Predictions in Genetics

Punnett Squares

Power-up:1. If a woman’s sex cells are a mixture of Bs and bs?

What was her original genotype for eye color? What was her phenotype?

2. If a man’s sex cells carry only bs? What was his original genotype? What was his phenotype?

3. A woman’s genotype is Ww for widow’s peak. What alleles will her eggs carry?

4. Dogs have 78 chromosomes in a body or somatic cell. How many chromosomes would a dog’s sex cells have? How many chromosome pairs are in a dog’s body cells?

5. For which genes below is the organism heterozygous? For which genes below is the organism homozygous

I Can…

I can use a punnett square to predict the outcome of genetic crosses

Probability

Probability is a number that describes how likely it is that a certain event will appear.

For example, the probability of a coin landings heads up is 1 in 2.

Mendel and Probability

After repeating his experiments many times, and always finding similar ratios of dominant to recessive traits in the F2

generation, Mendel concluded that the principles of probability applied to genetics.

Mendel and Probability

Mendel didn’t always find that 3 out his 4 F2 plants were tall--but he found that this was generally the case.

That’s why he used the word “probability” not words such as “for sure”, “always”, or “certain” to describe the ratios he obtained.

Punnett Squares

A punnett square is a chart that shows all the possible combinations of alleles that can result from a genetic cross.

Step 1

Draw a box and divide it into 4 squares.

Step 2

Write the male parent’s alleles along the TOP of the square and the female parents alleles on the LEFT side.B b

B

b

Step 3

Copy the female parent’s alleles into the boxes to the right. B b

B

b

B B

b b

Step 4

Copy the male parent’s alleles into the boxes beneath them.

BB Bb

bB bb

B b

B

b

Step 5:

Figure out what the phenotypes will be depending on the allele combination you see in each little box.

BBBrown eyes

BbBrown eyes

bBBrown eyes

bbBlue eyes

B b

B

b

Step 6 Calculate the probability for each allele combination or phenotype:– If the combination only shows up 1 time, the probability is 1/4 or 25%.

– If the combination shows up 2 times, then the probability is 2/4 or 50%.

– If the combination shows up 3 times, then the probability is 3/4 or 75%.

– If the combination shows up 4 times, then the probability is 4/4 or 100%.

BBBrown eyes

BbBrown eyes

bBBrown eyes

bbBlue eyes

Meiosis and Punnett Squares A punnett square is actually a way to show the events that occur at fertilization.

The punnett square shows the way alleles are seperated when sex cells are formed.

Practice Complete the worksheet with your lab group using the models provided.

Do NOT write on the actual worksheet. Each group member should have their own piece of notebook paper with– 4 punnett squares (completed)– Responses to Questions A and B

Practice 1

Build a punnett square for the offspring of a heterozygous Brown-eyed Dad and a homozygous brown eyed Mom. BB

Brown eyes

BbBrown eyes

BBBrown eyes

BbBrown eyes

B b

B

B

Practice 2

Build a punnett square for the offspring of a blue-eyed Dad and a heterozygous brown eyed Mom.

BbBrown eyes

BbBrown eyes

bbBlue eyes

bbBlue eyes

b b

B

b

Gummy Bear Practice: Part 1

Build Punnett Squares for each of the following sets of Gummy Bear parents. Use the table above to determine the phenotype of each offspring.

1. Dad (RR) x Mom (Rr)2. Dad (RR) x Mom (rr)3. Dad (heterozygous red) x Mom (orange)4. Dad (YY) x Mom (Yy)5. Dad (homozygous yellow) x Mom (clear)6. Dad (heterozygous yellow) x Mom (yy)

For each set of offspring calculate the probability of having a red, orange, yellow, or clear offspring (Hint: Use your punnett squares!!)

Phenotype Genotype Red RR or Rr

Orange rr Yellow YY or Yy Clear yy

Gummy Bear Practice: Part 21. Open your bag. Build a table to

record the number of each color gummy bear you have.

2. What percent of each color do you have? (Should show percent for red, orange, yellow, and clear)

3. What were the genotypes of the parents? (Hint: Look at the punnett squares you already completed in Part 1)