Title: INCOMPLETE DOMINANCE
1EXCEPTIONS 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).
2EXCEPTIONS 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.
3EXCEPTIONS 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.
4EXCEPTIONS 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.
5EXCEPTIONS TO MENDELS LAWS Incomplete Dominance
6EXCEPTIONS 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).
7EXCEPTIONS 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.
8EXCEPTIONS 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.
9EXCEPTIONS 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.