NON-MENDELIAN
GENETICS
REFRESHER
• Mendel kept things simple
• In his work on pea plants, genes
had two alleles which had clear
dominant-recessive
relationships
– Law of Dominance
• Mendel’s pea plants gave us the
core principles of inheritance
BEYOND
MENDEL
OVERVIEW
• Not all alleles behave as straightforward as Mendel’s experiment
• Allele pairs may have a variety of dominant relationships
– One allele of the pair may not completely ‘hide’ the other in a heterozygote pair
OVERVIEW
• There are often many different alleles of a gene in a population
• An organism’s genotype still determines the phenotype
• Yet a variety of alleles may interact with one another in different ways to
determine the phenotype
INCOMPLETE DOMINANCE
• Neither allele is dominant
• When an organism is
heterozygous for a trait, it will
show a third phenotype
• Heterozygous = blended
appearance
• The third phenotype is a blend
of the other two
• Example: Many flower colors
F2 phenotypic and genotypic ratios are 1:2:1 (1 red:
2pink:1white)
CODOMINANCE
• Codominance: When both
alleles are simultaneously
expressed in a heterozygote
• We use capital letters for both
alleles
Genotypic ratio:
CC : CS : SS
1: 2: 1
Phenotypic ratio:
Curly: Wavy: Straight
1: 2: 1
MULTIPLE ALLELES
• Mendel’s work
suggested that two
alleles existed for each
gene
• We know that this is
not always the case
• Individual humans (and
diploid organisms) can
only have two alleles for
a given gene
• Yet there may be
multiple alleles that can
exist in a population
MULTIPLE ALLELES:
BLOOD TYPES
• A good example of multiple
alleles in humans is blood
• There are 3 alleles for blood
type
– A, B, O
• A and B are codominant
• O is recessive