Download In genetics, it is possible to calculate the results that should appear

Name____________________________
Finding the Phenotypes and Genotypes for Two Traits
Remember that in genetics, it is possible to calculate the results that should appear in
offspring if the genotypes of both parents are known. These are called the “Expected
Results” and can be calculated by mathematics (made easier by the use of Punnett squares).
Expected results are specific numbers and are not the result of random events, but show the
mathematical probability of an event happening. Doing the actual cross and counting
number of specific offspring finds “Observed Results.” Observed results are due to chance
combinations of certain alleles.
We will now do a cross with regard to two traits, using individuals that are “dihybrid.” In
other words, the individuals will be heterozygous for two traits. The ability to roll your
tongue is genetic. Rolling is dominant to not rolling. Having free earlobes is also a dominant
trait. Free is dominant to attached.
(A)
Expected
Genotype for
16 offspring
(B)
Expected
Genotype for 96
offspring
(Column A * 6)
(C)
Toss results
–Put tally marks here
(D)
Observed
genotypes of 96
offspring (total
from column C)
LLRR
LLRr
LLrr
LlRR
In this investigation, you will:
a) Substitute coins for gamete cells, using heads and tails to represent dominant and
recessive alleles, respectively. We will use two different coins for the two different
genes.
b) Toss the coins 96 times as a partner tosses coins 96 times. This will represent the
result of 96 matings to produce 96 offspring. (This number is chosen because it is
divisible by 16 and is close to 100, a large enough number to get reasonable
variation). Note: each person will toss two coins simultaneously, resulting in 4
coins being tossed per mating.
c) Determine the expected numbers of genotypes for 96 offspring and compare them
with the observed number obtained by the coin tosses.
d) Determine the number of expected phenotypes for a genetic cross and compare
them with the observed phenotypes obtained by coin tosses.
Materials: 1 penny & 1 nickel per person, writing implement, a person to mate with.
Procedure: Part A, Determining Numbers of Expected Genotype:
How many of each genotype
combination are expected in
the offspring of a cross
between parents that are
heterozygous
for
traits
represented
by
LlRr?
Fill out the Table 1: Each
Gamete has both an L or l,
and an R or r. So the possible gametes are LR, lR, rL, and rl:
How many alleles in each zygote? _____________________
Table 2: Expected and Observed Genotypes
Fill in column A and B using your results from Table 1, (the dihybrid punnett square)
LlRr
Llrr
llRR
llRr
llrr
Part B, Determining Numbers of Observed Genotypes.
1) Obtain a penny and a nickel. This will represent your possible gametes. Being
heterozygous, you have equal chances of giving your offspring a big L (free ear
lobes) or a small l (attached earlobes) allele. You will also have an equal chance
of giving your offspring a big R (ability to roll your tongue) or a little r (no
tongue rolling ability). We will assume that a coin toss will provide equal chances
of landing on heads or tails and therefore will be an excellent representation of
the possible gametic combination of offspring between you and your mate.
2) Simulate having sex (recombining genetic information) by flipping the coins.
If both you and your partner flip all heads, make a mark in the LLRR row of
column C of Table 2, if both of you flip all tails, make a mark in the row llrr row.
Treat the nickel as the L/l chance, and the penny as the R/r chance. E.g. 2 nickel
Heads and 2 penny tails = LLrr Do this 96 times.
3) Add up the tally marks and record the numbers in column D of Table 2.
Part C, Determining Numbers of Expected Phenotypes.
With two traits there is a greater number of possible variations. An individual can
show the dominant trait for both genes, or show dominant for one and recessive
Name____________________________
Finding the Phenotypes and Genotypes for Two Traits
for another, or show recessive for one and dominant for the other, or show the
recessive trait for both genes. Assuming the genes are found on different
chromosomes, these choices will follow the expected values rather closely. Add the
numbers together from the Table 2 rows that show dominant for both traits and
record that data below in Table 3. Do the same for the other dominant/recessive
combinations, and finally transfer the data from the double recessive row in Table 2
to the appropriate spot in Table 3 below.
Table 3:
Phenotypes
Possible
Possible
Genotypes
For Those
Phenotypes
Expected
Phenotypes
for 16
Offspring
Expected
Phenotypes
for 96
Offspring
___________________________
5.
Analysis
1.
Why is it rare to find an individual who shows both recessive traits?
2.
What are the chances of getting a two recessive allele when you make a gamete
cell from a hybrid germ cell? __________
What are the chances of getting one dominant allele for one gene and one
recessive allele for another gene when you make a gamete cell from a hybrid
germ cell? _________
Why is it hard to find two people that have all of the same phenotypes?
___________________________________________
Observed
Phenotypes
for 96
Offspring
(Coin tosses)
Free earlobes,
unable to roll
tongue
Attached
earlobes, rolling
tongue
Attached
earlobes, unable
to roll tongue
If you looked at only one trait on each chromosome of a typical person, how
many coins would you need to flip simultaneously to represent fertilization?
6.
Free earlobes,
rolling tongue
3.
4.
Compare the expected and observed phenotypes for 96 offspring.
a.
Were the results exactly the same? ___________________
b.
If the results were not exact, is it wrong? _________________
c.
Explain: __________________________________________________
________________________________________
7.
Collect all the class data and put it in the table below:
Class total of offspring ___________ Number of students_________
Enter Class Totals Below:
LLRR:
LLRr
LLrr
LlRR
Expected
Phenotypes for
96 offspring
Free earlobes, rolling
tongue
Free earlobes, unable
to roll tongue
Attached earlobes,
rolling tongue
Attached earlobes,
unable to roll tongue
LlRr
Llrr
Expected
Phenotypes for
class total
llRR
llRr
llrr
Observed
Phenotypes for class
total