Exceptions to Mendelian Inheritance

Complete Dominance Mendel investigated complete dominance dominant gene totally masks the recessive

Complete Dominance Punnett squares are used to predict offspring genotypes and phenotypes.

Incomplete Dominance A cross between organisms with two different phenotypes can produce offspring with a third phenotype that is a blending of the parental traits. 

Incomplete Dominance It's like mixing paints Red + white will make pink Red doesn't totally dominate the white, instead there is incomplete dominance, and we end up with something in-between.

Incomplete Dominance

Incomplete Dominance How to recognize a question involving incomplete dominance: Notice that the offspring show a 3rd phenotype.  The parents each have one, and the offspring are different from the parents. Notice that the trait in the offspring is a blend of the parental traits.

Incomplete Dominance Can still use the Punnett square to solve problems involving incomplete dominance.  The letters used to represent each allele will be different, lowercase letters Why lowercase? Because neither allele dominates the other

r r Incomplete Dominance w w rw rw rw rw r = red flowers w = white flowers Cross red and white flowered plants Results? Genotype: 100% rw Phenotype: 100% pink r r rw rw w rw rw w

bb x ww Incomplete Dominance A cross between a blue bird & a white bird produces offspring that are silver.  What are the genotypes of the parent birds in the cross? b= blue w= white What is/are the genotype(s) of the silver offspring? bw What would be the phenotypic percent of offspring produced by two silver birds? 25% blue 25% white 50% silver bb x ww

ry x ry Incomplete Dominance When two plants with orange fruits are crossed the following phenotypic ratios are present in the offspring: 25% red fruit, 50% orange fruit, 25% yellow fruit.  What are the genotypes of the parent orange-fruited plants? ry x ry

red x white = red & white spotted Co-dominance Prefix "co-" means "together“ Cooperate = work together Coexist = exist together The heterozygous organism has a third phenotype in which both of the parental traits appear together.  red x white = red & white spotted

Co-dominance A very very very very very common phenotype used in questions about co-dominance is roan fur in cattle.  Cattle can be red, white or roan (red & white hairs together). 

Co-dominance Dominance

Co-dominance Can still use the Punnett square to solve problems involving co-dominance.  The letters used to represent each allele will be different, CAPITAL letters RR = all red hairs WW = all white hairs RW = red & white hairs together.

Co-dominance How to recognize a question involving co-dominance: Notice that the offspring show a 3rd phenotype.  The parents each have one, and the offspring are different from the parents. Notice that in the offspring BOTH of the parental traits are expressed, not a mix.

Co-dominance Predict the phenotypic ratios of offspring when a homozygous white cow is crossed with a roan bull. 50% WW - White 50% WR - Roan What should the genotypes & phenotypes for parent cattle be if a farmer wanted only cattle with red fur? Both parents should be RR, red

Co-dominance A cross between a black cat & a tan cat produces a tabby pattern (black & tan fur together). What pattern of inheritance does this illustrate? Co-dominance What percent of kittens would have tan fur if a tabby cat is crossed with a black cat? 0% tan 50 % Tabby 50% Black