1. Important to know before getting started:
Female
Male

2. Punnett’s Square
Dad’s alleles
Mom’s alleles
Scientists use a Punnett’s square to predict the probability of traits in offspring.
Each box represents 25%

3. Punnett Squares:
Each box represents the probability that alleles from each parent could combine in the offspring (babies).

4. Probability The likelihood of a particular event occurring. Chance
Can be expressed as a fraction or a percent.
Example: coin flip.

5. genotypes
Homozygous = when an organism possesses two identical alleles. ex.
YY or yy
Heterozygous = when an organism possesses different alleles. ex. Yy

6. Genotype
The specific combination of alleles (from mom and dad) are called the genotype.
Genotypes can be homozygous (aa or AA) or heterozygous (Aa)
They can also be dominant (capital letter) or recessive (lower case letter)
Example: BB Bb bb

7. Phenotype:
The phenotype is the physical appearance/expression of the alleles.
Example:
BB=brown hair
Bb=dirty blonde hair
bb= blonde hair

8. Monohybrid crosses show the crossing of one set of alleles from each parent. So we use a Punnett square with four squares/quadrants.

9. Black colored fur is the dominant trait.
We will use the Punnett’s Square to determine the offspring of guinea pigs. The offspring will either be black or white.
Black colored fur is the dominant trait.

10. Generation 1 B B b b
In this case we have a dad with black fur and a mother with white fur. Because black is the dominant gene, we write it with a capital ‘B’.

11. Generation 1 B B b b
White fur is a recessive trait. It is written with a lowercase ‘b’. It does not matter what letter we choose to represent a gene, but capital letter is always dominant and lowercase is always recessive.

12. Generation 1 B B Bb b b
To complete the Punnett’s square we combine the gene from mom with the gene from dad. We always write the dominant gene first.

13. Generation 1 B B Bb b b
We write the dominant gene first because it “masks” the recessive gene. Therefore, the color of the guinea pig with the genes Bb would be black.

14. Generation 1 B B Bb b b
Copy this Punnett’s square into your notebook. Try and fill out the remaining offspring on your own. When you are done, go to the next slide.

15. Generation 1 B B Bb Bb b Bb Bb b
We say an individual is heterozygous when it has two different genes.
What percentage of these offspring are heterozygous?

16. 100% are heterozygous black.
Generation 1 B B Bb Bb b Bb Bb b
100% are heterozygous black.
We will now take one female and one male from this generation to cross for our second generation.

17. What will the gene combinations be for these offspring?
Generation 2 B b B b
What will the gene combinations be for these offspring?
Copy this into your notebook and try to fill out the Punnett’s square. Continue when you are done.

18. Can you figure out what color these offspring will be?
Generation 2 B b BB Bb B bb Bb b
What you have completed is the genotype for all the offspring. Genotype refers to the gene combination that an individual has.
Can you figure out what color these offspring will be?

19. Generation 2 B b BB Bb B bb Bb b black black black white
What you have just determined is called the phenotype. The phenotype is what we see as a result of an individual's genes.

20. Generation 2 B b BB Bb B bb Bb b
We say an individual is homozygous when it has two of the same genes.
Can you find a homozygous black guinea pig?

21. Generation 2 B b BB Bb B bb Bb b This individual is homozygous black.
The entire Punnett’s square represents all possible outcomes. That means each small box represents 25% of the offspring.
What percentage of the offspring are homozygous black?

22. Generation 2 B b BB Bb B bb Bb b
25% of the offspring are homozygous black.
Try the next two on your own:
____% are homozygous white
____% are heterozygous black

23. Generation 2 B b BB Bb B bb Bb b
Notice: This will always add up to 100%.
25% are homozygous black.
25% are homozygous white
50% are heterozygous black

24. Now we will examine some human traits.
Earlobes can be either unattached (A) or attached (B)
Unattached
Attached
The unattached earlobe is the dominant trait.
The attached earlobe is the recessive trait.

25. What percent of offspring will have attached earlobes?
Copy this Punnett’s square into your notebook. Determine both the genotype and phenotype.
What percent of offspring will have attached earlobes?

26. 50% percent of offspring will have attached earlobes.Ee Ee ee ee e
50% percent of offspring will have attached earlobes.

27. The ability to curl your tongue is also genetic.
Tongue curling is the dominant trait.
Non-curling is the recessive trait.

28. Tongue Curling T t T T
Copy into your notebook and complete the Punnett’s Square.
What percentage of offspring will be able to curl their tongue?

29. 100% percent of offspring will be able to curl their tongues.
Tongue Curling T t T TT Tt TT Tt T
100% percent of offspring will be able to curl their tongues.

30. Incomplete dominance A situation in which neither allele is dominant.
When both alleles are present a “new” phenotype appears that is a blend of each allele.
Alleles will be represented by capital letters only.

31. W= white R = red WR = pink Incomplete Dominance
The petal color of pea flowers is an example of incomplete dominance. Both genes for white and red flowers are equally dominant, which results in a new phenotype.
W= white
R = red
WR = pink

32. Pea Flower Petals R R W R
Copy and complete this Punnett’s square in your notebook.
What percentage of offspring will have white flowers?

33. Pea Flower Petals R R WR WR W RR R RR white
0% of the offspring will have flowers.
50% will have red flowers.
50% will have pink flowers.
white