NAME___________________________________SECTION___________
6
Heredity
OBJECTIVES
1. Recognize dominant and recessive alleles and distinguish genotypes from phenotypes
2. Describe the phenotypic ratios among offspring in monohybrid crosses involving complete
dominance
3. Construct a Punnett square to predict phenotypic and genotypic ratios of a monohybrid cross
4. Construct a Punnett square to predict phenotypic and genotypic ratios of a dihybrid cross
5. Interpret pedigrees and determine specific genotypes through pedigree analysis
MATERIALS NEEDED
Corn from monohybrid cross
PREPARATION
Read Biology a Guide to the Natural World Chapter 9, 10, 11
INTRODUCTION
When organisms reproduce, the offspring often resemble their parents. Since two different individuals are
involved in sexual reproduction, it is natural to wonder about the contributions of each parent to the
offspring. Do these contributions somehow "blend" with one another to produce the characteristics we
observe in the offspring, or is some other process involved? Through a series of very simple crosses, an
amateur biologist named Mendel discovered the principles that govern the inheritance of simple physical
traits. In this unit, you will study the patterns and basic processes of heredity (genetics).
In this lab the Mendelian "laws" of inheritance will be illustrated using the results of genetic "crosses" in
corn. The use of the Punnett Square and the Chi-Square test will introduce techniques of statistical
analysis.
Analysis of Data: The Chi-Square Test for Goodness of Fit
Statistical tests are used to objectively determine whether or not a deviation from the expected result is
likely to be due to chance alone. When making genetic crosses, the ratio of offspring phenotypes is
usually predicted prior to the experiment. Rarely is the observed ratio identical to the expected one. In
fact, the observed (experimental) ratio may not be close; i.e., there is a certain amount of deviation from
the expected. This deviation may be due to chance, the hypothesis may be wrong, or some other factors
may alter the observed ratios beyond what would be expected from chance alone. Use of a Chi-Square
test allows us to assess how well the observed results fit the proposed hypothesis.
To acquaint you with this test, let's try it out on a simple problem, coin flipping. We know that a coin
has two sides, heads and tails, and logic tells us that if we flip the coin, one or the other side is going to
be up, i.e., either heads or tails. Also, if we flip a coin 100 times, we would expect 50 heads and 50 tails.
Yet when we do this it is rare to get exactly a 50:50 ratio. So if we get 45 heads and 55 tails, is this close
enough to support our hypothesis that the expected result is 50:50? Or is something wrong with the coin
because it falls with one particular side down more than with the other, i.e., a weighted coin? We have
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two hypotheses: 1) the 45:55 ratio is no different than the 50:50 ratio or 2) it is not due to chance
but some other factor or factors which may affect the ratio. To decide between these two
hypotheses, we use the Chi-Square (2) test as follows:
Deviation
(D = O -E)
Observed (0)
Expected (E)
N umber of heads
45
50
-5
Number of Tails
55
50
5
Now use the following formula to calculate the 2 :
Totals
 D2 = 25 + 25 = 0.5 + 0.5
E 50 50
2 = 1.0
Using the Chi-Square table (page 42) interpret the 2 value above. The 2 value is using a concept called
degrees of freedom. The number of degrees of freedom (DF) is defined as the number of categories
minus one (DF = n-1).
I. Punnett Square
The various alleles (i.e., alternate forms of a gene) which segregate during meiosis will appear in gametes
that eventually fuse to form a zygote. This makes it possible to predict the occurrence of specific -as of
alleles in a zygote. An individual that is homozygous for a trait (e.g., "PP" or "pp") can only produce
gametes with the allele in question (assuming no mutations occur). If the individual is heterozygous i.e.,
two different alleles, or "Pp") and a large number of gametes are produced, then you would expect equal
numbers of each allele. If you know the parental genotypes, you can predict all of the possible
combinations of gametes in the offspring through a graphic device known as the Punnett Square.
