Download Penny Genetics Lab

Names_____________________________________________________Due__________________
Penny Genetics (100pts)
How Well Does a Punnett Square Predict the Actual Ratios?
You’ve practiced Punnett Squares but why? What benefit does this give you in studying genetics? This
experiment is designed to test the usefulness of Punnett Squares and how it can help predict the
accurate genotype and phenotype of offspring for ONE specific trait. In this lab, you will make
predictions using Punnett Squares. Then you will then use coins to simulate the crosses as they could
occur randomly in nature. Finally, you will compare the Actual Ratios with the Predicted Ratios.
The Mendelian trait you are looking at is the gene that codes for a toe size in humans. T represents
the dominant allele (short big toe), t is the recessive allele (long big toe). The following genotypes are
possible for two heterozygotes. Fill in the phenotypes for them.
PART I: Comparing the Punnett square prediction to reality with two heterozygotes.
Genotype
Phenotype (description)
Allele
Side of the Coin
TT
T
Heads
Tt
t
Tails
tt
(_______5 pts)
Hypothesis: State what you will expect to find when you compare your predicted ratios (from the
Punnett Squares) with your actual ratios (from your coin flips).
(_______10 pts)
______________________________________________________
______________________________________________________
______________________________________________________
Chart A Punnet Square Prediction
(________10pts)
PREDICTED RATIO
Use a Punnett Square to predict the ratio of offspring in a cross where the parents are both Tt
(The Square is set up for you below)
This Punnet square is predicting the
Based upon the Punnett square, what will the predicted genotypic &
probabilities for 1 fertilization.
phenotypic ratios be for 100 offspring? Multiply the occurrence of
each box by 25 to represent 100 fertilizations. (25 x 4 boxes = 100
matings).
Predicted Genotypic Ratio for 100 offspring
T
t
___TTx25=____TT : ___Ttx25=____Tt : ___ttx25=____tt
T
Predicted Phenotypic Ratio for 100 offspring
t
STx25_____ Short toe : LTx25______ Long Toe
ACTUAL RATIO
Now you will determine the reality of probabilities by using penny flips to represent random crosses. You have
two pennies. The head side of the penny is the letter T, the tails side is the letter t. This penny represents a
parent’s genotype, which is Tt. A second penny represents the other parent’s genotype, which is the same.
One partner is going to play the role of the female & the other will play the role of the male. When the coin is
flipped, you are simulating what allele is in the sperm or egg being donated by each parent. Recall that during
meiosis, the 2 alleles for a gene have a 50/50 chance of ending up in the gamete. When you put the two flipped
coins together you are simulating random fertilization of the sperm and egg combining to create new life.
Practice flips. Flip the two pennies. The results show you what your offspring will be.
Did you flip “T-T”, “T-t” or “t-t” __________
What is the Phenotype of this practice flip (long or short)? ________________
(_______5pts)
Chart B
Coin Flip Actual ratios
(_______10pts)
Actual Ratio: To determine the actual ratio, you will flip your coins 100 times, recording in the table
below how often each combination came up. (Use tally marks to record your data then summarize
as a number in the total column.
Gene combination Tally marks
Total
TT
Tt
tt
Phenotypes
Short Toes (Add TT & Tt)
Long Toe (tt)
Total
These two charts
show your ACTUAL
Ratios
Comparing Actual to Predicted Ratios
Predicted Ratios
(from Chart A)
(________15pts)
Actual Ratios
(from Chart B)
TT
Tt
tt
Short Toe
Long Toe
Would you consider the actual and predicted ratios (circle the best choice)…
a. THE SAME?
b. CLOSE TO THE SAME?
c. NOT CLOSE AT ALL?
Part II: What if the Parents are Tt x tt? Using Percentages.
(________15pts)
1. First make your predictions by setting up a Punnet square for the parents.
(This one is not set up for you) T t x t t
What percentage are predicted to be:
Short Toe ______ Long Toe ______?
2. Keep one coin on tails and don’t flip (it should always be “t”, right?). Perform 100 flips with the other
coin & combine with the “t” penny to simulate your new set of parents. Record your data below.
Tally
Total
Tt
tt
Compare the Predicted Percentages of the cross to the Actual
Percentages.
Predicted (from square)
Based upon your flips, what
percentage should have:
_____short toes?_____ long toes?
Actual (from flips)
Short toe
Long Toe
Would you consider the actual and predicted percentages (circle the best choice)…
a. THE SAME?
b. CLOSE TO THE SAME?
c. NOT CLOSE AT ALL?
Part III. Analysis Questions (5pts Each)
(_______30pts)
1. Use a Punnet Square to predict the phenotypic ratios in this cross: T T x T t
Short toe _____
Long toe _____
2. If I told you to flip a coin 4 times, would you expect the coin toss method to give a similar ratio as the
Punnett square above? Think about the conclusions drawn from Part I & Part II.
3. What do the pennies or chips represent in the simulation?
4. When you toss a coin to see which side lands up, you are actually simulating what part of the
process of sexual reproduction?
5. When you put the two coins that are flipped together, you are simulating what part of the process of
sexual reproduction?
6. Overall, do you think Punnet squares are good predictors (models) of the way random allele
combinations occur during meiosis & fertilization based upon real-life probabilities over time? What
part of this simulation provides evidence for your claim? Explain in great detail in a few sentences.
Part IV. Dyhibrid Crosses
What you’ve tested is the validity of monohybrid crosses using Punnet Squares as predictors for the
likeliness of an offspring inheriting one trait. The law of independent assortment states that two
different genes have no influence on each other in heredity, as long as they are not linked. You will
briefly conduct a test on the probability of inheriting any one combination of 2 traits. This is called a
DYHIBRID CROSS.
The example to the right is what happens when two separate
genes are examined when two flies are crossed. The female
fly has a genotype of LlGg and the male has a genotype of
LLGg. Notice the egg and sperm show every possible
combination for all alleles both parents have. This accounts
for the law of independent assortment.
To construct the grid, you must know the genotypes of the
parents. Then, figure out every possible combination of their
alleles. Put mom’s possible allele combinations on top and
dad’s possible allele combinations on the side. Distribute the
alleles down and across, just like before but, this time, be
careful to account for all alleles. When finished, you will have
4 alleles in every box. Then simply count all the like boxes to
discover the ratios or percentages.
Sample problem. 20 extra credit points.
GG = gray hair
Gg = gray hair
gg = white hair
In rabbits, grey hair is dominant to white hair.
Also in rabbits, black eyes are dominant to red eyes.
BB = black eyes
Bb = black eyes
bb = red eyes
1. What are the phenotypes (trait descriptions) of rabbits that have
the following genotypes:
Ggbb ______________________
ggbb ______________________
ggBB _________________________
GgBb _________________________
2. A male rabbit with the genotype GGbb is crossed with a female rabbit with the genotype ggBb The square is set up
below. Fill it out and determine the phenotypes and proportions in the offspring.
How many out of 16 have grey fur and black eyes? ______
How many out of 16 have grey fur and red eyes? ________
How many out of 16 have white fur and black eyes? ______
How many out of 16 have white fur and red eyes? _______
Devise a strategy that could test if this cross matches what coin tosses would produce randomly.