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Observing Human Traits
Background:
Human traits, like those of other organisms, are inherited. In humans, you see
dominant, recessive, and sex-linked traits. For example, tongue rolling is a simple
dominant trait. A person who has the trait can roll his or her tongue into the shape of a U.
Another dominant trait is free earlobes. Red-green colorblindness is a sex-linked,
recessive trait in humans. A person with this trait cannot easily distinguish the color red
from the color green.
In the study of human genetics, scientists examine pedigrees, or family records, that
show the transmission of inherited traits. Studying a pedigree may enable a scientist to
determine a person’s genotype with respect to the traits being studied.
In this investigation, you will determine the possible genotypes and phenotypes of
certain crosses. You will also construct a pedigree.
Problem:
How can human traits be studied? How is a pedigree constructed?
Procedure:
Part A:
1. Complete the Punnett square for the possible offspring of a woman who has a
dominant and recessive gene for tongue rolling and a man who cannot roll his tongue.
Let T = gene for tongue rolling and t = gene for the inability to roll the tongue.
Tongue rolling Punnett Square
______
_______
______
______
2. What is the genotypic ratio of the cross above?____________________
3. What is the probability that the offspring will have the ability to roll their tongue? ___
4. Are you a roller or a nonroller? __________________________________
5. Based on your answer to the question above, what are your possible genotypes?
___________________________________________________________
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Part B:
In a pedigree, circles represent females, and squares represent males. If either shape is
completely colored in, it means the person has the trait being studied. Horizontal lines are
used to connect the parents, and the children are shown by vertical lines. The known
genotypes for each person may be written below each circle or square. The female
chromosomes are XX, while the male chromosomes are XY.
The diagram below represents the pedigree for colorblindness. Let N = gene for normal
visions and n = gene for colorblindness. If a female is colorblind, her genotype is XnXn.
However, a female may also be XNXn. She is not colorblind, but she is said to be a carrier
because she can pass the recessive gene to her offspring. On the other hand, a male is
said to be normal if his genotype is XNY and colorblind if his genotype is XnY.
XNXN
XC
XC
XC
XNY
XNXn
XnY
XNY
XNY
XnY
5. Using the sample above and the information in Part B, complete the pedigree below.
A
C
I
D
B
E
J
F
G
H
K
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Part C:
6. The diagram below shows the difference between free and attached earlobes. If a
female with the genotype of Ff married a male with the genotype Ff. What types of
earlobes could their children have? Complete the Punnet Square below. Use F as the
gene for free earlobes and f is the gene for attached earlobes.
Earlobes Punnett Square
______
Free
_______
Attached
______
______
7. What are the possible phenotypes of the offspring? ___________________
8. What is the genotypic ratio? _________________
9. Do you have free or attached earlobes? __________________________________
10. What are your possible genotypes? _____________________________________
Critical Thinking and Application Questions:
1. Can 2 parents who are not colorblind have a son who is colorblind? _________
Can they have a daughter who is colorblind? ____________. What would be the
genotype of the mother? ____________Father? ________________
Son? __________ Daughter? (2 possible genotypes) _________________
2. How might it be possible for you to show a trait that one of your grandparents has
when neither of your parents shows the trait? (think about their possible genotypes)
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
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