Since each kernel is the product of sexual reproduction, we know that each individual received genetic
information through the gametes from each parent. In other words, individuals have two bits of genetic
information for every trait. (That is a reflection of the diploid condition.) In this particular case, the
original parents (the P generation) were true-breeding for their characteristics (i.e., the two bits of
genetic information in a parent were identical). Designate these bits of information with symbols: use "P"
for purple color and "p" for the yellow condition. Since each individual has two bits of information and
since the parents were true-breeding, we know the genetic content of the original parents: The purple
kernel is PP and the yellow kernel is pp.
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GENERATION
DESCRIPTION
P
Original Parental Types: Yellow kernel plant x Purple kernal plant
F1
Individual that produced the cobs seen here
F2
Kernels that you are examining today
The Punnett square predicts all of the offspring that are possible from any particular set of parents. As an
example, consider the monohybrid cross you observed in Exercise 1. Since the parental generation was
drawn from two true-breeding populations, the genotype of each parent is known with certainty.
Likewise, the genotype of all individuals in the first generation (F1) is known to be "Pp". In order to
predict the results of a cross of the Fl individuals, the possible gametes that can be formed by individuals
are entered along the axes of the Punnett square. Then, the contents of the square are filled in, as indicated
in Fig. 1.
P
p
P
p
Figure 1. Punnett square for a monohybrid cross
It doesn't matter which parent's gametes go along either axis; the point is simply to consider all possible
combinations. Mating is simulated by entering the appropriate combinations of gametes from parents. In
this example, there are four boxes in the Punnett square to represent all the possible combinations of
gametes. The symbols in these boxes represent possible genotypes in offspring and phenotypes can be
predicted on the basis of dominance relationships among the alleles.
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1. What phenotypes are possible among the offspring in Figure 1? ________________________
______________________________________________________________________________
2. What is the ratio of phenotypes among these offspring? ______________________________
Based on this Punnett square, what is the likelihood (probability) of obtaining an offspring with:
3. genotype PP? _______________________________________________________________
4. genotype pp? ________________________________________________________________
5. genotype Pp? _______________________________________________________________
II. Monohybrid cross with corn (adapted from BMZ 115 laboratory manual)
Using the corn provided count the kernels on the cob. The original P generations had a genotype of PP =
dark color and pp = yellow color. The genotypes of the F1 generation are Pp.
6. From the Punnett Square, if the parents produced 1000 offspring, how many offspring would you
expect to have the dominant phenotype? _________________________
Carefully count the number of purple and yellow kernels and fill in the chart below
Observed (0)
Expected (E) Deviation (D = O-E)
D^2
D^2/E
Number of purple
Number of yellow
7. 2 = ______________________________________
8. How many degrees of freedom?_________________________________________________
9. Is the calculated 2 value greater or less than the value in the table at p = 0.05? ____________
10. Was there a significant deviation from the predicted ratio? ____________________________
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III. Punnett square for a dihybrid cross
The Punnett square can also be used to predict the outcomes of genetic crosses involving two traits
(dihybrid crosses). The only difference in this case is an increase in number of different gametes that can
be produced by either parent. You can examine this with the genetic corn used in Exercise 1. Imagine a
parental generation that is known to be true-breeding for two different traits. If you cross parents of
different phenotypes for each trait, the F1 genotype must be heterozygous for both characters.
Use "P" for purple kernels and "p" for yellow kernals; use "S" for smooth kernels and "s" for wrinkled
kernals.
11. List all of the possible gamete types that either parent can produce. (Remember: Both parents are
heterozygous for both traits.)
______________________________________________________________________________
Fill in the dihybrid punnet square below
PS
Ps
pS
ps
PS
Ps
pS
ps
12. What phenotypes are possible among F2 individuals in this dihybrid cross?
______________________________________________________________________________
______________________________________________________________________________
13. What is the ratio of phenotypes among these offspring? ______________________________
IV. Pedigree analysis
One application of Mendel's Laws can be seen in the study of pedigrees. A pedigree is a graphic method
of presenting the inheritance of particular traits in a group of related individuals. Males are represented by
square symbols and females are represented by circles. In addition, the trait of interest is indicated by a
shaded symbol.
