1. CROSSES

2. Monohybrid Crosses one pair of traits are considered in a cross
use a special chart called a Punnett square to predict the genotype of the offspring
phenotype ratio - appearance
genotype ratio - genes in square

3. How to Work a Genetics Problem
Follow these steps!

4. Read the problem. What information is provided in the problem?
What is the trait?
Cross a pure white flowered pea plant with a pure purple flowered plant.
What is dominant?
What letter will we use to represent this trait?
Flower Color P
What is recessive?
Dominant – purple -
What letter will we use to represent this trait?
Recessive – white - p

5. Determine the genotypes of the parents.
Cross a pure white flowered pea plant with a pure purple flowered plant.
Pure purple
Pure white
P – purple
p - white X pp PP X
Pure = homozygous

6. 3. The Principle of Segregation
gene pairs separate into different gametes during meiosis
each gamete has only one gene from the pair
ex: Tt separates into T and t
T t
Meiosis T t
Gametes

7. Law of Segregation: Determine the gametes
Cross a pure white flowered pea plant with a pure purple flowered plant.
P – purple
p - white
Pure white X
Pure purple PP pp X
Meiosis
Meiosis P P p p
Gametes
Gametes

8. Punnett Square: place the gametes across the top and down one side
Cross a pure white flowered pea plant with a pure purple flowered plant. p p
P – purple
p - white
pp X PP P p p P P P

9. Punnett Square: join the gametes
Cross a pure white flowered pea plant with a pure purple flowered plant. p p P p P p
P – purple
p - white
pp X PP P p p P P P P p p P

10. Determine the genotype and phenotype of the offspring.
Cross a pure white flowered pea plant with a pure purple flowered plant. p p
P – purple
p - white
pp X PP P p P p P
Genotype - Pp
purple
purple
purple P P
Phenotype - p p P
purple
purple

11. What is the genotype ratio? The phenotype ratio?
Cross a pure white flowered pea plant with a pure purple flowered plant. p p
P – purple
p - white
pp X PP P p P p P
Genotype
Pp to PP or pp
4 : 0 P P
Phenotype p p P
Purple to White
4 : 0

12. What percentage of the offspring are homozygous? Heterozygous?
Cross a pure white flowered pea plant with a pure purple flowered plant. p p
P – purple
p - white
pp X PP P p P p P
Genotype
Homozygous - 0%
Heterozygous -
100% P P p p P

13. What percentage of the offspring are purple? White?
Cross a pure white flowered pea plant with a pure purple flowered plant. p p
P – purple
p - white
pp X PP P p P p P
Phenotype
Purple -
100%
White - 0% P P p p P

14. Cross 2 heterozygous purple flowered pea plants.

16. 4. The Principle of Independent Assortment
Parents Genotype
applies to gene pairs on different chromosomes
separation (segregation) of a gene pair is not affected by gene pairs on other chromosomes
separate independently
Yy Tt
Possible
Gametes yt YT yT Yt