Download Worksheet - X-Linked Inheritance Practice Problems ANSWER KEY

Eastern Regional High School
Honors Biology
Name: _____________________________
Period: _________ Date: _______________
Unit 13 – Advanced Genetics: Exceptions to Mendel’s Rules
Worksheet – X-Linked Inheritance Practice Problems
Introduction:
Some families have a long history of hemophilia among male members of the line. The royal family of Great Britain is a
commonly mentioned example. Families with this kind of history generally want to know when and where the disease
will occur again.
Problems:
Imagine you are a genetic counselor. A man and woman have come to you for advice. They present the information
given in each problem below. Predict the genotypes and phenotypes that would be expected from each mating. Let XN
= the normal allele and Xn = the allele for hemophilia. Create a punnett square for each problem and show your work.
1. Normal female with no history of hemophilia anywhere in the family and a normal male with no history of
hemophilia anywhere in the family.
N
All Normal Offspring
XN
XN
2. Normal female whose father had hemophilia and a normal male
Phenotypes:
All females Normal
50% normal males, 50% Hemophilia males
50% of females are carriers
Y
X NY
X NY
X
X NX N
X NX N
XN
XN XN
XN Y
y
XN
Y
Xn
XN Xn
3. Normal female whose father had hemophilia and a male with hemophilia.
Phenotypes:
Females – 50% Hemophilia, 50% Normal
Males – 50% Hemophilia, 50% Normal
50% of females will be carriers
N
X
n
y
XnNY
X
y
Xn
X
XN Xn
Xn Xn
XN y
y
n
X
XNy
y
4. Female with hemophilia and a normal male.
XN
y
Xn
Xn
XN Xn
XN Xn
Xn y
y
n
X
XNy
y
Phenotype – 100% Normal Females
Males – 100% Hemophilia
All females will be carriers
5. Why is the cross in number 4 not likely to occur?
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Females
are not likely to have hemophilia – Two X chromosomes
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6. Why are male more likely to inherit X-linked diseases?
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Only
one X Chromosome
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___________________________________________________________________________________
7. A man with normal vision marries a women who also has normal vision. One of their sons is colorblind. Show
the cross that produced this child.
Man = XY
Woman = XXc
x
xc
x
y
xx
xy
xxc
x cy
Color Blind Male
8. A woman who is colorblind is married to a man with normal vision. The couple goes to a genetic counselor and
asks him if any of their children will be colorblind. What would you tell them if you were the genetic counselor?
___________________________________________________________________________________
___________________________________________________________________________________
___________________________________________________________________________________
All their male children will be colorblind. All the
female children will be carriers
9. In a large family in which all the daughters have normal vision and all the sons are colorblind, what are the
probable genotypes of the parents?
Most likely scenario: Father = XY and Mother = XcXc
10. Amelogenesis imperfecta is a dental abnormality, which is inherited as an X-linked recessive trait. Neither of
your parents have the disorder but your father’s brother does. (Assume your mother is homozygous). What is
the chance you will receive the recessive gene?
0% that you will get the disorder. The uncle
information is irrelevant.
11. In cats, one gene for coat color is X-Linked. Male cats are either black or orange, depending on which allele
they carry. Females are black, orange or tortoise-shell (Patches of black and orange), with the tortoise shell
resulting form the heterozygous genotype. Cross a black furred female with an orange male. What are the
possible phenotypes and genotypes?
Xb
Xb
Xo
Xb Xo
Xb Xo
Y
Xb
y
Xb
y
Let
Xb = Black
Xo = Orange
Phenotypes:
-Tortoise
Females
-Black Males
12. In cats, one gene for coat color is X-Linked. Male cats are either black or orange, depending on which allele
they carry. Females are black, orange or tortoise-shell (Patches of black and orange), with the tortoise shell
resulting form the heterozygous genotype. Two cats mate and have kittens. Of the females, ½ are tortoiseshell, ½ are orange. Of the male kittens, ½ are orange and ½ are black. What are the genotypes and
phenotypes of the parent cats?
Xo
Xb
Xo
Xo Xo
Xb Xo
y
Xo
y
Xb
y
Work backwards!
Fill in the square
first!
Parents
Mom = xbxo
Dad = xoy