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Patterns of Intermediate InheritanceExceptions to Mendel’s Principles
• Mendel’s 3 principles provide us with an important foundation in building our knowledge of genetics.
• However, there are many types of inheritance patterns that do not follow the principle of dominance.
• There are several types of intermediate inheritance patterns:– Co-dominance– Multiple Alleles– Incomplete Dominance– Polygenic Traits– Multifactorial Inheritance
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Co-dominance• A trait for which both alleles contribute equally
to the phenotype. – Example: Coat color in cows and horses– When a homozygous red cow is crossed with a
homozygous white cow they produce a heterozygous cow that is red and white (roan).
Genotype: RRGenotype: WW
Genotype: RW
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Practice Cross - Codominance• A horse that is roan is a blend
of red and white colors that result when a red horse is crossed with a white horse. Red (R) horses breed true; and white (W) horses breed true.
• What is the genotype of a red horse?
– RR• What is the genotype of a
white horse?– WW
• What is the genotype of a roan horse?
– RW
• What is the probability that two roan horses will have at least one roan offspring? – 50%
• What is the probability that two roan horses will have a white offspring? – 25%
RR RW
RW WW
R W
R
W
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Multiple Alleles
• When a gene has more than two possible alleles.• ONLY TWO alleles are inherited.
– Example: Rabbit Coat Color is controlled by 4 alleles - C, cch, ch, and c
CC, Ccch, Cch, Cc
cchcch, cchch, cchc
chc or chchcc
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Practice cross: Multiple Alleles• Practice cross: In rabbits there are four
different color alleles:– C is dominant over all C = wild type color– cch is dominant to ch & c cch = chinchilla color– ch is dominant to c ch = Himalayan color– C is recessive to all c = albino
• Each rabbit can inherit only two alleles. A wild colored rabbit is crossed with an albino and in the offspring there is a Himalayan rabbit.
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Show the cross between a wild type and albino rabbit that has a Himalayan baby.
• What are the genotypes of the parents?– Parent one : cc– Parent two : Cch
• What is the probability that they will have an offspring that is Himalayan?– 50%
• What is the probability that they will have an offspring that is albino?– 0%
• What is the probability that they will have an offspring that is wild colored?– 50%
C
c
ch
c
Cc
Cc
c
c
Mult. Alleles &
Co-dominance
ch
ch
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Incomplete Dominance
• Results in a heterozygous phenotype that is a blend of the two homozygous phenotypes.
– Examples: mirabilis (four o’clock) plants, coat color in horses
– When a homozygous chestnut horse is crossed with a homozygous white horse, they produce a heterozygous tan (palomino) horse.
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Genotype: cc (Chestnut) Genotype: ww
Genotype: cw (Palomino)
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Practice Cross – Incomplete Dominance
• In Four O’Clocks, the gene for red flowers (r) is incompletely dominant to the gene for white flowers (w).
• The heterozygous condition results in pink flowers.
• Red Plant Genotype?– rr
• White Plant Genotype?– ww
• Are the F1 flowers purebred or hybrid?– hybrid
• What is the phenotype of the F1 flowers?– pink
rr
rwrw
rw rww
w
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Polygenic
• A trait that is controlled by the interaction between 2 or more genes.– Examples: skin color, eye
color, height, hair color– Results in a continuum of
expressed phenotypes.
Humans & Plants
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Multifactorial Inheritance• The phenotype is a result of an interaction between your
genotype and certain environmental factors. • The expression of most all genes is influenced by
environmental conditions.• Examples:
– Diabetes – Height– Heart Disease
• Can inherit a predisposition to all of these diseases/characteristics.• Their development (phenotype) is influenced by environmental
factors such as proper nutrition, exercise, quality medical care, etc.
Enviro Factors
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Examples in plants:• In many plants, the
presence of light triggers the production of chloroplasts.
• Chemicals that stimulate germination are only produced in the presence of certain temperatures. http://www.dkimages.com/discover/previews/874/30020399.JPG
Review of Terms
