Download EXERCISE 4: Principles of Heredity: Human Genetics Learning

EXERCISE 4:
Principles of Heredity:
Human Genetics
Learning Objectives

Learn the meaning of the Hardy-Weinberg equations.

Apply these equations to predict allele and genotype frequencies in a population.

Learn how to read a simple pedigree chart.

Determine possible genotypes based on pedigree chart information.

Recognize common dominant and recessive human traits.

Demonstrate how human traits are inherited.
Materials
• Calculators and scrap paper for calculations
Pre-Lab Questions
1. What is the significance of the Hardy-Weinberg principle?
2. What does the term allele mean?
3. What is the difference between genotype and phenotype?
4. Knowing that an individual is of dominant phenotype, is it possible to determine its
genotype?
Introduction
Within a DNA molecule lie distinct regions called genes, most of which code for a protein.
Genes that are located on the same chromosome are usually inherited together and are
referred to as being linked. In contrast, those that reside on different chromosomes are
inherited separately, and thus follow Mendel’s Law of Independent Assortment. Every
individual produced by sexual reproduction will contain two genes for every trait
obtained from the male and female gametes. Early in the twentieth century, two scientists,
Godfrey H. Hardy and Wilhelm Weinberg, independently devised several mathematical
equations that could be used in two different ways of interest to biologists. These
equations are used to predict the allele and genotype percentages in a population. Then
these equations can be used to show changes in gene percentages in a population over
time, which is called microevolution. The Hardy-Weinberg principle states that in the
absence of evolutionary forces the percentage of alleles should remain stable, or in
equilibrium, from one generation to the next.
This exercise will investigate the first use of these equations. You will predict the
percentage of dominant and recessive alleles for certain traits in your lab class
population. You will further investigate the topic of human genetics by constructing
pedigree charts.
The Hardy-Weinberg equations
The first Hardy-Weinberg (H-W) equation is p + q = 1.00, where p stands for dominant
alleles and q for recessive alleles. Stated differently, this equation means that if you add
together all the dominant and recessive alleles in a population you will account for all
(100%) of the alleles in that population. The second equation is p2 + 2pq + q2 = 1.00. In
this equation, p2 is the percentage of homozygous dominant individuals, 2pq is the
percentage of heterozygous individuals, and q2 is the percentage of homozygous
recessive individuals. This equation says roughly the same thing as the first – if you add
together all of the individuals with the homozygous dominant, heterozygous, and
homozygous recessive genotypes, you will account for all the genotypes in the population.
In both equations 1.00 is another way of saying 100%.
Learning how to use these equations only takes a little practice. So for starters we will use
these equations to calculate the percentage of dominant and recessive alleles in a
hypothetical population of 50 people, 42 of whom have brown eyes and 8 of whom have
blue eyes. The brown eye allele (B) is dominant over the blue eye allele (b).
Therefore:
The potential genotypes for brown-eyed people are:
The potential genotypes for blue-eyed people are:
PROCEDURE
1. First, determine the number of eye-color alleles in the entire gene pool.
There are 50 people, each with two eye-color alleles. Therefore, there are 100 eyecolor alleles in the whole population.
2. Since you can’t tell the difference between homozygous dominant and
heterozygous people just by looking at their eyes, you have to begin solving this
puzzle using homozygous recessive people.
Eight of the people have blue eyes. This means their genotype is bb. Therefore
there are 16 “b” alleles in these 8 people. This number is equal to q2 from the H-W
equation p2 + 2pq + q2 = 1.
3. Next, turn q2 into a decimal fraction.
16 ÷ 100 = 0.16 (which is the same as saying 16%)
4. If you know q2, you can determine q by taking the square root of q2.
The square root of 0.16 = 0.4
q = 0.4 (or 40%)
5. Now that you know q (or the percentage of recessive alleles in this population), you
can use the other H-W equation (p + q = 1) to calculate p (or the percentage of
alleles that are dominant).
p+q=1
p + 0.4 = 1
p = 1 – 0.4
p = 0.6 (or 60%)
6. Now that you know both p and q you can go back and calculate the percentage of
brown-eyed people who are homozygous dominant and the percentage who are
heterozygous.
homozygous dominant = p2
(0.6)2 = 0.36 (or 36%)
heterozygous = 2pq
2 (0.6) (0.4) = 0.48 (or 48%)
7. To summarize what you have done:
a) Prediction of dominant vs. recessive alleles in this population of 50 people:

60% of the alleles are for brown eyes (dominant)

40% of the alleles are for blue eyes (recessive)
b) Prediction of homozygous dominant vs. heterozygous vs. homozygous recessive
genotypes in this population of 50 people:

36% (or 18 people) are homozygous dominant

48% (or 24 people) are heterozygous

16% (or 8 people) are homozygous recessive
Applying the Hardy-Weinberg equations
Now you are ready to apply these equations to the population that is your lab class. Table
1 lists several easily recognized human traits that are basically controlled by a single
gene. Using the same procedure outlined above, you will be able to predict how many
dominant and recessive alleles there are for each trait and how many of each genotype.
PROCEDURE
1. Observe the five traits described in Figure 1. Look at yourself and your lab partner
to determine which phenotype you have.
2. Complete a class count, recording how many students display the dominant
phenotype and the number who display the recessive phenotype for each trait.
Record this data in Table 1.
3. Determine the size of the gene pool. Refer to #1 in the previous exercise for a
reminder of how this is determined.
Gene pool = ____________________
4. When all the data is collected, calculate p, q, p2, 2pq, and q2 for your class
population. Record the data in Tables 1 and 2.
Figure 1: Common human traits.
QUESTIONS
1. In your classroom population for most traits, were there more students with the dominant
phenotype or the recessive phenotype?
2. Was there an instance where there were more students with the dominant phenotype BUT
at the same time there were more recessive alleles in the population (q was larger than
p)?
3. Explain how this is possible.