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Genetics After Mendel
Recall
Complete Dominance in F2 generation:
3:1 or 9:3:3:1
The following are some examples that do not fit
into Mendel’s patterns of inheritance:
1. Incomplete Dominance
2. Co-Dominance
3. Multiple Alleles
1. Incomplete Dominance
• Neither allele is dominance over the other.
• Heterozygous individual’s phenotype is a
BLEND of the two alleles.
• Alleles are written using a unique capital
superscript letter with a common base letter
Example
• Flower colour:
White (CWCW) flower X red (CRCR) flower
will produce
all CRCW offspring (F1) – pink in colour
Example Cont’d
• If self-pollinate F1, (CRCW)
Summary
F2 Generation
• Genotype is ¼ CRCR: 2/4 CRCW: ¼ CWCW
• Phenotype is ¼ red: 2/4 pink: ¼ white
2. Co-Dominance
• Exhibited in a heterozygous individual where
both alleles of a gene are expressed, not
blended.
• Alleles written the same way as incomplete
dominance
Examples
1. Black cat x tan cat = tabby cat (black & tan stripes)
1. Roan coat in horses (white and red hair)
3. Multiple Alleles
• When more than 3 phenotypes are possible,
then more than 2 alleles for that trait must
exist in the population.
• However, individuals have only two of those
alleles. Why?
• Because only one allele is inherited from each
parent
Example
• The ABO system of human blood type involves
three alleles (IA, IB, and i).
• IA and IB – co-dominant, where “i” is recessive
• As a result, there are four possible phenotypes
or blood types: A, B, AB, and O.
FYI
• Please note there are 32 human blood group
systems now recognized by the International
Society of Blood Transfusion.
• The most important of these:
– ABO blood group system
– Rh blood group system
What do the Alleles Code For?
• The blood types differ due to the molecules that are
present on the outside of the red blood cells (antigens)
• Antigens act as recognition factors for our immune
system
What do the Alleles Code For? …cont’d
• When white blood cells do not recognize the
antigen, it considers the cell an invader and
produces antibodies to attack that cell
• This is why receiving an incorrect blood type
will cause agglutination (clumping).
Normal Blood
Agglutinated Blood
Summary Table
Homework
• Genetics Word Problems:
– Incomplete Dominance & Co-Dominance #1-3
– Multiple Alleles #1-7
• Read pg. 194-196