Complete and Incomplete Dominance
How do alleles cooperate to create an offspring’s phenotype?
Thus far we have discussed how an organisms genotype (alleles from each parent) can lead to its phenotype (outward appearance).
We have also learned how to determine offspring genotype using Punnett Squares.
Types of Inheritance
There are a few different ways that these alleles can interact to produce a phenotype for a gene. These are called inheritance patterns.
There are four types of inheritance that you are expected to understand:
- Complete dominance
- Incomplete dominance
You will first learn about each type of inheritance. Then you will use your Punnett square skills to solve genetics problems related to each type of inheritance.
Complete Dominance Inheritance
Complete dominance is an inheritance pattern in which the dominant allele always masks the expression of the recessive allele. Therefore, wherever you see a heterozygous genotype, you will see the dominant phenotype.
Thus far, this is the only type of inheritance we have been looking at. Remember Mendel’s pea flower experiment? Pea flower color is inherited with complete dominance.
Look again at one of his crosses below:
Sample Complete Dominance Problem: Mendel allowed heterozygous flowers to self-pollinate. Recall that purple flower color (B) is dominant to white (b). What were the genotypes and phenotype ratios of the offspring?
Genotype ratio: 1BB: 2Bb: 1bb
Phenotype ratio: 3 purple: 1 white
You can see that when both the dominant (B) and recessive (b) alleles are present in the offspring (Bb), the flowers are purple. The heterozygote shows the dominant phenotype.
Incomplete Dominance Inheritance
In this type of inheritance, heterozygous offspring show intermediate traits. Each genotype, then, would have it’s own phenotype. In other words, the phenotype ratio is always the same as the genotype ratio.
Sample Incomplete Dominance Problem: In a the plant that produces four o’clock flowers, gene R (for red flowers) is incompletely dominant over gene r (for white flowers). Heterozygous individuals (Rr) are pink-flowered. A red (RR) and white-flowered (rr) four o’clock plant are crossed. What type of offspring will result?
Genotype ratio: 100% Rr
Phenotype ratio: 100% Pink
The heterozygote shows the intermediate phenotype. Note that this is different than complete dominance, where the heterozygote showed the dominant phenotype. One allele of a heterozygous pair only partiallydominates expression of its partner.
Also note: This does not support the blending model because parental phenotypes do reappear in F2 generation.
A Special Case: Lethal Alleles
When a genetic defect causes 100% mortality in the offspring it is called a lethal allele. When a lethal allele is present, we don’t “see” any offspring result from the cross (they die before birth) so the proportions in the offspring appear off compared to what we expect from a Punnett square.
Lethal alleles can be dominant or recessive. Recessive lethal alleles cause death in a recessive homozygote (aa). Dominant lethal alleles cause death in a dominant homozygote (AA).
Examples of dominant lethal alleles include Huntington’s disease or achondroplasia (a type of dwarfism). In achondroplasia, individuals with an homozygous dominant genotype die before or shortly after birth. Heterozygotes (Aa) show the dwarf phenotype, and homozygous recessives are of average stature (aa).
Stop and Think: Examine the picture below showing the inheritance of coat color in mice. It shows an example of a dominant lethal allele. What are the genotype and phenotype ratios you would see in the actual mouse population? (answer: P-R is 1 dead, 2 yellow, 1 white)