COMPLEX PATTERNS OF INHERITANCE

Exceptions to Mendel’s Laws

Mendel’s Laws

Law of Dominance--There are two forms of a gene--a dominant form and a recessive form. When the dominant and recessive are both present, the dominant gene (allele) will be expressed.

Mendel’s Laws

Law of Segregation--During gamete formation the alleles for a trait separate and each gamete receives one of the two alleles.

We call this meiosis.

Mendel’s Laws

Law of Independent Assortment-Genes for different traits are inherited independently of one another.

For example, hair color and eye color are inherited independently of each other.

EXCEPTIONS TO THE

RULES. . .

INCOMPLETE DOMINANCEAccording to Mendel, if pure red

snapdragons are crossed with pure white snapdragons, what would be the result in the F1 generation?

All the offspring should be heterozygous & show dominant phenotype.

INCOMPLETE DOMINANCEHowever, when red and white

snapdragons are crossed. . .The F1 generation are all PINK.PINK.This is referred to as

INCOMPLETE DOMINANCE.

INCOMPLETE DOMINANCE

The phenotype of the heterozygous individuals is intermediate between those of the two homozygous individuals.

CODOMINANCEWhen a pure black-feathered

chicken is crossed with a pure white-feathered chicken, what would you expect in the F1 generation?

The F1 generation should be heterozygous with dominant phenotype.

CODOMINANCE

However, when the black and white chicken were crossed. . .

The F1 chickens had both black black

and white feathers.and white feathers.This type of expression is known

as CODOMINANCE.

CODOMINANCE

The phenotypes of BOTH of the alleles are expressed in a heterozygous genotype.

MULTIPLE ALLELES

According to Mendel, how many different phenotypes could there be for a trait?

TWO, the dominant & recessive.However, in rabbits there are

FOUR different coat colors. How is this explained?

MULTIPLE ALLELESInstead of there being two

different alleles for coat color (a dominant and recessive). . .

There are 4 different alleles.Traits controlled by more than

two alleles are said to have MULTIPLE ALLELES.

4 GENES FOR RABBIT COAT COLORC--dominant

allele produces dark-gray coat.

cch--results in light gray coat

ch--results in white coat with black points

c--results in a white coat.

MULTIPLE ALLELES

Each rabbit will inherit two of these four alleles. . .

More than two phenotypes are possible.

C is dominant. cch is dominant to ch. c is always recessive.

DESCRIBE THIS RABBIT!

C cch

cch ch

ch cc cMore than two phenotypes are

possible!!!

POLYGENIC INHERITANCEFor many traits such as human

height, human hair color, human skin color, etc. there infinite possibilities.

What mode of inheritance explains an unlimited number of phenotypes?

POLYGENIC INHERITANCEPolygenic

inheritance is the inheritance pattern of a trait that is controlled by two or more genes.

POLYGENIC INHERITANCEThe result is that

the phenotypes usually show a continuous range of variability from the minimum value to the maximum value.

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