Modifications of Mendelian Ratios
Extensions and modifications
 Sometimes, inheritance patterns are more complicated than simple dominance
Incomplete dominance
 This is seen when heterozygotes have phenotypes that are intermediate between the
two homozygotes
 In incomplete dominance, the phenotypic and genotypic ratios are equal
 Each genotypic class has a separate phenotype
Codominance
 Heterozygotes simultaneously express the phenotypes of both homozygotes
Multiple alleles
 When there are more than two alleles controlling a trait, we have multiple alleles
 ABO blood group
o
This is a classic example of both codominance and multiple alleles
o
There are three alleles controlling this trait - IA, IB, i
o
The presence or absence of the different blood type antigens on the red
blood cells determines blood type
o
Your blood type also determines the type or types of blood that you can
receive in a transfusion
Lethal alleles
 1905 - Lucien Cuenot mated two yellow mice
 ~2/3 of the offspring were yellow; ~1/3 were not
 All of the yellow mice were heterozygous; none of the yellow mice bred true
 So there were two strange findings here
 The phenotypic ratio was 2:1, not a Mendelian ratio
 There were no homozygous yellow mice
 The 2:1 ratio is almost always indicative of the presence of a recessive lethal
allele

 Achondroplasia
o
Affects about 1 in 25,000 people
o
Head and torso develop normally but arms and legs are short
o
It is inherited in a dominant manner
In the dihybrid crosses previously considered, each gene exhibited simple
dominance/recessiveness
If either or both genes exhibits a different mode of inheritance, we see a modification of
the 9:3:3:1 ratio
Gene interaction - epistasis
 Sometimes one gene can mask the effect of another gene at a different locus in
determining a single characteristic
 The gene that does the masking is the epistatic gene; the gene that is masked is the
hypostatic gene
Recessive epistasis
 The Bombay phenotype and blood typing
 In very rare instances, a person can have an ABO blood type genotype that doesn’t
match their blood type (O) phenotype
 Individuals who are homozygous recessive (hh) do not produce the enzyme required
to produce the precursor to the A, B, and O antigens
 Without any antigen, they have the O phenotype
We can also consider inheritance of the more common mouse coat colors
 Two genes, two enzymes in a multi-step pathway
Dominant epistasis
 Sometimes the presence of only one allele can produce the masking effect
Bateson and Punnett looked at flower color in sweet pea, not the garden pea that Mendel
investigated
 They crossed two true-breeding strains of white-flowered plants and got all purple
F1 plants!
 Allowing the F1 plants to self-fertilize gave plants with both purple and white
flowers in a 9 purple: 7 white ratio
 In this case, at least one dominant allele of each gene is required to complete the
conversion of white flowers to purple
In the case of summer squash shape, you can cross plants with disc-shaped fruit (AABB)
with plants with long fruit (aabb)
 All of the F1 plants have disc-shaped fruit
 However, if you allow the F1 plants to self-fertilize, a new shape (sphere) is seen in
the F2 as well as the parental shapes
So, it really just new groupings of the 9:3:3:1 ratios
Complementation analysis
 Consider two mutants that display a similar phenotype
 This may be due to mutations in the same gene or in different genes
 Complementation analysis can distinguish between these two possibilities
Pleiotropy
 Sometimes, one gene can have multiple effects
 Human examples include Marfan syndrome and porphyria variegata
Sex-linked characteristics
 Most are X-linked; few are Y-linked characteristics
 Morgan noticed a lone white-eyed male among his thousands of red-eyed laboratory
flies
o
Naturally, he carried on a number of crosses with this white-eyed fly and
his offspring
 Morgan’s results
o
Pure-breeding, red-eyed female x white-eyed male gave all red-eyed F
1
(only 3/1237 had white eyes)
o
Conclusion: Simple dominance/recessive trait
o
However, a cross between two F flies produced all red-eyed F females
1
o
Half of the F males had red eyes; half had white eyes!
o
o
o
Conclusion: Male fruit flies are hemizygous for X-linked loci
Morgan was able to make predictions based on his hypothesis
Subsequent crosses confirmed the hypothesis
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 X-linked inheritance
o
Red-green color blindness
o
Cone cells that line the retina contain one of three pigments capable of
absorbing blue, green, or red light
o
The gene for the blue pigment is found on chromosome 7; the genes for
the red and green pigments are on the X
o
Affected women pass the X-linked recessive trait to their sons
o
Affected men pass the trait to their grandsons through their daughters
o
This pattern is known as crisscross inheritance
Sex and heredity
 Some characteristics, even though they are controlled by genes on autosomes, are
affected by the sex of an individual
Sex-limited characteristics
 Gene is only expressed in one sex
Sex-influenced characteristics
 Determined by autosomal genes
 Appears to have different inheritance patterns in females and males
 Pattern baldness in humans
o
Appears to be dominant in males; recessive in females
o
Related to the production of male sex hormones
Penetrance and expressivity
Incomplete penetrance
 Not all persons having the genotype for a particular trait will express the phenotype
 Penetrance is the percentage of individuals having that genotype and actually
expressing the phenotype
Variable expressivity
 Different affected individuals will display the phenotype to different degrees
 Expression of mutations in the eyeless gene in Drosophila can produce a range from
wild type to reduced to eyeless
 Polydactyly is a classic example of both incomplete penetrance and variable
expressivity
Genes and the environment
 In many cases, the genotype determines a range of phenotypes
 The environmental conditions determine the norm of reaction
 Himalayan rabbits
o
Black is seen at the extremities if the rabbit is reared at 20°C or less
o
No black is seen if the rabbit is reared at 30°C or above
Genomic imprinting
 Differential expression of genetic material depending on which parent contributed
the allele
 Birth weight in mice and humans is affected by a number of genes including Igf2
o
Oddly, only the paternal copy is expressed in the fetus and placenta; the
maternal copy of the gene is completely silent
 Genetic-conflict hypothesis
o
This seems to occur because the paternal copy promotes placental and
fetal growth by directing more of the maternal nutrients to the fetus
o
Non-expression of the maternal copy would select for smaller fetal size
 One deletion, two disorders
o
Prader-Willi and Angelman syndromes
o
There is a deletion in chromosome 15
o
If inherited from the father, Prader-Willi
o
If inherited from the mother, Angelman
Cytoplasmic inheritance
o Inheritance of mitochondria (and chloroplasts) occurs only through the egg
o The sperm contributes no cytoplasm
Mitochondria
o Contain DNA
o ~15,000 bp; 37 genes, most involved in energy metabolism
o Most cells contain 2-10 mitochondria, therefore 2-10 copies of mtDNA
o Because mitochondria segregate randomly when the cell divides, mitochondrial
disorders are highly variable
It also holds for the inheritance of cpDNA
o One variety of four-o’clock plants has variegated leaves and shoots
o Some branches of the variegated strain had all-green leaves while others had allwhite leaves
Genetic maternal effect
o The offspring’s phenotype is determined by the mother’s genotype
o Shell coiling in the snail Limnaea peregra is determined very early and depends on a
substance present in the egg
o In this case, all of the F are dextral
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o But, the F phenotypes would depend on the genotype of the mothers
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