Chapter 5 – Extensions and Modifications of Basic Principles

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Transcript Chapter 5 – Extensions and Modifications of Basic Principles

Chapter 5 – Extensions and
Modifications of Basic Principles
Dominance revisited
• Mendelian concept
– In the heterozygous condition, only one allele
(dominant) is expressed
• Incomplete dominance
– Heterozygote has phenotype intermediate to
homozygous phenotypes
Incomplete dominance
• Heterozygote has
• Does NOT have to
be phenotype
“right in the middle”
• Lighter shade of
red to very light
shade of pink
• Heterozygote expresses both alleles//both
• MN locus
– Codes of antigen on red blood cells
– Does not cause significant immune response
like ABO or Rh groups
– LM allele = M antigen; LN allele = N antigen
– LMLN individual has both antigens present
• “Dominance” can depend on which level you are
looking at
• Cystic Fibrosis – autosomal recessive disorder
– Normal allele produces carrier protein in plasma
membrane that allows Cl- passage in/out of cell
– Mutant allele produces defective protein that prohibits
Cl- from exiting cell
– Carriers of Cystic Fibrosis
• Physiological level – recessive
– Carriers have enough normal channels for unaffected
• Molecular level – codominant
– Carriers have both normal and mutant channel proteins
Incomplete penetrance
• Genotype does not always
produce expected phenotype
• Polydactyly
– Dominant allele
– Individuals with dominant allele
can occasionally have normal
number of digits, but have
affected children
• Penetrance
– % of individuals with a particular
genotype that express expected
– 42 individuals have polydacylous
allele; 38 express polydactyly
– 38/42 = 90% penetrance
Variable Expressivity
• Degree to which trait is expressed
• Polydactyly
– Some extra digits are fully functional; others are
just small skin tags
• Penetrance and expressivity are due to other
genes and environmental factors
– Mere presence of allele does not guarantee
expression, or standard “one size fits all”
Lethal alleles
• Cause death at an early
age of development
(usually before or shortly
after birth) so some
genotypes are appear
among progeny
• Recessive – need to be
homozygous to be lethal;
heterozygote will have
different phenotype
• Dominant – lethal in both
homozygotes and
– Only transmissible when
lethal after individual has
passed reproductive age
Multiple alleles
• One gene may have more than 2 possible
– Regardless of possible alleles in a population, an
individual can only have a maximum of 2 different
• ABO blood type
– Codes for antigens of surface of red blood cells
– 3 possible alleles
IA – puts “A” antigen
IB – puts “B” antigen
i – puts no antigen
i is recessive to both IA and IB; IA and IB are co-dominant
– Served as primitive means of paternity testing
Gene interaction
• More than one gene
contributes to a single
• Polygenic inheritance
• One gene masks the effects of another
• Can be dominant or recessive
• Albinism
– Lack of pigment melanin
– Very light skin and hair; pink or very light blue
Albinism cont
• Duplicate recessive epistasis
– Since pigment production is a multi-step process
requiring multiple enzymes, different genes can
each result is albinism
• P generation aaBB (albino) x AAbb (albino)
• F1 AaBb (normal pigmentation)
• F2 9A_B_:3aaB_:3A_bb:1aabb
9 normal pigmentation:7 albino
Complementation test
• Test to determine whether two different
mutations are at the same locus or different
• Cross homozygous individuals with different
• D. melanogaster
– both apricot (a) eye color and white (b) eye color
is recessive to normal wild-type red
Complementation test cont
• If same locus, all
F1 will have a
mutant phenotype
• If different loci, F1
will have wild-type
Interaction between sex and
• Sex-influenced traits
– Autosomal Mendelian
inheritance, but
expressed differently
in sexes
– Beards on goats
• Dominant trait in males
– Expression requires
only one allele
• Recessive trait in
– Must be homozygous
to have a beard
Sex influenced traits cont
• Human male pattern baldness
– NOT x-linked
– Dominant in male; recessive in females
• Affected males can be homozygous or heterozygous
• Affected females must be homozygous
– Usually results in “thinning” – variable expressivity
– Difference due to presence of male sex hormones
• Males castrated prior to puberty do not exhibit pattern
baldness, even with genotype
Sex-limited traits
• Autosomal inheritance
• Trait is only expressed in one
sex; zero penetrance in other
• Domestic chickens
– H = hen plumage; h = cock
– Male hh = cock feather tail
– Female hh = hen feather
• Cock plumage never expressed in
Cytoplasmic inheritance
• Inheritance of DNA in cytoplasm
(mitochondria or chloroplasts)
• Inherited from mother only
– Sperm contributes nucleus, but no
• Characteristics exhibit extensive
phenotypic variation
– Each cell can contain hundreds of
mitochondria, and may not have
same genetic information
– Homoplasmy – all the same
– Heteroplasmy – different genetic
• Ratio of “normal” to “mutant”
Genetic maternal effect
• Genotype inherited from
both parents, but phenotype
is determined by
MOTHER’S genotype
• Limnaea peregra
– Dextral coiling (to the right) is
dominant over sinistral (to the
left) coiling
– Phenotype determined by
mother’s genotype (not her
Genomic Imprinting
• Differential expression of
gene depending of
whether it was inherited
from mother or father
• Due to different
methylation patterns of
• Microdeletion of 15p
– Deleted from father –
Prader-Willi syndrome
– Deleted from mother –
Angelman syndrome
• Genetic trait becomes either more strongly
expressed or expressed at an earlier age
as it is passed from generation to
• Due to an unstable region of DNA that
tends to increase in size in next generation
Environmental Effects
• Himalayan allele in
– Produces dark fur –
nose, feet, ears
– Develops at
temperatures less
than 20°C
– Enzyme is inactivated
at temperatures over
• Environmental factors produce a phenotype that mimics the
phenotype of another genotype
• PKU – phenylketonuria
– Autosomal recessive
– Phenylalanine can not be broken down; build-up causes brain
– Affected child put on restricted diet - prevents retardation
• Can go off diet after nervous system is fully formed (early 20s)
– Affected woman when pregnant must be diet restricted
• If not, excess phenylalanine can cross placenta and give child PKU
phenotype, even if genotypically unaffected
Inheritance of Continuous
• Discontinuous – few, distinct phenotypes
• Continuous – wide range of phenotypes
– Often form bell-shaped curve when plotted
• Height, skin color
– Usually due to multiple genes contributing to a
single trait
• Polygenic inheritance
• One gene affects multiple characteristics
– Mental retardation, light skin and eye color