Download Mechanisms of Heredity Sex

Mechanisms of Heredity
Sex-linked Traits
Read the paragraph. Then answer the question.
Have you ever seen a calico cat? It has areas of black and areas
of yellow on its coat. This coloring is caused by a pair of alleles
(B = black and b = yellow) that combine to give Bb = calico.
This trait is also sex-linked, which means that the genes for this
trait appear only on the X chromosome. Females may have the
genotype Bb and show the calico phenotype, but males only have
one X chromosome, so their genotype is either B (black) or b (yellow).
Whatever allele is on their X chromosome will be expressed.
1. To do a sex-linked Punnett square, link the alleles with the chromosome
they are on. So, instead of just showing alleles, show the X and Y chromosomes as
well. Show the following cross:
A calico female has a litter of kittens with a yellow male cat.
XB
Xb
Xb
X
X
X
X
Y-
X
Y
X
Y
2. Give the gender and color probability of this cross:
_____________________________________________________________________
3. Why were there no black female kittens? ____________________________________
_____________________________________________________________________
4. Why were there no calico males? __________________________________________
______________________________________________________________________
A yellow female cat has a litter of four kittens. Three of them are calico and one is
yellow. Show the cross:
5. What is the likely color of the kitty-daddy? __________________________________
6. What sex are the calico kittens? ___________________________________________
7. What sex is the yellow kitten most likely to be?_______________________________
*8. How could a male cat be calico? Discuss. __________________________________
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
____________________________________________________________________
Additional Sex-Linked Practice Problems:
SHOW YOUR WORK!
9. Red-green colorblindness is a sex-linked recessive trait. A woman who homozygous
normal for this trait marries a man who is colorblind.
What is the genotype of the woman? _________ the man? __________
What can you predict about their children? (show your work!)
______% of their kids will be colorblind.
X
X
X
X
X
X
X
Y-
X
Y
X
Y
10. If a colorblind woman has a child with a man with normal vision, give the probability
that their child will be a
a) girl with normal vision? _________
girl that is colorblind? _______
b) boy with normal vision? _________
boy that is colorblind? _______
11. Hemophilia is a blood-clotting disorder caused by a recessive trait on the X
chromosome. There are preventative treatments now, but without them a person can
bleed excessively from very minor injuries. Neither Pam nor Mike have hemophilia,
and they are planning on having kids together. Pam is concerned because her father
had hemophilia.
What could you tell them about the chances that their kids would have hemophilia?
Does the probability differ for the boys vs the girls?
12. A certain type of muscular dystrophy (Becker muscular dystrophy) results from a
mutated gene on the X chromosome, causing the body to improperly produce a muscle
protein called dystrophin. The results in difficulty with anything that requires muscles,
of course, like walking, standing, etc. Becker muscular dystrophy is less severe than
the more common type, Duchenne muscular dystrophy.
If Dana, who is a carrier* for the mutated muscular dystrophy gene, were to
have kids with Jim, and he has muscular dystrophy, what could you tell them about
the overall probability of their kids having muscular dystrophy?
Does the probability differ for the boys vs the girls?