I-13
Chance in Genetic Variation
EXPECTATIONS VS. REALITIES Making predictions is a part of the scientific problem- solving
process. As an example, consider the toss of a coin. If a coin is tossed 10 times,
what head/tail count do you predict?
Heads: 5
Tails: 5
What is your reason for this prediction?
 Because there are one head side and one tails side, the probability of it being either or
would be 50 percent. So I just did a 50/50 probability, with each side possibly being
tossed.
Now toss a coin 10 times and record your results:
Heads: 6
Tails: 4
If the results of the trial do not match your prediction, what are possible reasons?
 The results of the trial did not match my prediction, but it was relatively close. Possible
reasons the results didn’t match was because of just pure luck. Because there’s a 50/50
chance, it could be any outcome.
When experimental results match one’s expectations perfectly, there may be cause for
rejoicing. Often there is not a perfect match, however, and doubts arise about the accuracy and
adequacy of the data. Would you demand a perfect result of 5 heads and
5 tails to consider the trial to be a fair one? Not necessarily.
If not, how far off would you allow
the result to be before you became suspicious of a trick? Probably, a 6/4 toss for either heads or
tails.
How did you decide that’s acceptable?
 I decided that’s acceptable based off the fact that it wasn’t a perfect outcome for each
heads or tails, as long as it strays away from a perfect outcome, I feel that it is acceptable.
VARIATION IN FAMILIES We are familiar with many examples of variation within a species and
even within a family. A strong element of chance enters with each reproductive event – a
chance that a certain ovum and a certain sperm, each having received a random mix of
chromosomes, will meet in fertilization. To simulate these chance events, we will use playing
cards.
Monohybrid Cross Let the two colors of cards represent genes. Shuffle the deck well, then split
it into two equal piles. One pile represents sperm, the other ova. Letting red cards represent
dominant genes (R) and black cards represent recessive genes (r), you have represented parents
containing Rr genes. What is your expectation or prediction of the variation possible in the
offspring?
 I feel as if there would be more (Rr) genotypes recorded, and possibly an equal amount
of (RR) dominant genes and (rr) recessive genes.
Draw one card from each pile, representing meiosis, and record the genotype resulting from
fertilization. Keep a tally (IIII II) of at least 20 fertilizations, and complete the data table to show
how your results compare to your expectations. Calculate the ratios as percents or decimals for
easy comparison with the expected, predicted ratio.
Genotype
Tally
RR
Rr
rr
1111111
6
11111111111 14
1111111
6
Total
Ratio
Expected
Phenotype
Ratio
Expected
6/26
14/26
6/26
6/26
14/26
6/26
Red
20/26
20/26
Black
6/26
6/26
Do your results match your expectations satisfactorily?
 Surprisingly enough, my results actually matched my expectations satisfactorily.
Dihybrid Cross
This time pile hearts and spades separately from diamonds and clubs, and
shuffle both piles. Let hearts = gene A, spades = a, diamonds = B, and clubs = b. Using parents
both AaBb and presuming complete dominance as suggested by letter size, predict the variation
within the offspring of this mating. What phenotypes will be produced, and how many of each?
 Because this takes into account a Dihybrid cross, it’s harder to predict what the
outcomes will be. I assume that AABB will be produced the least and AaBb to be
produced the most. I feel as if those that are recessive will be also produced the least as
well.
Draw one card from each pile to form a sperm, then draw again one from each pile to form an
ovum. Place these together representing a fertilization, and tally the genotype in the space
below. Continue for at least 32 fertilizations, re-shuffling as needed, and complete the table.
Calculate the ratios as percents or decimals for easy comparison.
Hand
HHDD
HSDD
HHDC
HSDC
HHCC
HSCC
SSDD
SSDC
SSCC
Genotype
AABB
AaBB
AABb
AaBb
AAbb
Aabb
aaBB
aaBb
aabb
Tally ////
1111
1111
11111
11
11111
111111
1111
1111
111
Phenotype
Total Ratio Expected Ratio
18/40 18/40
11/40 11/40
8/40
8/40
3/40
3/40
Do your results match your expectations satisfactorily?
If they do not, what
are possible reasons?
 The results did not match my expectations satisfactorily. I
chose that AaBb to be produced the most, an in fact it was
the least produced. AABB on the other hand was chosen to be
one of the most produced, but it was on average with the rest
of the genotypes. Because this was a dihybrid cross, there are
more chance of outcomes, making the probability much less
than a monohybrid cross.
Trihybrid Cross
Studying three gene pairs simultaneously is more work, but gives us a more realistic view of
variation. Given the genotype AaBbCc in a parent, determine how many different gene
combinations can exist in each sex cell. Remember that only one member of each gene pair goes
into sex cell formation, so each cell has a set of three different genes: A or a, B or b, C or c.
 AABBCC, AABBCc, AABbCC, AABbCc, AaBBCC, AaBBCc, AaBbCC, AaBbCc, AABBCc,
AABBcc, AABbCc, AABbcc, AaBBCc, AaBBcc, AaBbCc, AaBbcc, AABbCC, AABbCc, AAbbCC,
AAbbCc, AaBbCC, AaBbCc, AabbCC, AabbCc, AABbCc, AABbcc, AAbbCc, AAbbcc, AaBbCc,
AaBbcc, AabbCc, Aabbcc, AaBBCC, AaBBCc, AaBbCC, AaBbCc, aaBBCC, aaBBCc, aaBbCC,
aaBbCc, AaBBCc, AaBBcc, AaBbCc, AaBbcc, aaBBCc, aaBBcc, aaBbCc, aaBbcc, AaBbCC,
AaBbCc, AabbCC, AabbCc, aaBbCC, aaBbCc, aabbCC, aabbCc, AaBbCc, AaBbcc, AabbCc,
Aabbcc, aaBbCc, aaBbcc, aabbCc, aabbcc
Considering all genes
Assuming that you do not have an identical twin, do you suppose that your genes could
entirely match those of another person?
 Personally, I don’t believe that my specific genes could entirely match those of
another person. Because my genes are fairly specific, it would be almost
impossible for my genes to match someone’s of another person. This is evident
when the text includes that “…with the exception of identical twins, each
person’s DNA is unique and it is possible to detect differences between human
beings on the basis of their unique DNA sequence.” (199). Essentially our DNA is
what makes us unique, so the possibility that someone’s genes could entirely
match another’s without it being an identical twin is fairly complicated.