Transcript 04-01 B11.2 PP Sex-Linked Traits

Sex Determination, Sex-Linked Traits, &
Sex Inf luenced Traits
April 17, 2008
 Gender is determined by a pair of chromosomes called
the sex chromosomes.
 two types: X and Y – not always homologous
 females have two X chromosomes
 males have one X and one Y chromosome
 In humans, the remaining 22 pairs of chromosomes
are homologous and are referred to as autosomes.
 X chromosome is larger than Y
 ‘X’ contains 900-1400 genes
 ‘Y’ contains fewer than 100 genes –
mostly related to “maleness”
 In females, one of the X
chromosomes is inactivated.
 X-inactivation occurs randomly in
each body cell.
 Form of dosage compensation.
 Inactivated X-chromosome
condenses into a darkly staining
region called a Barr body.
 Example:
 Calico color in cats
Calico cat
 Traits controlled by genes on the sex chromosomes are called sex-
linked traits.
 Mostly X-linked traits; very few Y-linked traits.
 Males disproportionately affected by recessive alleles on the X
chromosome.
 Examples:
 Red-green color blindness
 Hemophilia
 Traits located on the autosomes can sometimes be affected by the
proportion of sex hormones produced by the body.
 Ultimately sex hormone production is prescribed by the sex
chromosomes.
 These are called sex-influenced traits
 Example:
 Male pattern baldness
Other
Colorblindness
Tests
 A recessive X-linked trait
 Affects 8-12% of males in the United States; less than 0.5% of females.
 Results when the color-detecting cones in the retina of the eyes
function ‘poorly’ in discriminating between red and green colors.
Normal vision
Red-green colorblind vision
PROBLEM:
 A man with normal vision and a
woman who is heterozygous for the
colorblind allele want to have a
child.
 ‘B’ is the allele for normal vision
 ‘b’ is the allele for color blindness
 What is the probability that their
child will be colorblind?
 Could they have a colorblind
daughter?
XB
Xb
XB
Y
XBXB XBY
XBXb XbY
SOLUTION:
 25%
 No, they cannot have a colorblind daughter. The only child that could be
affected would be a boy.
Hemophilia
 A recessive X-linked disorder
 Results in a delayed clotting of blood
 Will it be more prevalent in males or females? Why?
 A man with hemophilia has children with a woman who
is a carrier for hemophilia. What is the chance that their
next child will have hemophilia? Their next son? Their
next daughter?
Male pattern baldness is a sex-influenced trait. The gene, B1, for baldness is
dominant in males, but recessive in females. The gene, B2, produces normal
hair growth in both men and women. The baldness gene is also an autosomal
trait.
If a man with male pattern baldness
and genotype B1B2 marries a woman
who is not bald with genotype B2B2,
what are the chances that they will
have a son with male pattern
baldness? (HINT: This requires a
dihybrid cross.)
XB1
XB2
XB2
XB2
XB2
XB2
YB1
YB2
XXB1B2 XXB2B2 XYB1B2 XYB2B2
XXB1B2 XXB2B2 XYB1B2 XYB2B2
XXB1B2 XXB2B2 XYB1B2 XYB2B2
XXB1B2 XXB2B2 XYB1B2 XYB2B2
50% female (none bald); 25% male (balding); 25% male (not balding)
Did the balding sons get their baldness gene from their mother or their father?
ADAM Inc., Male Pattern Baldness. 14 Apr 2008. U.S. National Library of Medicine 16 Apr 2008
<http://www.nlm.nih.gov/medlineplus/ency/imagepages/17083.htm>
ADAM Inc., Various Tests for Color Blindness. 2007. New York Times Company. 16 Apr 2008
<http://www.nytimes.com/imagepages/2007/08/01/health/adam/9962Colorblindnesstests.html>.
Biggs, Alton, et. al. Biology. New York: The McGraw Hill Companies, Inc., 2007.
Color in Computer Graphics. 25 Feb 1998. Cornell University Program in Computer Graphics. 16 Apr 2008
<www.graphics.cornell.edu/online/tutorial/color/>.
Waggoner, Terrace L.. "About Color Blindness (Color Vision Deficiency)." Colors for the Color Blind. U.S. Naval Hospital,
Pensacola, FL. 16 Apr 2008 <http://www.toledo-bend.com/colorblind/aboutCB.html>.
"X chromosome." Genetics Home Reference: Your Guide to Understanding Genetic Conditions. 14 Apr 2008. U.S. National
Library of Medicine. 16 Apr 2008 <http://ghr.nlm.nih.gov/chromosome=X>.
“X chromosome.” Photo Researchers, Inc. 16 Apr 2008.