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Chapter 4
MULTIPLE ALLELES
• When a given gene has several
alleles, not just two
• A diploid individual still has a
maximum of 2 alleles, one on each
homologous chromosome
MULTIPLE ALLELES
• ABO Blood Groups
–Discovered in early 1900s
–Important when considering
tranfusions
–4 types; 3 alleles
MULTIPLE ALLELES
• ABO Blood Groups
A: IAIA; IAi
B: IBIB; IBi
AB: IAIB
O: ii
MULTIPLE ALLELES
• ABO Blood Groups
–Antibody – protein molecule that
recognizes and binds to foreign
material
–Antigen – molecule that is recognized
as foreign and stimulates antibody
production
• Don’t stimulate antibody formation in
organism expressing them
(exception: Autoimmune diseases)
MULTIPLE ALLELES
• ABO Blood Groups
–IA : specifies the ‘A’ antigen; antibodies
against ‘B’ and will clump onto the IB
–IB : specifies the ‘B’ antigen; antibodies
against ‘A’ and will clump onto the IA
–AB : have both antigens, but no “anti-”
antibodies
–O (ii) : have no antigens and no “anti-”
antibodies
MULTIPLE ALLELES
• ABO Blood Groups
–Safe Transfusions:
• A (IAIA /IAi) – can receive A or O
–Can give to A or AB
• B (IBIB /IBi) – can receive B or O
–Can give to B or AB
• AB (IAIB) – can receive AB or O
–Can give to only AB
• O (ii) – can receive only O
–Can give to any blood group, A, B or O
MULTIPLE ALLELES
• What does this have to do with
molecular genetics?
–The base pair sequence of a gene
specifies amino acid sequence of a
protein…this protein function depends
on the sequence of amino acids
–So, a simple change in the base
sequence can drastically change the
protein function
Modifications of Dominance
• Complete Dominance
–One allele is dominant to another, so
the heterozygous individual shows the
dominant phenotype
• Incomplete Dominance
–One allele is not completely dominant
to another (partial dominance),
heterozygous individual shows a new
intermediate phenotype
Modifications of Dominance
• Incomplete Dominance
–Ex. Plumage color in chickens
• Cross a true-breeing black (CBCB) with a
true-breeding white (CWCW) and the
heterozygous F1 offspring (CBCW) exhibits
a bluish-grey plumage
–C : color
–B : black
–W : white
• Can’t be true-breeders…why?
Modifications of Dominance
• Incomplete Dominance
– Explanation: believed to occur for this
reason…
• CB : produces color gene expression
• CW : produces no gene expression
• So a heterozygous individual produces
“half” a dose of gene expression
–Insufficient
–Heterozygotes that produce a “normal”
dominant appearance are described as
haplosufficient
Modifications of Dominance
• Codominance
– one allele is not dominant to another,
instead the phenotype produced
exhibits both dominant phenotypes
• Ex. ABO blood grouping (AB)
– Explanation…believed to occur because
• Both alleles for competing phenotypes are expressed
In Review
• Complete dominance
– A/A & A/a produce the same phenotype and can
be written as A/- because the second allele does
not change the expression of the gene
• Incomplete dominance & Codominance
– A/A & A/a do not produce the same phenotype so
they must be written out as they appear
Modified Mendelian Ratios
• Production of NEW Phenotypes
– Ex. Comb Shape in Chickens (may be true-breeders)
• a) R/- p/p
• b) R/- P/• c) r/r P/• d) r/r p/p
– Assuming recessives
do not take any action,
we can assume the
single comb is a
product of other genes
while the others are due
to the activity of the
R and P alleles
Modified Mendelian Ratios
• Production of NEW
Phenotypes
– Ex. Fruit Shape in
Summer Squash –
available in long, sphere,
and disk-shaped
• Sphere – A dominant
allele of either gene and
homozygous recessive of
the other
• Disk-shaped – A dominant
allele of both genes
• Long – double
homozygous recessive
Modified Mendelian Ratios
Produces NO new phenotype
• Epistasis
– Involves a gene masking or modifying the
phenotypic expression of another gene
– Interaction between 2 or more genes to control a
single phenotype
– Confined to dihybrid crosses where two pairs of
alleles assort independently
– Does not produce a new phenotype, only masks
• Epistatic gene – the gene that masks another
• Hypostatic gene – the gene that is masked
Modified Mendelian Ratios
• Epistasis
– Recessive – must be homozygous
• Ex: coat color in rodents – natural coat color in wild
