INCOMPLETE DOMINANCE

1
EXCEPTIONS TO MENDELS LAWS

• INCOMPLETE DOMINANCE
• The pairs of alleles studied by Mendel all
  exhibited a dominant-recessive relationship.
• In 1760 the German scientist Josef Kölreuter
  crossed white carnations (rr) with red
  carnations (RR).
• He found that all of the offspring from his
  crosses had pink flowers (Rr).

2
EXCEPTIONS TO MENDELS LAWS

• INCOMPLETE DOMINANCE
• Surprisingly the hybrids had a phenotype that
  was intermediate between those of the parents.
• It appeared as if the parents alleles had
  blended
• together.
• Kölreuter then crossed his pink F1 hybrids with
  each other to form an F2 generation.

3
EXCEPTIONS TO MENDELS LAWS

• INCOMPLETE DOMINANCE
• In the F2 generation 1/4 of the plants had red
  flowers 1/2 had pink flowers and 1/4 had
  white flowers.
• This 121 ratio is what Mendel got from his F1
  crosses but the heterozygous state is not
  dominant.

4
EXCEPTIONS TO MENDELS LAWS

• INCOMPLETE DOMINANCE
• In carnations the R allele which codes for an
  enzyme that makes red pigment is
  incompletely dominant over the r allele
  which codes for a defective enzyme that cannot
  make pigment.
• Incomplete dominance is when the active allele
  does not compensate for the inactive allele and
  the heterozygous phenotype is somewhere in
  between the homozygous phenotypes.

5
EXCEPTIONS TO MENDELS LAWS Incomplete Dominance
6
EXCEPTIONS TO MENDELS LAWS

• CODOMINANCE
• Codominance is a condition in which both
  alleles of a gene are expressed.
• Unlike incomplete dominance where only one of
  the alleles is active here both alleles are
  active. Codominant alleles are written as
  capital letters with subscripts (for example B1
  and B2) or superscripts (for example R and R).

7
EXCEPTIONS TO MENDELS LAWS

• CODOMINANCE
• Codominance is seen in many organisms.
• For example red hair (HR) is codominant with
  white hair (HW) in cattle. Cattle that have the
  genotype HRHW are roan or pinkish brown in
• color because their coats are a mixture of red
  and white hairs.

8
EXCEPTIONS TO MENDELS LAWS

• POLYGENIC INHERITANCE
• Not all traits are produced by single genes.
  Many traits are produced by the interaction of
  many genes.
• Traits controlled by two or more genes are said
  to be polygenic.
• For example at least three enzymes - each of
  which is produced by a different gene - are
  involved in making the reddish-brown pigment
  in the eyes of fruit flies.

9
EXCEPTIONS TO MENDELS LAWS

• POLYGENIC INHERITANCE
• Complicated traits such as the shape of your
  nose height and weight are the result of
  interactions between large numbers of genes.
• The effect each gene has on a particular trait
  usually depends on the number of genes involved
  with that trait.