Transcript SEX-LINKED TRAITS

SEX-LINKED TRAITS
Solving pedigrees for traits with genes
located on the X and Y chromosome
AUTOSOMES
VS.
SEX CHROMOSOMES



Humans have 23 pairs of
chromosomes
22 pairs of chromosomes are not
related to gender (male or female).
These are called AUTOSOMES
1 pair of chromosomes determines
gender. These are called SEX
CHROMOSOMES
KARYOTYPE
chart of homologous
chromosomes (XX)
KARYOTYPE
chart of homologous
chromosomes (XY)
AUTOSOMAL TRAITS



Most of the traits in humans are controlled
by genes on the autosomes.
Autosomal traits are inherited the same way
by both males and females.
All of the traits we have studied so far have
been autosomal – they are the simplest type
of genetics problems to solve
SEX CHROMOSOMES






The sex chromosomes determine gender
The sex chromosomes are also called the X
and Y chromosome
Females are XX
Males are XY
The Y chromosome is much smaller, and
does not have the same genes as the X
Most “sex-linked” traits are “X linked”
INHERITANCE OF SEX
CHROMOSOMES

Since Females are XX
– They inherited an X chromosome from
both of their parents
– They will only produce eggs with X
chromosomes
– All of their children will inherit one or the
other of her X chromosomes (with equal
probability) – but all of her children, both
boys and girls, will inherit an X
chromosome from her
INHERITANCE OF SEX
CHROMOSOMES

Since males are XY
– They inherited the Y from their father

must be – their mother did not have a Y
chromosome because she is female
– They inherited the X from their mother

process of elimination – if the Y came from
Dad, the X came from Mom
Females vs. Males
X-Linked Traits

Since females are XX
– They have 2 alleles for the trait. It works out
exactly like autosomal traits


Homozygous dominant, heterozygous, and
homozygous recessive are all possible
Since males are XY
– They only have one allele for the trait. There is
no corresponding allele on the Y chromosome
– Whatever alleles males have on the X
chromosome will be expressed.
Eye Color in Fruit Flies
White eyes in fruit flies is X linked recessive
(the normal “wild type” eye color is red)
Predict the offspring of a cross between
a white eyed male and a purebred red
eyed female
Predict the offspring of a cross between
a white eyed male and a red eyed female


All offspring will
have red eyes
Females will all be
carriers
Predict the offspring of a cross between
a normal male and a heterozygous
female
Predict the offspring of a cross between
a normal male and a heterozygous
female


All female offspring will
have red eyes, but half of
them will be carriers
Half of the males will be
normal, and half will have
white eyes
Colorblindness: X linked
recessive


People affected by red-green colorblindness
are not able to distinguish red from green
The pictures above are used to test for
colorblindness
Colorblindness:
Genotypes and Phenotypes

Females:
– XNXN
– XNXn
– XnXn

normal vision, not a carrier
normal vision, carrier
colorblind
Males
– XNY
– XnY
normal vision
colorblind
Colorblindness:
Pedigree solving





All males are XY
All females are XX
Males inherit X from
mother and Y from
father.
Males give X to
daughters and Y to
sons
Trace the X and Y
through the
generations
Colorblindness:
Pedigree solving


All males are XY
All females are XX
Colorblindness:
Pedigree solving




#3 is colorblind
Colorblindness is X
linked recessive
#3 must have a
recessive allele for
colorblindness on his X
chromosome
All other males are
normal, and must have
dominant alleles for
color vision on their X
chromosomes
Colorblindness:
Pedigree solving

All the females in
this pedigree have
normal color vision,
so all of them must
have at least one
dominant allele
(maybe both) for
normal color vision
on an X
chromosome
Colorblindness:
Pedigree solving



#3 inherited his Y
chromosome from his
father (his mother
doesn’t have one
because she’s female)
He inherited his X
chromosome from his
mother
#2 (mother of #3) is a
carrier
Colorblindness:
Pedigree solving



#5 is the daughter of
#3 (colorblind male)
and #4 (normal
female)
#5 has 2 X
chromosomes, one
from her Mom and one
from her Dad
#5 is a carrier because
she inherited her Dad’s
X chromosome
Colorblindness:
Pedigree solving


The only person left
is #4 – a normal
female
Without knowing
more about her
parents, we can’t
determine if she is
a carrier or not.
Her genotype is
unknown