1. GENETIC NOTES
Mrs. Callan
2017

2. GENOTYPE Genotype – an organism’s genetic makeup.
The letters used in a Punnett Square
Three types:
1. Homozygous Dominant – two dominant alleles.
Example: BB
2. Homozygous Recessive – two recessive alleles.
Example: bb
3. Heterozygous – One dominant and one recessive allele.
Example: Bb
*Always write the capital (dominant) trait first!

3. PHENOTYPE Phenotype – an organism’s physical appearance.
The traits you CAN SEE.
Example: Flower color, plant height, eye color

4. Punnett Square
A way to find the expected proportion of possible genotypes/phenotypes in the offspring of a cross.
Monohybrid Cross – a cross in which only one characteristic is tracked.

5. Example 1: Homozygous x Homozygous
Cross a homozygous dominant flower with a homozygous recessive flower. P P
P = Purple
(Dominant
Trait)
p = White
(Recessive Trait) Pp Pp p Pp Pp p

6. Example 1: Homozygous x Homozygous
Cross a homozygous dominant flower with a homozygous recessive flower. P P
Genotype:
4/4 or 100% Pp
Phenotype:
4/4 or 100% Purple Pp Pp p Pp Pp p
RATIO: 1:0 for both Genotype and Phenotype

7. Example 2: Homozygous Dominant x Heterozygous
Cross a person homozygous dominant for brown eyes with a person heterozygous for brown eyes. B B
B = Brown (Dominant Trait)
b = Blue (Recessive Trait) BB BB B Bb Bb b

8. Example 2: Homozygous Dominant x Heterozygous
Cross a person homozygous dominant for brown eyes with a person heterozygous for brown eyes. B B
Genotype:
2/4 or 50% BB
2/4 or 50% Bb
Phenotype:
4/4 or 100% Brown BB BB B Bb Bb b
Genotype Ratio: 2:2 Phenotype Ratio: 1:0

9. Example 3: Homozygous Recessive x Heterozygous
Cross a person homozygous recessive for blue eyes with a heterozygous for brown eyes. b b
Genotype:
2/4 or 50% Bb
2/4 or 50% bb
Phenotype:
2/4 or 50% Brown
2/4 or 50% Blue Bb Bb B bb bb b
Genotype Ratio: 2:2 Phenotype Ratio: 2:2

10. Example 4: Heterozygous x Heterozygous
Cross a heterozygous purple flower with another heterozygous purple flower. P p
Genotype:
¼ or 25% PP
2/4 or 50% Pp
¼ or 25% pp
Phenotype:
¾ or 75% Purple
¼ or 25% White PP Pp P Pp pp p
Genotype Ratio: 1:2:1 Phenotype Ratio: 3:1