Non-Mendelian Genetics
Alleles are not always dominant/recessive and
many traits are controlled by more than one
gene.
Note: different texts, people use different letter
types for the different patterns of inheritance.
As long as you get the right outcome it isn’t
really important.
1. Incomplete Dominance – neither allele is
dominant. In this case, the heterozygous
individual’s phenotype is in between the two
different homozygous parent phenotypes.
Ex. White carnation x red carnation
RR x R’R’
All offspring will have the genotype RR’ and
have a pink phenotype.
2. Codominance - In this case, the heterozygous
individual displays both phenotypes of two
different homozygous parent phenotypes.
Ex. black chicken x white chicken
BB x WW
All offspring will appear “checkered”, with both
black and white feathers.
3. Multiple alleles – trait is controlled by more
than two alleles , so there are more than two
possible phenotypes. See page 273, Fig. 1112. (more later in human genetics)
4. Polygenic traits – These traits are controlled
by more than one gene and show a wide
range of phenotypes. Ex. Human height, skin
color.
Remember that environment can also play a role
in the expression of a trait!