Extensions to Mendel

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• Extensions to Mendels Observation
• Types of Dominance Relationships Between Alleles
  of Same Locus
• Complete Dominance
• Incomplete Dominance
• Codominance

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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

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Summary of dominance relationships
Fig. 3.2
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• Complete Dominance
• Difficult to explain at the molecular level.
• Recessive allele may be inactive (no product)

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• 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

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• Codominance
• Each allele in the hybrid expresses itself
  clearly
• Coat color in lentils
• Blood groups A and B.

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• 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 gt i
• Seed coat patterns in lentils one gene locus, 5
  alleles,
• marbled-1 gt marbled-2 gt spotted dotted gt
  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

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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 gt1. 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.

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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 gt at gt a A is the wild
type allele and its frequency in nature is gt99.
This is because at and a make the mice easily
seen by predators so they die before they reach
the reproductive age.
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Polymorphic 1. ABO blood groups IA, IB and i
each has a frequency of gt1. 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
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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.
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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 neuro-toxic 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.
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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 (9331) b) flower color in sweet peas
result from complementary genes (97) 2.
Alleles of one gene locus mask the effects of
alleles from a different gene locus
(Epistasis) a) Recessive epistasis (934) b)
Dominant epistasis (1231 or 133)
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• 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

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• 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.

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