Overview
This lecture covers the three types of genetic dominance—complete, incomplete, and co-dominance—explaining their definitions, how to represent alleles, and key examples for each.
Types of Dominance in Genetics
- There are three kinds of dominance: complete, incomplete, and co-dominance.
- Dominance describes how alleles (gene versions) interact to determine an organism's phenotype (physical trait).
Complete Dominance
- Complete dominance: one allele is dominant and completely masks the other (recessive) allele in heterozygous individuals.
- Use the same letter for both alleles, uppercase for dominant (e.g., B = black fur, b = brown fur).
- Homozygous dominant (BB) and heterozygous (Bb) individuals show the dominant phenotype; homozygous recessive (bb) shows the recessive phenotype.
- Governed by Mendel's law of dominance: crossing two homozygous parents with contrasting traits yields offspring showing only the dominant trait.
Incomplete Dominance
- Incomplete dominance: neither allele is dominant; heterozygous individuals have an intermediate phenotype.
- Use different capital letters to represent each allele (e.g., R = red, W = white in snapdragon flowers).
- Homozygous red (RR) and white (WW) flowers produce pink (RW) flowers.
- Intermediate phenotypes indicate an incomplete dominance cross.
Co-dominance
- Co-dominance: both alleles are equally dominant and both are fully expressed in the heterozygous phenotype.
- Use a letter (e.g., C) with superscripts for each trait (CB = black, CW = white).
- Heterozygous chickens (CBCW) are black and white speckled, not a new blended color.
- Co-dominance is seen in other animals like cows with spotted fur patterns.
Blood Group Dominance (Preview)
- Blood types show both complete dominance and co-dominance: alleles A and B are co-dominant, O is recessive.
- Further details will be covered in a separate lesson.
Lettering Conventions in Crosses
- Complete dominance: capital letter for dominant, lowercase for recessive (B/b).
- Co-dominance: capital C with superscripts (CB, CW).
- Incomplete dominance: different capital letters for each trait (B/Y).
- Always provide a key for chosen allele symbols.
Key Terms & Definitions
- Allele — Different versions of a gene.
- Dominant — Allele that masks the effect of a recessive allele.
- Recessive — Allele masked by a dominant allele.
- Heterozygous — Having two different alleles for a gene.
- Homozygous — Having two identical alleles for a gene.
- Phenotype — Observable physical trait.
- Genotype — The allele combination (genetic makeup) for a trait.
- Law of Dominance — Mendel's principle that dominant alleles mask recessive ones in heterozygotes.
- Incomplete Dominance — Both alleles blend to create an intermediate phenotype.
- Co-dominance — Both alleles are fully expressed in the phenotype.
Action Items / Next Steps
- Review blood group dominance and co-dominance examples in the upcoming video.
- Create flashcards using the key terms for exam preparation.
- Practice genetic crosses using proper allele notation and Punnett squares.