Quiz 7E
Variations in Mendel's Theories
Sex-linked Traits
Variations in Mendel's Theories




Incomplete dominance
Codominance
Multiple gene inheritance
Sex-Linked traits
INHERITANCE OF SEX CHROMOSOMES
Humans have 23 pairs of chromosomes
Autosomes: 22 pairs are non-sex determining
chromosomes
the 23rd pair determines the sex of the
offspring
female sex chromosomes - XX
male sex chromosomes - XY
( y is
smaller
than x and
has
different
genes)
Who determines the child's sex?
the male
SEX- LINKED TRAITS
an inherited trait that has a gene on the x
chromosome but no
corresponding gene
on the y chromosome
(genes that only exist
only on the sex
chromosomes)
Examples of human sex-linked traits
 Hemophilia
 Red-green color blindness
 Muscular dystrophy
 Male pattern baldness
Inherited Disorders
inherited disorders - an abnormal trait
passed on through genes
examples: red-green colorblindness,
some handicaps
How to work sex-linked trait problems
Hemophilia: is a rare, inherited bleeding
disorder in which your blood doesn’t clot
normally. If you have hemophilia, you may
bleed for a longer time than others after an
injury. You also may bleed internally,
especially in your knees, ankles, and elbows.
This bleeding can damage your organs or
tissues and, sometimes, be fatal.
All possible genotypes & phenotypes:
genotypes
XHXH
XHXh
XhXh
XHY
XhY
phenotypes
Normal female
Female carrier
Female hemophiliac
Normal male
Male hemophiliac
What is a carrier?
carrier - expresses the normal
characteristic but has both
dominant and recessive genes
(have the gene for a trait but do
not express the trait themselves)
H
h
X X
for sex-linked traits, males cannot be carriers
How to work sex-linked trait problems:
start with this type of punnett square
X
X
X
X X
X X
Y
X Y
X Y
How to work sex-linked trait problems:
put the genes on the X chromosome
X
X
H
Y
H
H H
X
H
h
X X
X X
H
h
X Y
h
X Y
Cross a normal husband with a carrier wife