Extensions to Mendel’s Observation
Types of Dominance Relationships Between Alleles of
Same Locus:
• Complete Dominance
• Incomplete Dominance
• Codominance
Dominance is not always complete
• Crosses between true-breeding strains can
produce hybrids with phenotypes different
from both parents
– Incomplete dominance
• F1 hybrids that differ from both parents express an
intermediate phenotype. Neither allele is dominant or
recessive to the other
• Phenotypic ratios are same as genotypic ratios
– Codominance
• F1hybrids express phenotype of both parents equally
• Phenotypic ratios are same as genotypic ratios
Summary of dominance
relationships
Fig. 3.2
Complete Dominance:
• Difficult to explain at the molecular level.
• Recessive allele may be inactive (no product)
Incomplete Dominance:
• Hybrid phenotype is midway between the two parents
•In snapdragon,
red allele produces pigment
white allele produces no pigment
Hybrid (red/white) produces half the amount of
pigment, hence pink in color
Codominance:
Each allele in the hybrid expresses itself clearly
• Coat color in lentils
• Blood groups A and B.
Multiple Alleles:
A gene locus may have more than two alleles but only
two can be present in a diploid individual.
Examples:
• ABO blood groups: one gene locus, 3 alleles, IA = IB > i
• Seed coat patterns in lentils: one gene locus, 5 alleles,
marbled-1 > marbled-2 > spotted = dotted > clear
• Histocompatibility antigens: 3 gene loci HLA-A, HLA-B
and HLA-C. Each gene locus has 20-100 alleles. Every
pair of alleles within a locus show co-dominance
creating an astronomic number of variations
How do new alleles in any one gene arise?
By mutations. When a mutation causes one specific change in a
gene (such as a single nucleotide substitution at the DNA level) this
produces a new form of the gene i.e. a new allele.
Distribution of alleles in nature:
A wild type allele for any gene locus has a frequency of >1%.
Based on that, two types of genes exist in natural populations:
1. Monomorphic: have only one wild type allele.
2. Polymorphic: have more than one wild type allele.
Monomorphic:
The agouti gene in mice is one of the main genes
determining coat color in mice.
Three alleles are known for this gene
A
gives rise to agouti color (each hair striped with yellow & black)
at
gives rise to black belly/yellow body fur
a
gives rise to all black fur
Dominance series: A > at > a
A is the wild type allele and its frequency in nature is >99%. This is because at
and a make the mice easily seen by predators so they die before they reach the
reproductive age.
Polymorphic:
1. ABO blood groups: IA, IB and i each has a frequency
of >1%. So they are all wild type alleles.
2. Self incompatibility gene in tomato and petunia has a
series of alleles.
This series promotes out-crossing and encourages the propagation
of new mutant alleles in this gene locus. So there are several wild
type and mutant alleles for this gene
Pleiotropy:
One gene contributes to several visible characteristics
that are not related.
1. Example is a single recessive allele causes respiratory
problems and sterility in men of a tribe in New Zealand.
How?
This recessive allele encodes for a defective protein
required for the motion of cilia in the respiratory tract
and also for flagella movement in sperms.
2. A recessive lethal allele in mice AY also produces a
dominant yellow coat color phenotype
In humans, Tay-Sachs disease is caused by a recessive
lethal that encodes a defective hexoseaminidase, an
important enzyme needed to rid the body from neurotoxic metabolites. Heterozygotes (carriers) can be
easily detected by assaying the levels of
hexoseaminidase activity in the blood. They have half
the amount in a homozygous normal person.
Gene Interactions:
Two genes can interact to produce one trait.
1. New phenotypes are produced by the combined
action of the alleles of two different gene loci.
a) seed coat color in lentils (9:3:3:1)
b) flower color in sweet peas result from
complementary genes (9:7)
2. Alleles of one gene locus mask the effects of alleles
from a different gene locus (Epistasis)
a) Recessive epistasis (9:3:4)
b) Dominant epistasis (12:3:1 or 13:3)
Heterogenous Traits:
• homozygosity of mutations at many gene loci
can cause the trait (e.g. deafness in humans)
• complementation test is useful in heterogenous
traits because it can reveal whether two
defective individuals (for the trait) have
mutations in the same gene locus or in different
loci
Quantitative traits:
• Unlike a Mendelian trait which is controlled by
a single gene locus (monogenic), a quantitative
trait is controlled by 3 or more gene loci
(polygenic).
• The contribution of each gene to the
quantitative trait is additive leading to the
appearance of many phenotypic classes in F2.
With many genes it becomes difficult to separate
the phenotypic classes in F2 and the trait is
known as continuous. Example is human height
and crop yield.