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Extensions of Mendelian Genetics
 Mendelian genetics seems to be relevant to only a
small set of heritable features
 For only a few characters there are…
 Only 2 versions of an allele (green or yellow)
 1 gene codes for a single external character
 1 allele is completely dominant to the other
 The basic patterns of segregation & independent
assortment apply to more complex patterns of
inheritance
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Different Types of Dominance
 Complete dominance
 Phenotype of heterozygote & HomoD are
indistinguishable
 The pattern with which you are already familiar
 Incomplete dominance
 Phenotype of heterozygote is in between the 2 Homo
phenotypes
 Example: pink snapdragons
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Snapdragons – Incomplete Dominance
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So incomplete
dominance does NOT
provide evidence for
“blending” theories
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Codominance
 Codominance
 Phenotype of
heterozygote is separate
& distinguishable from
Homozygous Dominant &
Homozygous Recessive
 Example: AB blood type
or Rhododendron flower
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Dominance & Phenotype
 The observed dominance/recessiveness of alleles
depends on the level of the investigation
 Consider Tay-Sachs disease
 Brain cells of the baby do not metabolize certain lipids
 As lipids accumulate, seizures, blindness, and mental
degeneration
 Death occurs within a few years of conception
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Tay-Sachs Disease

At the Organismal level, the disease is recessive
 Only children with 2 copies of the recessive trait will have the
malady

Heterozygote is not afflicted – they produce some lipidmetabolizing enzyme, though not as much as in HomoD
 So intermediate enzyme production

This suggests that @ the biochemical level, the disease is an
example of incomplete dominance

Which is Tay-Sachs: dominance or incomplete dominance?
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Prevalence & Dominance

Polydactyly
 Extra fingers or toes

1 of 400 in the US

The allele for polydactyly is dominant, but rarely present

Recessive homozygotes (HomoR) are found 399 out of 400
instances
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Blood Typing
 Only 2 alleles existed for Mendel’s peas, but this is not
typical for most traits
 Consider ABO blood group in humans
 A refers to the “A” membrane carbohydrate & type A
blood
 B refers to (seriously, I’m not writing this down)
 O means neither A or B carbohydrate is found
 AB means both A & B are found
 BUT the A and B alleles are codominant and are both
expressed if an individual inherits both alleles
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Epistasis
 A gene at one locus alters the phenotypic expression of a
gene at a second locus
 Example: Mouse fur color
 Bb or BB = Black bb = brown

If HomoR for (C) gene [cc], then no fur color (albino or white)
 Regardless of fur color specified by brown-black gene

If NOT HomoR for (c) gene [Cc or CC], then can be brown (bb)
or black (Bb or BB)
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What is the phenotype of…

BBcc?

BbCc?

bbCC?

Bbcc?

BBCC?

bbcc?
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Pleiotrophy
 Single gene has multiple effects
 Should be unsurprising given intricate molecular and
cellular interactions for development of an organism
 Phenylketonuria
 Mental Retardation
 Reduced skin and hair pigmentation
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Polygenic Inheritance
 The additive effect of 2 or more genes on 1 phenotypic
character
 Called quantitative characters since there is a continuum
of gradations
 Normal curve of phenotypes
 Example: human skin pigmentation is determined by at
least 3 separately inherited genes



AABBCC = Dark
AaBbCc = Intermediate
aabbcc = Light
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Pedigree Analysis
 Family tree describing the interrelationships of parents &
children across the generations
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Recessively Inherited Disease
 Requires 2 copies of the recessive allele (Homozygous
Recessive) to express the mality
 Heterozygotes are called carriers
 Normal phenotype, but may transmit disease to
offspring
 Examples:
 Cystic Fibrosis
 Tay Sachs disease
 Sickle-cell disease
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Cystic Fibrosis
 Recessive autosomal disease
 Common in those of European descent
 1 of 2,500 affected, but 1 of 25 are carriers
 Affects Chloride ion transport between a cell and
extracellular fluid
 If untreated, most die before 5th birthday
 Typically, patients live until their 20s or 30s with
efficacious treatment
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Sickle-Cell Anemia
 Recessive autosomal disease
 African descent
 Affects Hemoglobin protein in RBCs
 Low blood oxygen = hemoglobin molecules
clump together forming sickle shaped RBCs
 Sickle-celled RBCs clump together creating
chronic vascular occlusion of small vessels
 Example of incomplete dominance

Heterozygotes are usually normal but will show
some symptoms during prolonged periods of
reduced blood oxygenation
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Dominant Alleles
 Most harmful alleles are recessive, but some human
diseases are due to dominant alleles
 Only require one copy of the allele to be expressed
 Hypothesis: if there is a lethal disease carried on a
dominant allele, it would have burned out its carriers
by now.
 UNLESS, the lethal disease carried by a dominant allele
is one that affects organisms of advanced age

Like Huntington’s disease
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Examples of Dominant Allele Disease
 Achondroplasia – form of dwarfism
 Heterozygous individual = dwarf
 1 in 25,000 have achondroplasia, so 99% of the
population are HomoR
 Huntington’s disease
 Caused by a lethal dominant allele
 Degenerative disease of nervous system
 Usually only affects those > 40 yrs old
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Genetic Testing
 Pedigree analysis gives some info about risk to
offspring
 There are also tests to identify carriers of certain
genetic diseases
 Amniocentesis – amniotic fluid is removed and then
cells contained in the fluid are cultured to identify
certain chromosomal defects via karyotype
 Chorionic villus sampling (CVS) – placental tissue is
removed for same purpose as amnio, but results are
available far faster, & can be performed earlier in
pregnancy
 Karyotype can be immediately obtained
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