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INCOMPLETE DOMINANCE

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He found that all of the offspring from his crosses had pink flowers (Rr) ... Cattle that have the genotype HRHW are roan, or pinkish brown, in ... – PowerPoint PPT presentation

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Title: 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.
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