BIOL 1106
GENETICS
Lecture 3
Dr. Lawrence Ramsden
Department of Botany
The Complications
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Multiple alleles
Incomplete dominance
Codominance
Epistasis
Lethal alleles
Expression
Continuous variation
Environmental effects
Multiple Alleles
• A gene may have many alleles not just 2
• But, a single diploid individual can only
carry 2 alleles
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+
1.0000
ch
ch
w /w
0.0410
w/w
0.0044
a3
a3
w /w
0.0632
wt/wt
0.0062
ww/ww
0.0650
w /w
a
a
0.0197
co
co
w /w
0.0798
bl
bl
w /w
0.0310
sat
sat
w /w
0.1404
we/we
0.0324
wcol/wcol 0.1636
w /w
Dominance
• Where there is complete dominance the
phenotype of the heterozygote is the same
as that of the homozygote dominant.
• Incomplete or partial dominance can lead to
the expression of intermediate phenotypes.
• Andalusian blue, colour produced by CB/CW
heterozygote from Black x White chickens.
Codominance
• In codominance both phenotypes are
expressed in the heterozygote, not an
intermediate type as in incomplete dominance
• In ABO blood group system heterozygous
individuals produce both A & B antigens.
Gene Interactions
• No gene acts by itself in determining
phenotype.
• Genes can interact to produce new
phenotypes.
• or, Genes can interact to hide the expression
of phenotypes,- Epistasis.
Chicken Combs
• Phenotype determined by interaction of
alleles of 2 genes, R & P
• 4 comb types;
• Rose
R/- p/p
• Walnut R/- P/• Pea
r/r P/• Single r/r p/p
• True breeding if the alleles are homozygous
F2 9:6:1 ratio in Squash
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2 genes A & B can interact
3 phenotypes
Spherical
A/- b/b or a/a B/Long
a/a b/b
Disc
A/- B/-
Epistasis
• Genes interact to mask another genes
phenotype.
• No new phenotypes are produced.
• Can be dominant or recessive.
Agouti 9:3:4 ratio
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Rodent coat colour and pattern.
Agouti, albino or black.
Albino lacks any coat colour.
Black lacks yellow colour in agouti pattern.
But, you can’t have a coat pattern without
any colour! so even A/- c/c will be albino,
and the recessive c/c masks the A allele
Flower colour, 9:7 ratio
• White variety crosses sometimes produce
purple flowered F1
• Purple F1 x F1 gives 9:7 purple:white
• Due to interaction between dominant alleles
• Purple C/- P/• Duplicate recessive white
Lethal Genes
• Yellow coat colour in mice is an essential
gene.
• Yellow heterozygotes bred to non-yellow
mice give 2:1 yellow non-yellow ratio.
• Yellow AY/AY homozygotes are aborted.
• Yellow dominant for coat colour.
• Recessive with respect to lethality.
Environmental Effects
• Fur colour in Himalayan rabbits;
• above 30°C all white
• at 25°C normal pattern with dark
extremeties.
• Cooled below 25°C, more dark patches.
Neurofibromatosis
• Penetrance, the phenotype does not always
reflect the genotype.
• Expressivity, the degree to which the
phenotype is expressed for a given
genotype.
Continuous Variation
• Many phenotype characters are the product
of the action of a large number of genes.
• So the phenotype is the product of
independent assortment in the individual
genes.
• In a population we will see gradual
differences, or continuous variation.
Linkage
• Non-independent assortment of genes
• Linked genes, located on same
chromosome
• Genetic recombination, where progeny
show new combinations of alleles not
present in the parents
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Total flies = 2,441
Total parental phenotypes = 1,541
Total recombinant phenotypes = 900
% recombinants = 36.9
If there was independent assortment we
would predict 50% recombinants
• Conclusion, the genes are linked
Crossing Over
Linkage Maps
• Use linked genes to determine physical
location on chromosome.
• The greater the separation between 2 genes
the higher the probability that crossover will
occur.
• 1% recombination = 1 map unit (mu or cM)
• Multiple cross-overs can also occur
Avoiding multiple cross-overs
• Use closely-linked genes (within 10 mu)
• Use a three-point test cross including a third
allele.
• The products of a double crossover will be
the least frequent.
• Can help establish the order of the genes on
the chromosome.
• Interference can happen between cross-overs.