Consider the pedigree in Fig. 2, showing the inheritance of freckles. How is this pedigree interpreted?
First, look at Row 1. Neither individual has freckles (both symbols are unshaded). Their son (Row 2) also
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lacks freckles, but he married a woman who has freckles (note the shaded symbol). The son and his wife
have three children and two of them (a boy and a girl) have freckles.
Figure 2 Pedigree showing the inheritance of freckles
How can we tell whether freckles are dominant or recessive? Look at the second family (starting on the
right half of Row 2). Both of these individuals have freckles, but some of their children do not have
freckles. This tells us that freckles must be dominant. If freckles were recessive, then the two freckled
parents in Row 2 could not produce any children without freckles. On the other hand, if freckles were
dominant, then the two freckled parents in row 2 could produce children without freckles (How? If the
freckled parents were heterozygous, then they would have freckles, but they could also contribute the
recessive allele to an offspring. If both were heterozygotes, a homozygous recessive child is possible.)
Now that we know that freckles are dominant, it is possible to examine genotypes for individuals in Row
3.
14. What genotypes would result in the freckle phenotype? ______________________________
15. Can the parents in Row 2 produce either of these genotypes? __________________________
16. Can you state with certainty the genotypes of the two freckled individuals who mate in row 3?
____________________________________________________________________________
17. What are the possible genotypes of the individuals in row 4? Can you state their genotypes with
certainty? __________________________________________________________________
V. Inheritance of human traits
A number of human traits are known to be governed by a single gene. These include:
Widow's peak. A pointed hairline is dominant (W) to a straight hairline (w).
Tongue rolling. The ability to roll your tongue (T) is dominant to the inability to roll the tongue (t).
Attached ear lobes. The allele for free ear lobes is dominant (A) to the recessive allele for attached ear
lobes (a).
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Mid-digital hair. Each of your fingers has 3 joints. If you have hair on the middle joint of any finger you
have the dominant allele (M) for mid-digital hair versus if the hair is absent (m).
Examine yourself for each of these traits. List your phenotype in the following table and, if possible, list
your genotype. If you have the dominant trait, but are uncertain of the genotype, use an asterisk (*) for the
second allele. For example, if you are not certain whether you are homozygous dominant or heterozygous
for Widow's peak, write "W*".
Characteristic
Phenotype
Genotype
Widow’s peak
________
________
Tongue rolling
________
________
Attached ear lobes
________
________
Mid-digit hair
________
________
18. If you have the dominant trait for any of these characteristics and one of your parents shows the
recessive trait, do you know your genotype? _______________________________________
19. What is it, and how can you be so certain of this? ___________________________________
______________________________________________________________________________
20. What do you know about your parent’s genotypes if one of your brothers or sisters shows the
recessive trait, but neither of your parents does? _______________________________________
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GLOSSARY OF TERMS USED IN UNIT 4
Allele - alternate forms of a gene
Dihybrid cross – a genetics cross in which the inheritance of two traits is studies
Dominant – the allele that is expressed in the heterozygote
F1 – the offspring of the parental generation
F2 – the offspring of a cross of two F1 individuals
Genetics – the study of inheritance patterns and processes
Genotype – the alleles that an individual has for a particular gene
Heterozygous – a genotype consisting of two different alleles; not true-breeding
A genotype consisting of two identical alleles; true-breeding
Monohybrid cross – a genetics cross in which the inheritance of one trait is studies
P – Parental or original generation in a series of genetic crosses
Pedigree – a family tree in which the phenotypes of family members are presented by standard symbols
Phenotype – the physical appearance or expression of a genotype
Punnett square – a graphic device in which all possible outcomes of a cross between two individuals are
depicted
Recessive – an allele that is masked in the heterozygote
Test cross – a cross between and individual of unknown genotype and an individual with the
homozygous recessive genotype
True breeding - homozygous
Wild type – the most common phenotype in natural populations; generally taken as the standard or the
“normal” phenotype
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CRITICAL V ALVES OF X1.
