Download Making and Interpreting Pedigree Studies

Making & Interpreting Pedigrees
Name:________________________________________________Period:_____Date:__________
Background
A pedigree is a diagram showing the phenotypes of a trait for a group of related organisms. A pedigree
may help you determine the genotypes of an organism. Geneticists can use pedigrees to determine how a
trait is carried on through a family and how to predict the genotypes of future generations. An example of
a pedigree is shown in Diagram A. On this pedigree, squares are used to represent males, circles are used
to represent females, and shaded areas show an individual that exhibits the trait being studied.
Diagram A
I.
II.
2
1
3
4
5
A line is used to connect the parents. Children appear below the parents as a family line. The numerals I
and II represent the generations and 1 through 5 represent individuals. This example shows only two
generations of a family while most pedigree studies will show at least three generations. The accuracy of
a pedigree depends on the number of individuals who provide information about their phenotypes. The
more individuals that are included in the study the more accurate the pedigree will be.
Many traits are controlled by several genes, but in this lab you will examine traits that are controlled by
two genes. Observing these traits will help you determine the genotype for the trait.
Objectives



Determine whether a genotype is dominant or recessive
Determine the phenotype and genotype, when possible, for a group of traits
Show how genotypes can be determined using pedigrees
Materials
Strips of PTC paper
Procedure A
Determining a genotype
1. Obtain a piece of PTC paper from your teacher. The ability to taste the chemical
phenylthiocarbamide (PTC) is dominant over the inability to taste it. Chew on the paper and then
discard it. If you are a PTC taster you should be able to taste the bitterness right away.
Remember, if the paper tastes bitter that means you have the dominant trait but you may be
homozygous (TT) or heterozygous (Tt). This could be determined by testing whether your parents
possess the trait. If both of your parents could taste the paper your genotype would probably be
TT but if only one of your parents could taste the paper your genotype would be “Tt.” For now,
simply record T? on line, if you could taste the paper. But if you cannot taste the paper, you have
the recessive trait, so record “tt.”
Genotype: ______________
1
2. Record your phenotype and genotype for each of the traits used in the table below. If you have a
DOMINANT phenotype, then you should record your genotype as R?.
Traits
DOMINANT
RECESSIVE
Curly Hair
Straight Hair
Free Ear Lobes
(as in A)
Attached Ear Lobes
(as in A)
Dark Hair
Light Hair
Long Eyelashes
Short Eyelashes
Non-Red Hair
Red Hair
Brown Eyes
Blue or Grey Eyes
Hazel or Green Eyes
Blue or Grey Eyes
Tongue-Roller
(as in B)
Non-Roller
Widow's Peak
(as in C)
Straight Hair Line
Straight Thumb
Hitchhiker's Thumb
(as in D)
PTC Taster
Non-Taster
Straight First Joint
of
Little Finger
Bent First Joint of
Little Finger
A) Free
Ear lobes
Attached
Ear lobes
Your
Phenotype
B) Tongue Rolling
Your
Genotype
C) Widow’s Peak
D) Hitchhiker’s Thumb
2
Procedure B : Determining Genotypes From a Pedigree
1. Nearsightedness – or myopia – is a recessive trait. Use the symbols N and n to label the genotype for each of
the numbered individuals. The shaded regions show individuals who are homozygous recessive for myopia.
They exhibit the trait being studied.
Diagram F: Myopia (Nearsightedness)
I.
II.
2
1
3
4
10
9
8
7
III.
6
5
IV.
12
11
13
2. Free earlobes are a dominant trait. Attached earlobes are a recessive trait. Use the symbols E and e to label
each of the numbered individuals. The shaded regions show individuals who are homozygous recessive for
attached ear lobes. They exhibit the trait being studied; they have attached ear lobes.
Diagram G: Earlobes
I.
II.
3
III.
5
IV.
2
1
6
7
4
8
9
10
3
Analysis
1. If the genotype, “R?” is used, what does that genotype represent?
______________________________________________________________________________________
2. What are some of the practical applications of pedigree studies?
______________________________________________________________________________________
3. Without looking at anyone’s chart, do you think there is anyone in the class who has exactly the same
genotypes as you? Explain.
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4. If you were to do a population study, when do you think you would find the least amount of variation
of phenotypes?
______________________________________________________________________________________
Going Further: A SEX-LINKED Pedigree
Hemophilia is a disease in humans that causes the blood to clot slowly. It is a sex-linked trait that is caused by a
recessive gene on the X chromosome. Label each of the numbered individuals Using H for normal blood clotting
and h for hemophilia. Remember to label both the X and Y chromosome.
Hemophilia
I.
II.
III.
IV.
2
1
3
7
4
8
5
9
6
10
11
4