Human Genetics Lab Name: _________________________
Polygenic Inheritance and Pedigrees
Polygenic Inheritance and Pedigrees
PURPOSE
❑ To explore how traits that are determined by multiple genes are inherited
❑ To demonstrate how variation in quantitative traits arises through random chance
❑ To practice reading and writing pedigrees
PART I – HUMAN HEIGHT INTRODUCTION
Polygenic traits are traits that are controlled by more than one gene, i.e. height, weight, hair color, skin color (basically, anything that deals with size, shape and color). This allows for a wide range of physical traits. For example, if height were controlled by one gene A and if AA= 6 feet and Aa = 5 feet 7 inches and aa= 5 feet, then people would be one of three different heights. Since height is controlled by more than one gene, a wide range of heights is possible. This range is expanded even further because variations in nutrition also impact height.
Like most polygenic traits, it is unclear exactly how many genes determine height. However, several height-related genes have been identified. Among these are HMGA2, a gene that controls cell growth, GDF5, which is involved in bone growth, and two growth hormone genes (GH1 and GH2). For the first activity, we will simulate the distribution of heights in a population based on random selection of six alleles for three different height genes.
In this exercise, the coin toss represents each allele for a gene that determines height. Three genes are represented, and each gene has two alleles, so you need to flip the coin six times for each round. “Heads” represents the dominant (tall) allele for each gene and “Tails” represents the recessive (short) allele. The tallest individuals will be homozygous dominant for all three genes (6 Heads and 0 Tails). The shortest individuals will be homozygous recessive for all three genes (0 Heads and 6 Tails). The height chart below shows all the possible coin flips and the resulting heights.
HEIGHT CHART
Coin Flip
Height
6 Heads and 0 Tails
6 feet 1 inch
5 Heads and 1 Tail
5 feet 11 inches
4 Heads and 2 Tails
5 feet 9 inches
3 Heads and 3 Tails
5 feet 7 inches
2 Heads and 4 Tails
5 feet 5 inches
1 Head and 5 Tails
5 feet 3 inches
0 Heads and 6 Tails
5 feet 1 inches
PROCEDURE
1. Carefully flip the coin six times and record the number of heads and tails that result from the flip in Table 1 of your lab. Repeat for each of the 10 rounds. Determine the height of each round.
TABLE 1
Rounds
1
2
3
4
5
6
7
8
9
10
# of Heads
# of Tails
Height
PART I – Discussion Questions:
1. If a male is 5 feet 7 inches and a female is 5 feet 5 inches, is it possible for their child to be 5 feet 11 inches tall? Explain your answer.
2a. If 2 parents are 5 feet 7 inches, is it possible to have a child that is 6 feet tall? Explain how this is possible.
2b. If the male is 5 feet 5 inches tall and the female is 5 feet 3 inches tall, what is the tallest height that their child could attain? Explain.
3. If the male is 5 feet 7 inches tall and the mother is 5 feet 3 inches tall, what is the shortest height their child could attain? Explain.
4. Why do you think that some children are taller than their parents?
PART 2 – Interpreting a Human Pedigree
Use the pedigree below to answer 1-3. The trait shown is hemophilia, an X chromosome gene.
1a. In a pedigree, a square represents a male. If it is darkened he has hemophilia; if clear, he has normal blood clotting.
How many males are there? ________ How many males have hemophilia? ________
1b. A circle represents a female. If it is darkened, she has hemophilia; if open she is normal.
How many female are there? _______ How many females have hemophilia? _______
1c. A marriage is indicated by a horizontal line connecting a circle to a square.
How many marriages are there? _______________
2. A line perpendicular to a marriage line indicates the offspring. If the line ends with either a circle or a square, the couple had only one child. However, if the line is connected to another horizontal line, then several children were produced, each indicated by a short vertical line connected to the horizontal line. The first child born appears to the left and the last born to the right.
a. How many children did the first couple (couple in row I) have? ______________
b. How many children did the third couple (couple in row III) have? _____________
3. Level I represent the first generation, level II represents the second generation.
a. How many generations are there? _______________
b. How many members are there in the fourth generation? _____________
Use the pedigree below to answer 4-6.
Shaded individuals have Achondroplasia (Short limbed dwarfism)
I.
II.
III.
4a. Which members of the family above are afflicted with achondroplasia? ____________________________
4b. There are no carriers for achondroplasia–you either have it or you don’t.
With this in mind, is achondroplasia caused by a dominant or recessive trait? _______________
5a. How many children did individuals I-1 and I-2 have? ________________________________
5b. How many girls did II-1 and II-2 have? _______ How many have achondroplasia? ________
6a. How are individuals III-2 and II-4 related? _____________________
6b. How are individuals I-2 and III-5? ______________________________
Use the Pedigree below to answer 7-9.
I.
II.
III.
IV.
7. The pedigree above shows the inheritance of Hitchhiker’s Thumb in a family. Is this trait
dominant or recessive? ____________________ How do you know? ______________________
8a. How are individuals III-1 and III-2 related? ________________________
8b. Name 2 individuals that have hitchhiker’s thumb. __________________
9. Name 2 individuals that were carriers of hitchhiker’s thumb. ______________. Explain.
Determining Inheritance Patterns
10. When working through a pedigree, the first thing you need to do is figure out which characteristic is dominant – the shaded one or the un-shaded one. Then you need to choose a letter (let’s use A for albino) and begin assigning genotypes. Remember that recessive individuals are always homozygous, so assign their genotypes first. Then go back and look at all of the dominant individuals. For some, you will only be able to determine one allele of the genotype, so just write the one capital allele followed by a question mark (A?).
a. Which characteristic is dominant? ________________________
b. Which characteristic is recessive? ________________________
c. Determine the genotypes of all individuals. TIP: You will have the genotype “A?” three times. Write the Genotypes beneath each individual in the pedigree below.
11a. Is the trait in the pedigree below dominant or recessive? _____________________
Explain.
11b. Is the trait in the pedigree below dominant or recessive? _____________________
Explain.
Making Conclusions
12. Can autosomal recessive traits skip generations?
13. If a child has an autosomal dominant trait, what can you say about the parents?
14a. If two parents have an autosomal dominant trait, what can you say about their children?
14b. If two parents have an autosomal recessive trait, what can you say about their children?
15. If two parents do not have an autosomal recessive trait, what can you say about their children?
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Polygenic Inheritance and Pedigrees
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