Incomplete Dominance & Codominance

Review of Mendel’s Principles Genes are passed parents  offspring; get one allele from each parent During Meiosis, the alleles for a gene segregate from each other. During Meiosis, genes independently assort with each other.

Exceptions to Mendel’s principles Sometimes, there is no dominant or recessive gene, or the trait is controlled by many alleles or genes.

Incomplete Dominance In Incomplete Dominance, every genotype has its own phenotype. (One allele not completely dominant over the other.) Third phenotype that is a blending of the parental traits. (2 alleles produce 3 phenotypes.) Result: Heterozygous phenotype somewhere in between homozygous phenotype.

1. Incomplete Dominance Examples: Trait: Flower Color Expressions: Red x White  Pink RR= Red; RW= pink; WW= white straight hair, wavy, curly

Incomplete Dominance

Codominance In codominance, neither allele are dominant; both are expressed. A cross between organisms with two different phenotypes produces offspring with has both phenotypes of the parental traits shown. 

2. Codominance Both alleles contribute to the phenotype. Example: In come chickens Black Chicken x White  Speckled Chicken

YOU tell me which type of dominance… Codominance!

Type of Dominance? Incomplete Dominance!

Type of Dominance? Incomplete Dominance!

Test Cross (Back Cross)… Used to determine whether an individual is homozygous dominant or heterozygous. RULES: Always cross unknown genotype with a homozygous recessive. Observe (count) large numbers of offspring to insure accuracy in determining the unknown genotype. Then… if ANY offspring show the recessive trait, the unknown genotype is heterozygous if ALL the offspring have the dominant trait, the unknown genotype is homozygous dominant

Quick Review

What about multiple alleles? two or more possible alleles for the same gene within a population Thus multiple alleles However, individuals within the population are only able to hold two of them Non-human examples: rabbit fur color, mice skin color, eye color in flies, wing size in flies

Multiple alleles Blood type in humans The _four_ different blood types: A, B, O, and AB Blood types are produced by three_ different alleles: A, B and O

I and i used in genotype I used with A and B i used with O A and B are dominant over O: A and B are codominant Ex: Genotype: AA is IAIA

Blood Type Chart Blood type AB is an example of codominance in humans

Blood Type Punnett Square Draw a Punnett square showing all the possible genotypes for the offspring produced by a type “O” mother and an a Type “AB” father. Cross: ii x IAIB

Blood Type Punnett Square Cross a female with blood type A and a male with heterozygous B blood type. Complete ratios (phenotypic and genotypic) A male with blood type B has a child with a woman who has blood type A. The child is blood type O. What is the genotype of the male and female? Show your work.

Blood Transfusions

How do they work?