How common are certain inherited traits?
Introduction
     Can you roll your tongue? Do your Fingers have mid-digital hair? Do you have a widow's
peak? These 3 traits are known to be inherited in humans. Every human being has 46
chromosomes. Each chromosome contains thousands of genes. Although we do not know
the function of most of these genes, there are a few genes that produce easily identifiable
traits. During this lab you will study the 3 inherited traits mentioned above. You will find your
own phenotype for each of these traits. Then you will pool your data with other students' data.
For one of these traits, you will compare your data with national figures for that trait.
In this lab you will:
      1. determine how many of your classmates display certain inherited traits,
      2. discover whether a dominant trait is always found in a majority of a population,
      3. analyze your data by comparing them with data for larger populations.

Prelab Preparation Before doing this lab, review your notes on Mendel's inheritance principles, in particular how dominant and recessive genes are inherited and expressed. Make sure you know how to describe the phenotype of a person whose genotype is given to you.You will be studying 3 pairs of human traits in this lab. The ability to roll your tongue is dominant to the inability to roll your tongue. If you cannot roll your tongue into the shape shown, you are a nonroller.

The presence of mid-digital hair is dominant to the absence of mid-digital hair. Look at your fingers. Notice that each finger has 3 segments. All humans have hair on the backs of their fingers on the segments nearest to the palm. However, some people also have hair on the middle segment. You should check each finger and, if you have hair on the middle segment ofyour fingers, consider yourself as having mid-digital hair regardless of how many mid-digital hairs you have. (Some people have a lot; others have only a few.) Look at the illustration to to the right determine whether you have mid-digital hair.

The presence of a widow's peak is dominant to the absence of a widow's peak. If your hair comes to a point in the middle of your forehead, you have a widow's peak. Use a mirror to compare your forehead with the illustration to the right.

Procedure
Part I
A. Try to roll your tongue.
      1. Are you a roller or a nonroller?
      2. Record your phenotype for tongue-rolling in the data table.

B. Now you will gather data for your entire class. Then you will calculate the percentage of
      students in your class who exhibit each trait.
      3. Count how many people are able to roll their tongues and how many people are
          not. Enter these numbers in the appropriate columns on the chart for individual
          and class data.
      To calculate the percentage of the class who are rollers, first find the total number of
      students in the class. Then divide the number of rollers by the total number of students.
      The answer will be a decimal fraction between 0 and 1. Multiply this answer by 100 to
      get the percentage. For example, if there are 18 students in the class and 5 are rollers,
      the percentage of rollers is:
                      5   x  100  =  0.28  x  100  =  28%
                     18
      4. Perform this calculation with the tongue-rolling data you collected. Record the
          percentage of students who are tongue-rollers and the percentage who are not.
      5. Now collect class data for mid-digital hair. Follow the same procedure for
          calculating the class percentages and entering data on the data table as you did
          for tongue-rolling.
      6. In the same manner, gather class data for widow's peak. Follow the same
          procedures you used for recording the preceding traits.

C. Review the data you collected for these 3 traits.
      7. Does the same percentage of students always display the dominant trait?
          Explain your answer.
      8. For which trait(s) do the majority of students display the dominant phenotype?
      9. For which trait(s) do the majority of students display the recessive phenotype?
      10. Why is it possible that a majority of people may display a recessive phenotype?

Part II
D. In Part I you examined yourself and members of your class for 3 inherited traits.
      Now you will concentrate on one of these traits, tongue-rolling. You will
      obtain more data and do some calculations that will enable you to measure how
      many genes in a sample population are R (rolling) and how many genes are r
      (nonrolling).

As a class, obtain the tongue-rolling       phenotypes of 100 people. Begin your       data collection with the members of your class.       Then each class member should test enough       additional people (friends and relatives) so that       you have a total of 100 different people.       Construct a table like the one shown to the right      that includes the name and phenotype of      everyone in your sample. (Remember to include      your class data on the table.)      11. Why do you (and the class as a whole) need            to keep track of which people are asked?      12. Enter your compiled data in the data            chart for 100 subjects. Nationally, about            70% of the population are rollers, and            approximately 30% are nonrollers.

     13. How does your data compare with these figures? Discuss possible reasons for
           any differences between your data and the national data.

E. Now you can calculate the frequency of R and r genes in your sample by using an algebraic      equation, the Hardy-Weinberg Principle. According to this principle, the distribution of      dominant and recessive alleles in a population remains constant from one generation to
     the next as long as no external factors such as mutation and migration arise. Further,
     mating must be random, and the population must be large for the formula to work. There
     must be no natural selection. Specifically, the equation for a gene with two alleles is:
                           P2 + 2pq + q2 = 1

                   where p = frequency of R (dominant gene), expressed as a decimal fraction
                   q = frequency of r (recessive gene), expressed as a decimal fraction
                   P2 = number of RR individuals
                   2pq = number of Rr individuals
                   q2 = number of rr individuals
                   and p + q = 1

The Punnett square to the right       illustrates the formula in a more familiar way.       Together the 4 squares represent the total       population under study. In other words,       if you were to assign values to each of the       genes in the squares, these values would       add up to one, or 100% of the population.            Now return to the algebraic formula.       Knowing the percentages of individuals       possessing each trait, rolling and nonrolling,       does not in itself tell us the frequency of R       and r genes in the population. To determine       these frequencies, do the following:

      14. Express the percentage of nonrollers as a decimal fraction (e.g., 36% = 0.36).
      We know from Hardy-Weinberg that q2 = rr. The square root of q2 will give us the
      frequency of  r. Using our example, the square root of 0.36 equals 0.6.
      15. Calculate q for your sample.
      Now, since Hardy-Weinberg states that p+q = 1, then p = 1- q. Therefore, using
      our example, p = 1- 0.6 = 0.4.
      16. Calculate p for your sample.
      We can check these calculations by plugging the example values into the
      Hardy-Weinberg equation.
                P2  +  2pq  +  q2  =  1
                (0.4)2  + 2 (0.4 X 0.6)  +  (0.6)2  =  1
      17. Check your values by plugging them into the Hardy-Weinberg equation.
      18. Considering your findings in Questions 15 and 16, what is the frequency
           of the gene for tongue rolling, R? What is the gene frequency for
           nonrollers r?

Postlab Analysis

19. What percentage of people did you find in your sample of 100 who could roll their tongues?
20. How does this figure compare with the national average of 70%?
21. How might your frequency values change if you increased your sample from 100 to 200
      people?
22. Do you think there was any bias in your sample?       Explain.