Transcript Heredity - Doc Charron's Science & Biology Website

Heredity
BEYOND MENDEL’S LAWS
Incomplete Dominance
 Review:
 Inherited traits were either dominant or recessive
 The dominant allele in an individual was always expressed,
even if the recessive allele was present
 However...
...there are exceptions to Mendel’s work
 In some cases, neither allele is completely dominant
 Therefore, in a heterozygous individual, there is a
blending of 2 traits
 This is called Incomplete Dominance
Example
 R = Red
 R’ = White (R’ is used rather than “r” to indicate incomplete
dominance)
A pure red snapdragon flower (RR) is crossed with a
pure white snapdragon flower (R’R’). Neither trait is
dominant.
R
R
R’
RR’
RR’
R’
RR’
RR’
RR’ is neither red nor white,
but rather, pink
Co-dominance
 In other cases, both alleles may be dominant and are
both expressed
 This is called Co-dominance

Ex. Feathers of black birds and white birds
Multiple Alleles
 Many genes have more than 2 different alleles, or
multiple alleles
Example: Human blood types
 3 different possible alleles for blood type
 IA
IB
IO
 IA and IB are co-dominant and are dominant over IO
 There can only be two alleles in blood type, but there are three alleles to
choose from
 Possible genotypes: IAIA, IBIB, IAIB, IAIO, IBIO, IOIO
 Possible phenotypes: Type A, Type B, Type AB, Type O