rodents is a greyish color (produced by alternating
bands of black and yellow – agouti pattern)
– Aids in camouflage
– Found in mice, squirrels, etc
– Other colorations exist, but are recessive to agouti
» (A/– agouti; a/a nonagouti)
» (C/– pigment; c/c albino)
» (B/– black; b/b brown)
» c is epistatic when homozygous (recessive gene)
» A is hypostatic
Modified Mendelian Ratios
• Epistasis
– Recessive - must be homozygous
• Ex: coat color in labrador retrievers – available in black,
yellow, & chocolate
– One gene specifies black pigment (B/-) or brown (b/b)
– An independent gene either allows (E/-) or hides (e/e) the
expression of the Black / brown gene
» Black: B/- E/» Chocolate: b/b E/» Yellow: -/- e/e (B/- black noses; b/b brown noses)
» e is epistatic when homozygous (recessive gene)
» B is hypostatic
Modified Mendelian Ratios
• Epistasis
– Dominant
• Ex: fruit color in summer squash – available in white,
yellow, or green
– W/-, -/- white
– w/w, Y/- yellow
– w/w, y/y green
» W – epistatic (homozygous or heterozygous)
» y - hypostatic
Modified Mendelian Ratios
• Epistasis
– Dominant
• Ex: Greying in horses
– It doesn’t matter what color the horse’s base is (sorrel, black,
bay, etc) over time the Grey gene will mask that phenotype
– It is a progressive process
– Grey does not affect skin or eye color, only hair
Modified Mendelian Ratios
• Epistasis
– Duplicate Genes
• When a gene at one locus produces a phenotype identical
to that produced at another locus
• Ex: sweet peas flower colors
–
–
–
–
C: colored
c: no color
P: purple
p: white
» Purple flowers: C/- P/» White flowers: c/c -/- OR C/- p/p
(duplicate recessive epistasis OR complementary gene action)
when 1 or both loci are homozygous recessive
Modified Mendelian Ratios
• Essential & Lethal Genes
– Mutations not only change phenotypes, they can also
cause death (which I guess technically does change the phenotype)
• Alleles resulting in death are lethal alleles, caused
by essential genes (essential to the normal functioning
of the organism)
• When caused by a dominant lethal allele both the
heterozygous and homozygous individuals will
show the lethal phenotype
• When caused by a recessive lethal allele, only the
homozygous individual will show the lethal
phenotype
Modified Mendelian Ratios
• Essential & Lethal Genes
– Lethal alleles
• Ex: Yellow body color in mice
– Acts dominant in determining body color, but acts recessive in
determining lethality (only heterozygotes survive to birth)
• Ex: Huntington’s disease in humans
– Autosomal dominant (can’t be studied until reproductive age)
– Onset doesn’t appear until early-thirties, and death in forites
• Ex: Hemophilia in humans
– X-linked recessive
Modified Mendelian Ratios
• Gene Expression
– Penetrance – frequency with which a gene manifests
itself in individuals in the population
• Depends on genotype and environment
– Expressivity – degree to which a gene or phenotype
are expressed in an individual
– Environment –
• Age of onset: creates internal environmental changes
– Genes are not “on” all the time; genes can be activated or
deactivated over time
» Pattern baldness
» Muscular Dystrophy
Modified Mendelian Ratios
• Gene Expression
– Environment –
• Sex – expression of genes are influenced by gender
– Sex-limited traits –autosomal genes that affect only 1 gender and
not the other
» Ex: milk production
» Ex: appearance of horns in some species
» Ex: facial hair
• Temperature – Reactions are catalyzed by enzymes, which
function in a certain range.
– Ex: fur color in Himalayan rabbits
» >30 C – all white
» <25 C – typical coloration (black paws, ears, nose tail) and
anywhere it is artificially cooled
Modified Mendelian Ratios
• Gene Expression
– Environment –
• Chemical – can have significant effect on an organism
– EX: Phenylketonuria (PKU): autosomal recessive, trouble
metabolizing amino acid phenylalanine, diet determines severity
(proteins)
– Nature vs Nurture
• What are the relative contributions of genes and the
environment to the phenotype?
– Ex: Height – influenced by genes (potential) and environment (diet,
overall health, hormones)
– Ex: Alcoholism – influenced by genes (susceptibility) and
environment (choice)
– Ex: Intelligence – influenced by genes (potential) and environment
(learning, challenges)