|
|
|
OAC Biology GENETICS - INCOMPLETE DOMINANCE Review MR. GALAJDA
P1 phenotype Red X White genotype RR rr F1 phenotype _________ genotype _________
allow the F1 to self-pollinate
F1 phenotype ________ X ________ genotype ________ ________
gametes
F2 phenotype genotype
The absence of the 3:1 phenotypic ratio when two heterozygous plants are pollinated is apparent and appears to indicate a deviation from Mendel's simple dominant inheritance patterns. Thus, a 1:2:1 phenotypic ratio would be expected when two heterozygous traits are crossed in INCOMPLETE DOMINANCE. In the case above, it appears that the homozygous RR produces more red pigment than the heterozygous Rr. Thus, the amount of pigment produced in Rr is much less than the RR, therefore the Rr flower is pink while the RR flower colour is red. The plant with the homozygous recessive phenotype rr produces little or no pigment, therefore it is white.
INCOMPLETE DOMINANCE: the heterozygous form results in a phenotype, which is intermediate between the two pure homozygous forms, partially expressing both alleles. Other examples of incomplete dominance include: -feather colour in the Andalusian fowl -in cattle, the alleles for red coat colour (R) and for white coat colour (r) behave as codominants with the heterzygote having a roan coat colour (Rr).
SAMPLE PROBLEM: #1. In the four o'clock, a flower rather like petunia, the allele for red flower colour is incompletely dominant over the allele for white flower colour. When a red flower is crossed with a white one, an intermediate pink flower results. (a) Show the genotypes of the parents and the F1 generation of a cross between a red and a white four o'clock flower.
(b) What would be the anticipated offspring if the F1 plant from above (a) was to be: (i) test crossed?
PEDIGREES: A rather useful way to follow a particular trait throughout several generations is to systemically list the ancestors of a given trait in a family tree or PEDIGREE. This chart can be used to determine the probability of inheritance of the trait (particular allele) by a person not yet born. Thus a family pedigree would be a particularly useful tool of a genetic counselor. Typically, a square is used to represent a male and a circle to show a female on a pedigree chart. As well, the squares and circles are generally filled in when the person has the condition being studied or shaded if the person is known to be a carrier of the condition being studied. Some examples of pedigrees include:
DOMINANT TRAITS: the trait for widow's peak hairline is dominant over other hairlines. It can be shown in the following pedigree:
RECESSIVE TRAITS: the genetic disorder, cystic fibrosis is caused by a recessive allele. It can be shown in the following pedigree: |
|
E-mail Mr. Galajda at: david.galajda@sac.on.ca |