Download X –Linked Traits

X –Linked Traits
Definitions:
• Autosomes - Chromosomes #1 – 22
• Sex chromosomes - #23
• In males, the 23rd pair of chromosomes is NOT
homologous
• Females have a homologous 23rd pair.
• Traits located on the 23rd pair are sex-linked or Xlinked.
• Males: XY
Females: XX
X –Linked Traits
• Genes for sex-linked (X-linked) traits are
carried on the X chromosome but not on the Y
chromosome
– Therefore in a male, the gene on the X
chromosome is expressed whether it is
dominant or recessive
– In a female, she must have two recessive
alleles to have the recessive phenotype
Thomas H. Morgan:
Gender and Inheritance
• Breakthrough work done by Thomas Hunt
Morgan – American Geneticist
• 1908 – Work on fruit flies (Drosophila
melanogaster) provided deeper
understanding of heredity based on gender
Drosophila melanogaster
Ideal for genetic experiments for several
reasons:
• Rapid life cycle (10-15 days)
• Hundreds of eggs at one time
• Crossing experiments can be repeated
many times
• Offspring mature quickly
• