RR : 2 RW : 1 WW; Phenotypic ratio: 1 red : 2 roan : 1 white.

Summary Comparison

• Complete Dominance: Phenotype of Heterozygote is the same as the dominant allele, Tall pea plants (Tt) are the key example
• Incomplete Dominance: Phenotype of Heterozygote is Intermediate/blended trait, pink snapdragons (RW) example
• Co-Dominance: Phenotype of Heterozygote contains both traits fully visible, AB blood type (IAIB) is the key example