Mendelian Genetics
Concept 2: Analyzing the effects of complex genetic crosses such as incomplete/co- dominance, multiple
alleles, pleiotropy, epistasis, polygenics, and lethal alleles.
For each of the following terms, define them and provide at least one example.
Use Holtzclaw & Holtzclaw page 108-110 and Campbell page 271-273 to help you.
Term
Definition
Example
Complete Dominance
When one offspring looks like one of the parents
because one allele is a pair showed complete
dominance over the other. The phenotypes of the
heterozygote and the dominant homozygote are
indistinguishable.
Incomplete Dominance
Neither allele is completely dominant. The F1 hybrids
have a phenotype somewhere between those of the
tow parental varieties. Appears to be the ‘blending’
theory of inheritance. But F2 offspring with phentopyic
ratio of 1:2:1 red:pink:white
Example: Red snap dragons
crossed with white snap
dragons give you white
snap dragons
Codominance
Two alleles both affect the phenotype in separate,
distinguishable ways. There is not an intermediate;
they the heterozygous have both of the alleles present
in the phenotype.
Multiple Alleles
Only 2 alleles exist for Mendel’s pea plants but most
genes exist in more than 2 allele forms.
IA - Carbohydrate A
IB – Carbohydrate B
I – none
The human MN blood group.
Individuals who are MM have
red blood cells with only M,
individuals with NN have only
N. But both M and M
molecules are present on the
red blood cells of individuals
wo are MN.
The ABO blood type of
humans. It can be IA IB and i
6 genotypes:
4 Phenotypes: A, B, AB, O.
These letters refer to
carbohydrates on the surface
of a red blood cell.
Pleiotropy
Most genes have multiple phenotypic effects. A single
gene can affect a number of characteristics in an
organism.
In the garden pea, the gene
that determines flower colour
also affects the colour coating
on the outer surface of the
seed.
Epistasis
A gene at one locus alters the phenotypic expression of
a gene at a second locus.
Black fur is dominant to brown
fur. A mouse might have Bb
but there is another allele that
determines the deposition of
pigment. If that is not
dominant, than Bb doesn’t get
expressed and the hair is
white.
Polygenic Inheritance
Some traits are either/or. Example, either pink or red.
Others, such as skin colour, are a continuum. These are
quantitative characters that usually indicate polygenic
inheritance. – an additive effect of two or more genes
on a single phenotypic character (the opposite is
pleiotropy – a single gene affects multiple traits)
Skin pigmentation.
Lethal Alleles
Require only one copy of the allele in order for the
disorder to be expressed. Usually, only late-acting
lethal alleles are passed on.
Huntington’s disease.
Degenerative disease of the
nervous system. Usually
doesn’t affect individuals until
they are over 40 years old.
Environmental Effects
The outcome of the genotype lies within the norm of
the reaction – depends on the environment.
Hydrangeas. Acidity of the soil
determines the colour.
Genetic Testing
May be used on a fetus to detect certain genetic
disorders.
Amniocentesis – physicians
remove amniotic fluid from
around the fetus.
Chorionic villus sampling – a
sample of the placenta is
taken.