Punnett Squares

Phenotype and Genotype Practice

1. Describe the genotypes given. The first one is already done.Example: DD homozygous, dominant A. ss ______________________B. Yy ______________________C. dd __________________ D. Ww ____________________

2. In humans, brown eye color (B), is dominant over blue eye color (b). What are the phenotypes of the following genotypes? In other words, what color eyes will they have?A. BB ________________________B. bb ________________________C. Bb ________________________

Punnett Squares

• Used to organize all the possible combinations of offspring from particular parents

Making a Punnett Square

1. Draw a square; divide it into four sections.

Making a Punnett Square

For this example, we will show the cross between two plants that produce round seeds.

R=round seedsr=oval seeds

Making a Punnett Square2. Write the letters that represent alleles

(genes) from one parent along the top of the box.

R r

Making a Punnett Square3. Write the letters that represent alleles

(genes) from the other parent along the side of the box.

R r

R

r

R

r

R r

RR Rr

Rr rr

R

r

R r

Example #1

G G

G

g

Example #2

A A

a

a

Probability

• The mathematical chance that something will happen.–Example: If you toss a coin, the probability of tossing heads is ½, or 50%. You will get heads half the time.

What is the probability of having a baby with blue eyes?

BB Bb

BB Bb

B b

B

B

Example #1:

• The allele for a cleft chin, C, is dominant among humans. Using a Punnett square, what are the results of a cross between parents with genotypes Cc and cc?–What is the probability that the

offspring will have a cleft chin?

Example #2:• This Punnett square

shows the alleles for fur color in rabbits. Black fur, B, is dominant over white fur, b.

– Given the combinations shown, what are the genotypes of the parents?

Bb Bb

Bb Bb

Problem #1:

TT TT

Tt Tt

1. What is the unknown genotype?

2. If T represents the allele for tall pea plants and t represents the allele for short pea plants, what is the phenotype of each parent and offspring?

T

t

? ?

Problem #2

• The allele for freckles, F , is dominant among humans. Using a Punnett square, what are the results of a cross between parents with genotypes FF and ff?

–What is the probability that the offspring will have freckles?

Whiteboard Practice Problems

Greetings Scientists!!

• In rabbits, black fur is dominant to white fur. If you cross a BB male with a Bb female, what are the possible genotypes and phenotypes of the offspring? What is the percent chance for each type?

• In cabbage butterflies, white wings are dominant to yellow wings. If a Ww butterfly is crossed with a ww butterfly, what are the possible genotypes and phenotypes of the offspring and the percent chance for each?

• In dogs, there is a hereditary type of deafness caused by a recessive gene. Two dogs who carry the gene for deafness but have normal hearing are mated. What are the possible genotypes and phenotypes of their offspring and the percent chance for each?

• In guinea pigs, short hair is dominant over long hair. If a short haired SS guinea pig is crossed with a long haired ss guinea pig, what are the possible genotypes and phenotypes of their offspring and the percent chance of each?

• Can you curl your tongue up on the sides? Tongue-curling in humans is a dominant genetic trait. Suppose a man who is Tt for tongue-curling marries a woman who is also Tt for this trait. What are the possible genotypes and phenotypes of their children and the percent chance for each?