Punnett Squares

Why do we all look just a little like our moms and dads?

What – How – Why?

• What: Utilize a Punnett square to predict the patterns of a cross. (2.7d) RWC -3.2f/g, 4d

• HOW: Given a word problem, complete and analyze a Punnett square to predict and justify the resulting patterns of a cross.

• Why: What is the chance that you may have had different color eyes?

What you have to do at the end of today!

• One cat carries heterozygous, long-haired traits, and its mate carries homozygous short-haired traits. Long hair is DOMINANT.

• Use a punnett square to analyze and fill in the chart. Phenotype Genotype % of

offspring

What do we need to know in order to answer this question?

• One cat carries heterozygous, long-haired traits, and its mate carries homozygous short-haired traits.

• Use a Punnett square to determine the genotype and phenotype probability of their offspring.

Steps to get there

1. Pick the following statements out of the question:

“One cat carries heterozygous, long-haired traits”“its mate carries homozygous short-haired traits”

• Allele – different forms of genes (ONE Letter)– Examples: Dominant and recessive

Heterozygous: Two DIFFERENT alleles.Homozygous: Two of the SAME alleles.

Practice

• Write down the key statements in the following genetics problem:– Suppose a man who is heterozygous for tongue-

curling marries a woman who is also heterozygous for this trait. What are the possible genotypes and phenotypes of their children?

Practice

• Pick out the key statements in the following statement:– Suppose a man who is heterozygous for tongue-

curling marries a woman who is also heterozygous for this trait. What are the possible genotypes and phenotypes of their children?

2. Using the information copy and fill in the following chart:

Phenotype Allele Genotype

Red R RR or Rr

White r rr

Looking at the chart, what do you think phenotype means?

Vocab!!

• Phenotype – Physical Characteristic/Trait (Color, tongue curling, etc..)

• Genotype – Genetic make up (TT, Tt, tt)• Homozygous – TT, tt

» Dominant – TT» Recessive - tt

• Heterozygous - Tt

Practice - TogetherLet’s put the two steps together

• 1st – pick out the key statements– If one parent is homozygous dominant for free

earlobes, while the other is heterozygous for free earlobes. Use a Punnett square to show the genotype and phenotype for their offspring.

• Homozygous dominant for free earlobes• Heterozygous for free ear lobes

• 2nd – Fill out the chart

Phenotype Allele Genotype

Free ear lobes F FF or Ff

Attached ear lobes f ff

Complete BOTH steps in your notebooks!!

• A man that is heterozygous for a widow’s peak marries a woman is homozygous recessive for no widow’s peak.

• 1. Write down KEY INFORMATION.• 2. Fill in the chart.

Phenotype Allele Genotype

Steps to get there

3. Show the genotypes of the parentsRR x rr

4. Place on a punnett square

R R

R

Rr

r

rr

Or

KFed is heterozygous for brown hair, so….

His genotype is

Bb

B

b

And Britney is homozygous for blonde hair, so….

Her genotype is

bb

b b

Bb Bb

bb bb

Steps to get there• Predict what the top row of letters (other

parent’s genotype) should be and write the rule for crossing.

B

b

BB Bb

Bb bb

Practice

• In humans, free earlobes are dominant over attached ear lobes. If one parent is homozygous dominant for free ear lobes, while the other has attached earlobes, can they produce any children with attached earlobes? Justify your answer.

• 1. YES 2. NO

Dominance (based on PHENOTYPE!!!)

• Simple = Rr Big letter always dominant– Black hair vs. Blonde

• Incomplete = RW two dominant so there is blending– End up with a third phenotype

• Red + White Pink

• Co-Dominance = BW– Black cow + white cow Spotted cow

Which is which?

Independent Practice

• Fill in and analyze these punnett squares!!

What – How – Why?

• What: Utilize a Punnett square to predict the patterns of a cross. (2.7d) RWC -3.2f/g, 4d

• HOW: Given a word problem, complete and analyze a Punnett square to predict and justify the resulting patterns of a cross.

• Why: What is the chance that you may have had different color eyes?

DOL: 6pts!!

• One cat carries heterozygous, long-haired traits, and its mate carries homozygous short-haired traits. Long hair is DOMINANT.

• Use a punnett square to analyze and fill in the chart. Phenotype Genotype % of

offspring