More Genetics Questions
Colorblindness is a sex-linked trait found on the X chromosome. Use this information to
do the following problems.
1. A male who is colorblind marries and has children with a female who is not color
blind. None of their offspring are colorblind. What genotype was the mother?
What genotype was the father? What is the genotype of the male offspring and the
female offspring?
2. A man who is colorblind marries and has children with a female who is a carrier
for the colorblind gene. What percent of the males will be colorblind? What
percentage of the females will be colorblind? What percentage of the females will
be carriers? What percentage of the males will be carriers?
Answers at the end
More problems can be found at:
http://biology.clc.uc.edu/courses/bio105/geneprob.htm
http://www.learn4good.com/science/biology/genetics_practice_problems_single_trait_cr
osses.htm
http://faculty.mdc.edu/mdelcamp/BSC%202010%20Notes/Chpt_15_Genetics_Practice_P
roblems.doc
There are many many more problems online (I assume that your lab instructor gave you
some as well. Just google genetics practice problems. Many even give you the answers.
Answer #1
Xa
Y
XA
XAXa
XAY
A
XAXa
XaY
X
Females both
not colorblind,
but are carriers
Male not
colorblind
Male
colorblind
Answer #2:
Xa
Female not
colorblind, but
carrier
Y
XA
XAXa
XAY
a
XaXa
XaY
X
Male not
colorblind
Male
colorblind
Female
colorblind
50% of the females (1 out of 2 females) will be colorblind
50% of the females will not be colorblind and will be carriers
50% of the males will be colorblind
50% of the males will be colorblind
50% of the offspring will be colorblind
50 % of the offspring will not be colorblind
** none of the males will be carriers. Males in this case can not be carriers because
they only have one X, so they don’t have another X to mask a recessive a.
** As you can see for sex-linked genes you must draw the sex chromosomes and put
the alleles as small letters to the right of the chromosomes. Note there are no alleles
on the Y chromosome because colorblindness is a X linked gene. It is not found on
the Y chromosome only the X chromosome. If you don’t write in the chromosomes
and forget that males don’t have 2 alleles in this case you will come out with a
completely different, and wrong, answer. So take care when filling these out!