Incomplete dominance - Condition in which neither of allele is dominant over the other in heterozygous . Carl correns did an experiment with four o'clock plant which explains incomplete dominance by showing blended phenotype pink heterozygous flowers . Tay - Sachs Disease - Autosomal recessive genetic disorder . Chromosome 15 carry homozygous recessive rr alleles for disease . Andalusian fowls is an example of incomplete dominance , in F1 generation we get all blue Andalusian fowls and F2 generation shows same phenotype and genotype ratio i.e 1: 2: 1

1. Biology – Let’s Simplify

2. Incomplete Dominance
Today’s Topic :

3. Incomplete Dominance
Incomplete Dominance : The condition in which neither
of allele is dominant over the other in heterozygous
organisms.
Also called Mosaic or Partial Dominance.
Heterozygous express entirely new Phenotypes that are not
like the parent organisms.

4. Carl Correns Experiment
Incomplete Dominance : Four o’clock Plant
RR
Red
rr
White
R R r rGametes
P
F1
r r
R
R
Rr Rr
Rr Rr
F1 – all pink ( Rr)

5. Explanation
color is due to mixing of and colors in F1.
The two alleles both produce proteins, but one is non-functional. This
results in heterozygotes producing only half the amount of protein
produced by a homozygous dominant individual.
The wild type R allele codes for an enzyme vital for the conversion of
the precursor in the flower into the xanthophyll pigment.
The mutant r allele produces an enzyme incapable of catalyzing the
reaction.

6. F1 self cross – Rr Rr
Gametes
F2
R r
R
r
RR Rr
Rr rr
F2 (genotype) –
1 RR : 2 Rr : 1 rr
F2 (phenotype)-
1 Red
2 Pink
1 White

7. It is due to expression of a single gene for pigmented flower
which produces only pink color.
(e.g. RR ) are required for the expression of complete
dominant trait.
Phenotypic and Genotypic ratios are the same in
Conclusion : Four o’clock Flowers

8. Andalusian fowls : Incomplete Dominance

9. Incomplete dominance: Tay Sachs disease
(humans)
Autosomal Recessive Genetic Disorder.
Chromosome 15 carrys homozygous recessive (rr) Alleles.
Due to deficiency of Beta- Hexosaminidase- A enzyme .
Healthy individual – both A & B form
Affected individual – only A form

10. Beta- Hexosaminidase – A helps breaking down lipids.
In its absence , Sphingolipids & GM2 Ganglioside
accumulate
Developing Brain & Peripheral Nervous System
Result : Brain damage , Retardation ,Death by age five.

11. Aa Aa
A a
A
a
AA Aa
Aa aa
AA = 25% healthy
Aa = 50% carriers ,
(half amount of enzyme
produced but sufficient for
normal growth)
aa = 25% recessive affected,
(No functional enzyme)
Cross between Carrier Parents : Tay - Sachs Disease
Same Genotype & Phenotype : 1 : 2 : 1