Probabilities
..... NOT SIGNIFICANT ..... ......... SIGNIFICANT ........
DF
0.5
0.1
1
2
3
4
5
6
7
8
9
10
0.46
1.39
2.37
3.36
4.35
5.35
6.35
7.34
8.34
9.34
2.71
4.61
6.25
7.78
9.24
10.61
12.02
13.36
14.68
15.99
......HIGHLY SIGNIFICANT ...
0.05
0.025
0.01
0.005
0.001
3.84
5.99
7.82
9.49
11.07
12.59
14.07
15.51
16.92
18.31
5.02
7.38
9.35
11.14
12.83
14.'45
16.01
17.54
19.02
20.48
6.64
9.21
11.35
13.28
15.09
16.81
18.48
20.09
21.67
23.21
7.88
10.60
12.84
14.86
16.75
18.55
20.28
21.96
23.59
25.19
10.83
13.82
16.27
18.47
20.52
22.46
24.32
26.12
27.88
29.59
Find the value on the appropriate line \specified by the degrees of freedom) in the table that
most closely approximates the calculated value for X. and read the matching probability. If the
probability is equal to or less than 0.05, the difference between the observed and the expected
values is considered statistically significant and is probably NOT due to chance, so the null
hypothesis that the observed and expected values are "essentially" identical must be rejected. If
the probability is greater than 0.05, then the difference in the. observed and expected values is
more likely due to chance and the null hypothesis cannot be rejected. The investigator
arbitrarily chooses the level of significance, usually 0.05 (or 0.01). Keep in mind that, at these
probabilities, the hypothesis will be incorrectly rejected 5% (or 1 %) of the time.
Another way to use the table is to compare the calculated value with the table value found on
the appropriate DF line under one of the probabilities (e.g., 0.05). If the calculated value is less
than the table value, then the probability is less than 5% that the observed and expected values
are significantly different.
If the probability is equal to or less than 0.05 then the difference is considered significant. If
the probability is equal to or less than 0.01 then the difference is considered highly significant.
[This table is abridged from the Chi-Square Distribution, p. 34 of Geigy Scientific Tables,
volume 2, C. Lentner (ed.), Ciba-Geigy Limited, Basle, Switzerland.]
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ADDITIONAL PEDIGREE PROBLEMS:
1. In the pedigree chart below, determine whether the characteristic is dominant or recessive.
2. Consider the following pedigree showing the ability to change skin color in the tree frog,
Hyla versicolor.
Is this trait due to recessive allele or a dominant allele?
If the F1 generation, what is the genotype of the male indicated with an arrow?
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HEREDITY WORKSHEET
Answers are on BLACKBOARD under “Course Documents”
The following questions (1-4) concern the nucleus drawing labeled A.
1. The diploid number of this cell? _____________
2. The haploid number of this cell? _____________
3. Draw a picture of the resulting cell if A. underwent MITOSIS. Then draw a picture of the
resulting cell if cell A. underwent MEIOSIS (make sure you label your cells).
4. Match the HOMOLOGOUS chromosomes by drawing lines connecting them.
5. In seals, the gene for the length of the whiskers has two alleles. The dominant allele (W)
codes long whiskers & the recessive allele (w) codes for short whiskers.
a) What percentage of offspring would be expected to have short whiskers from the cross of two
long-whiskered seals, one that is homozygous dominant and one that is heterozygous?
b) If one parent seal is true breeding long-whiskered and the other is short-whiskered, what
percent of offspring would have short whiskers?
6. Mendel found that crossing wrinkle-seeded plants with pure round-seeded plants produced
only round-seeded plants. What genotypic & phenotypic ratios can be expected from a cross of a
wrinkle-seeded plant & a plant heterozygous for this trait (seed appearance)?
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