D3.2.5 Effects of dominant and recessive alleles on phenotype Notes

Dominant and Recessive Alleles Determine Traits

1. Recall that genes exist in different forms called alleles, and these alleles determine the traits expressed in an organism:
   1. Dominant Alleles: Mask the effect of recessive alleles. Represented by a capital letter (e.g., T for tall plants).
   2. Recessive Alleles: Expressed only when both alleles are recessive. Represented by a lowercase letter (e.g., t for dwarf plants).

Understanding Homozygous and Heterozygous Genotypes

1. Homozygous-Dominant (TT): Two dominant alleles result in the dominant trait being expressed.
2. Heterozygous (Tt): One dominant allele is enough to mask the recessive allele, producing the dominant phenotype.
3. Homozygous-Recessive (tt): Two recessive alleles are required for the recessive trait to appear.

Why Recessive Traits Are Hidden in Heterozygotes

1. Recessive alleles require both copies to be recessive (tt) to express the trait.
2. In heterozygotes, the dominant allele ensures the production of functional proteins, overshadowing the recessive one.

Using Punnett Grids to Predict Offspring Phenotypes

1. A Punnett grid visually represents how alleles combine during reproduction:
   1. Identify Parent Genotypes: Determine the alleles for each parent.
   2. List Gametes: Write the possible gametes along the top and side of the grid.
   3. Fill in the Grid: Combine the gametes to show potential offspring genotypes.
   4. Determine Phenotypes: Use the genotypes to predict traits.

Real-World Applications: Genetic Disorders

1. Many genetic disorders follow a recessive inheritance pattern.
2. Individuals with one normal allele and one recessive allele are carriers, they don't show symptoms but can pass the allele to their offspring.