Name________________________________

Penny Lab

Predicting Genes Of Offspring

Observed Results vs. Expected Results

Punnett squares are used to predict expected results from a genetic cross.

B b

B

b

The expected results:

BB=_________

Bb=_________

bb=_________

In Mice, black coat color is dominant over white coat color. Remember dominant traits are represented by capitalized letters and recessive traits are represented by lower case letters. The Punnet square on the left shows the cross of two mice that are heterozygous (Bb) for coat color. Fill out the Punnet square to see the expected results of the Bb X Bb cross.

Observed results do not always agree with the expected results. It could be possible that the four offspring of the Bb X Bb cross to all be white. In this case the observed results (that which actually happened) are not what we expected.

Part 1: Procedure:

∑ Drop 2 coins on the desktop and make a (/) mark in the data table under the correct combination of genes (These are the observed results). Heads=B Tails=b. Repeat this 40 times. These coins represent the combination of genes you might have observed in the offspring of several litters.

Data Table

Coin Combinations	Head-Head	Head Tail	Tail-Tail
Gene combinations	BB	Bb	bb
Total of expected results in 40 tosses
Tally of observed results in 40 tosses
Total of observed results
Coat color of mice

Part 2:

B b

b

Suppose you mate a female mouse that is heterozygous (Bb) with a male that is pure recessive (bb). Predict what kind of offspring she will have by completing the Punnett square.

b

∑ Place tape on both sides of the two coins and mark both sides of one coin with a b. Mark one side of the other coin with a b and the other side with a B. These coins now represent the mice in the punnett square above.

∑ Again drop the coins on the desktop and make a (/) mark in the data table below under the correct gene combinations.

Coin combinations	B-b	b-b
Gene combinations	Bb	bb
Tally of observed results in 40 tosses
Total observed results in 40 tosses
Total of expected results in 40 tosses
Coat color of mice

1. Compare your observed results with your expected results: How close were they?

2. What is the expected result of a cross that is pure dominant and pure recessive. Use the punnett square.

BB= %

Bb= %

bb= %

3. A scientist made a cross between two black mice. The cross was repeated between the same two mice several times. The data chart showed the color of all 42 offspring to be black. Use the punnett squares below to show what you think the gene combinations were of both parents.

B ____ B ___

B B

__ __

What are the two possible gene combinations of the parents?