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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