What Mendel Didn’t Know . . . Types of Dominance What Mendel Didn’t Know . . .

Types of Dominance Not all alleles follow the dominant recessive pattern that Mendel described in his first law. Since Mendel we have discovered . . . Incomplete dominance Codominance Multiple Alleles Polygenic Traits Sex-Linked Traits

Incomplete Dominance In Mirabilis flowers, when you cross a red flower with a white flower the offspring will have pink flowers When neither trait is dominant, but instead the heterozygotes fall somewhere in between the two traits, that is considered incomplete dominance.

Incomplete Dominance Punnett Even though there is a Capital Letter R which should be Dominant, in this case it is not. The Red and White flowers mix to create Pink.

Co-Dominance Co-dominance is very similar to incomplete dominance, but in co-dominance both alleles are present at the same time. Some cattle have reddish brown hair and some have white hair. When the two mate their offspring have both red and white hairs making the cow look pinkish brown.

Co-Dominance Punnett Squares This is done the same way the incomplete dominance punnett squares are done. Both alleles get their own letter.

Multiple Alleles Many genes have more than just two alleles. An individual can only have two alleles, but by increasing the variety of alleles there are to choose from, this increases the variety of different expressions possible. Blood types are created by multiple alleles. One letter is chosen to represent the gene, and then a superscript letter above it represents the allele.

Multiple Alleles Punnett Squares

This is how Blood Types are crossed

Polygenic Traits Sometimes a traits is produced by several genes interacting with one another. There are many pigments involved in creating eye color. Each pigment is controlled by a different gene. That is why we get so many varieties of eye color including different shades and eyes that seem to change colors. Polygenic traits lead to a wide variety of phenotypes. Since there are many different genes they would require many different punnett squares (which I won’t make you do).

Sex-Linked Traits Another complication that can occur is when a trait is sex linked. This means that the gene for that traits is located on the sex chromosome (X or Y). The Y-chromosome (found only in males) is shorter than the X chromosome and therefore contains less genetic information. So, sometimes the X chromosome will contain a gene that the Y does not. For example it is far more likely for men to be colorblind than women because that gene is found on the X chromosome and men don’t have a spare X chromosome to trade genes with.

Sex Linked Traits For these punnett squares you need to use X and X for the mother and X and Y for the father. Their alleles for a given gene are written smaller next to the X’s if it is an X linked trait.