1. Understanding Genetic Heredity & Probability Through Punnett Squares

2. Question- Is it possible for the two Highland Cattle on the left produce either of the calves on the right?

3. Understanding Punnett Squares
What is a Punnett Square?
A chart that allows us to determine the different genotypes that offspring can inherit from their parents
The size of the square depends on the number of genes you’re looking at

4. Let’s Try It! First we must get the genotype of each parent Example:
B: allele for black hair
b: allele for red hair
Tt – Bull’s genotype
Tt- Cow’s genotype

5. Set Up the Punnett SquareB b B B B B b b b b B b

6. Now Let’s Talk About Probability
What is probability?
The chance that an event will occur
How do you find the probability of an event?
You must know:
the total number of possible outcomes
the number of each individual outcome

7. BB Bb Genotypic Ratio B b B b
The offspring can inherit three different genotypes:
BB, Bb, & bb B b BB Bb bb B
The Genotypic Ratio would be:
¼ BB: ½ Bb: ¼ bb OR
25% BB: 50% Bb; 25% bb b

8. BB Bb Phenotypic Ratio B b B b
The offspring can inherit three different genotypes, but those produce only two phenotypes. B b BB Bb bb B
The Phenotypic Ratio would be:
¾ Black:¼ Red OR
75% Black: 25% Red b

9. Is it possible for two black cattle to produce a red calf?
YES! If both parents are Bb and can pass on the recessive genes, there is a 25% chance of having a red calf!
If they had 8 calves, you should expect (hope) that 2 of those are red!

10. Practice
Wrinkled seeds are recessive to smooth seeds. Show a plant that always produces wrinkled seeds crossed with a heterozygous smooth seeds producing plant. Use S for allele
Yellow seeds are dominant to green seeds in peas. Show a cross between a homozygous yellow seed with a green seed. Identify the genotypic and phenotypic ratios. Use Y for allele

11. What If We’re Looking at More Than One Trait?
We use what’s called a dihybrid cross!

12. Dihybrid Example
A male guinea pig that is heterozygous for both fur color and coat texture is crossed with a female that has light fur color and is heterozygous for coat texture.
What possible offspring can they produce?

13. Start With Writing A Key
Dark fur color (D) is dominant
Light fur (d) is recessive.
Rough coat texture (R) is dominant
Smooth coat (r) is recessive

14. Figure Out the Genetics of Each
The male guinea pig is heterozygous for both color and texture.
Male = “DdRr”.
The female guinea pig has light fur.
Since that is a recessive trait the genotype = “dd”.
It is also heterozygous for fur texture, which means a genotype of “Rr”.
Female= “ddRr”.

15. DdRr X ddRr

16. Determine Gene Combinations DdRr X ddRr
During the formation of sperm a “D” could go with a “R” producing a sperm “DR”, or a “D” could go with a “r” forming a sperm with “Dr”.
DdRr
Possible Combinations: DR, Dr, dR, or dr
ddRr
Possible Combinations: dR, dr, dR, or dr

17. Set Up Your Punnett Square
DdRr
DR Dr dR dr dR dr
ddRr

18. DdRr ddRr Combine the Genetics DR Dr dR dr dR dr DdRR DdRr ddRR ddRr

19. Calculate Genotypic Ratio
2 DdRR : 4 DdRr : 2 ddRR : 4 ddRr : 2 Ddrr : 2 ddrr

20. Calculate Phenotypic Ratio
2 Dark/Rough : 4 Dark/Rough : 2 Light/Rough : 4 Light/Rough : 2 Dark/Smooth : 2 Light/Smooth OR
6 Dark/Rough, 6 Light/Rough, 2 Dark/Smooth, 2 Light Smooth
2 DdRR : 4 DdRr : 2 ddRR : 4 ddRr : 2 Ddrr : 2 ddrr

21. Try One On Your Own
KEY:
G= Black Hair, g= White Hair
B= Black eyes, b= Red eyes
A male rabbit with the genotype GgBb is crossed with a female rabbit with the genotype ggbb.
Fill it out and determine the genotypic ratio and the phenotypic ratio of the offspring.