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 DOMINANCE  According to Mendel, if pure red snapdragons are crossed with pure white snapdragons, what would be the result in the F 1 generation?  All the offspring should be heterozygous & show dominant phenotype.

INCOMPLETE DOMINANCE  However, when red and white snapdragons are crossed... PINK.  The F 1 generation are all 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.

CODOMINANCE  When a pure black-feathered chicken is crossed with a pure white-feathered chicken, what would you expect in the F 1 generation?  The F 1 generation should be heterozygous with dominant phenotype.

CODOMINANCE  However, when the black and white chicken were crossed... black and white feathers.  The F 1 chickens had both black 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 ALLELES  Instead 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 COLOR  C--dominant allele produces dark-gray coat.  c ch --results in light gray coat  c h --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. c ch is dominant to c h. c is always recessive.

DESCRIBE THIS RABBIT!  C c ch  c ch c h  c h c  c c  More than two phenotypes are possible!!!

POLYGENIC INHERITANCE  For 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 INHERITANCE  Polygenic inheritance is the inheritance pattern of a trait that is controlled by two or more genes.

POLYGENIC INHERITANCE  The result is that the phenotypes usually show a continuous range of variability from the minimum value to the maximum value.