Genetics
Objectives:
By the end of this exercise you should be able to:
1. Demonstrate the principles of heredity and to understand the statistics involoved when traits
are passed from parents to offspring;
2. Differentiate between dominant and recessive characteristics and to understand the difference
between a genotype and phenotype; and
3. Increase you genetic vocabulary.
Introduction
In today’s lab, we will investigate the principles of heredity. We will be looking at some phenotypes of
many people and from that we will attempt to determine the genotype for each of the traits we examine.
Many traits are controlled by a complex relationship of genes so to keep things simple we will examine
traits which seem to be inherited in a relatively straightforward manner.
Sex-linked traits: The gene for a sex-linked trait can be located either on the X or Y chromosome and
contains many more genes; most sex-linked traits are on the X chromosome. Females carry two X
chromosomes while males carry the X and Y chromosomes. The Y chromosome carries a gene that shifts
fetal development into a male.
Red-green colorblindness is inherited as a sex-linked recessive trait. We will do another
experiment to demonstrate the effects of sex linked traits. Once again we will use beans. This time, red
beans represent the dominant ability to see red-green color (B), white beans represent the recessive gene
for colorblindness (b), and black beans represent the Y chromosome that does not carry the gene.
Materials:
5 cups
red beans, white beans and black beans
Maternal
A. Mix 20 red beans and 20 white beans for maternal genotype. XBXb
B. Have a container with 10 red beans for maternal genotype XBXB
C. Have a container with 10 white beans for maternal genotype XbXb
Paternal
D. Mix 20 red beans and 20 black beans for paternal genotype XBY
E. Mix 20 white beans and 20 black beans for paternal genotype XbY
Now we will mix the gene pool and tally the results.
1. Without looking, pull one bean from A and one bean from D. Write down the result (XBXB,
XBXb, XBY, XbY) AND record the sex of the offspring and the phenotype in the table below.
Place the beans back into the bag from which you selected.
2. Repeat the above step until you have made 40 trials. Record your results and get percentage of
each genotype occurrence as before.
3. Do the same thing for each possible mix. Ultimately you will mix all of the maternal with the
paternal gene pool. These are the gene pool combination you should have: AD, AE, BD, BE, CD,
and CE.
Questions:
1. What is the possibility of a female child inheriting colorblind genes from parents A and D?
2. What is the possibility of a male child inheriting colorblind genes from parents C and D?
3. What is the possibility of a female child inheriting colorblind genes from parents A and E?
4. What is the possibility of a male child inheriting colorblind genes from parents B and E?
5. Could a female child inherit colorblindness from parents who were no colorblind? Could a male
child?
Gene
Pheno.
Sex
AD #s
AD %
AE #s
AE %
BD #s
BD %
BE #s
BE %
CD #s
CD %
CE #s
CE %
XBXB
XBXb
XbXb
XBY
XbY