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CP1 Biology
Unit 8 Genetics
Activity:
Name: ___________________
A Human Pedigree
Introduction –
A sex-linked characteristic is determined by an allele that is carried only on the X chromosome.
The shorter Y chromosome does not carry an allele for a sex-linked trait. Most sex-linked traits are
recessive. Since there is only one X in his genotype, XY, a male who carries a particular recessive allele
on the X chromosome will have the sex-linked condition. A female who carries a recessive allele in one
X chromosome will not show the condition if there is a dominant allele on her other X chromosome. She
will express the recessive condition only if she inherits two recessive alleles, one from each parent. Her
chances of inheriting the condition are thus greatly reduced.
One sex-linked trait is hemophilia, a condition in which the blood does not clot properly. Most
people who have hemophilia are men. In this investigation you will determine why this is so.
Procedure:
1. Study the pedigree for hemophilia shown in Figure A. In this pedigree, a square represents a
male. If it is darkened, he has hemophilia, if clear then he does not and has normal blood
clotting.
How many males represented by this pedigree have hemophilia? How many are normal?
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2.
A circle represents a female. If it is darkened, she has hemophilia, if clear, she is healthy.
How many females in this pedigree have hemophilia? How many are normal (healthy)?
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3.
A marriage is indicated by a horizontal line connecting a circle to a square.
How many marriages are indicated in the pedigree? __________________________
4.
A line perpendicular to a marriage line indicates offspring of that marriage. 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 eldest child ALWAYS appears to the left and the
youngest child to the right.
How many children did the first couple have? ________________________________
5.
Level B represents the second generation. Level C represents the third generation, and so on.
How many generations are represented in this pedigree? _________________
6.
The genotypes of the males in a pedigree for a sex-linked trait are easy to determine. Normal
blood clotting (N) is dominant and hemophilia is recessive (n). Since these alleles are only on the
X chromosome, a male represented by a clear square will have the genotype X NY. A male
represented by a darkened square will be X nY. Label the genotypes of all the males in the Figure
A.
How many males have the genotype XNY? _______________________
7. Females who have the hemophilia have an easy genotype to identify. They are all XnXn. Both
recessive alleles must be present for a female to express the disease. If one dominant allele is
present, the individual would appear normal for blood clotting. Label all the females with
hemophilia as XnXn on the pedigree in Figure A.
How many women have genotype XnXn in this pedigree? ________________________
8. Females who do not show the trait for hemophilia may be homozygous dominant (XNXN) or
heterozygous (XNXn). A heterozygous female is called a carrier. Examination of offspring can
often determine which genotype the parents have. If any child (son or daughter) has hemophilia,
then the female must be heterozygous (XNXn). If her son has hemophilia, he has the genotype
XnY. He inherited the Y from his father, and so the X chromosome carrying the recessive allele
n must be from his mother. If a daughter has hemophilia, she inherited one Xn from her mother,
and the other from her father, thus indicating that the mother has AT LEAST one recessive n.
Label all females XNXn that have children with hemophilia.
What is the genotype of the female in the first generation? ___________________
9. Females who have more than four sons, with none exhibiting hemophilia, are likely to be genotype
XNXN. If she has four or fewer than four sons, her genotype is less certain and cannot be
determined exactly. In such cases, her genotype is labeled as XNX? to indicate that the other
allele is unknown. Label the rest of the females in the pedigree as either XNXN or XNX?.
How many females in the pedigree have the genotype XNXN? _________________
10. All individuals on the above pedigree should now be labeled.
11. Using the same rules as above, label ALL individuals on the pedigree in Figure B.
10. Fill in Table B based on the information you filled in the pedigree.
Questions about the pedigree in Figure B
Number of generations
Number of men with hemophilia
Number of women with normal blood clotting
Number of marriages
Number of men with genotype XNY
Number of women with genotype XnXn
Number of single women
Number of people that never married
Number of women with genotype XNX?
Number of couples with only one child.
Answer
Analysis Questions:
11. Which sex usually inherits the sex-linked condition? Explain why. ______________________
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12. How can you tell whether a female has a genotype XNXN, XNXn, or XNX? ? _______________
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13. List all the possible genotypes in a pedigree of sex-linked traits. ______________________
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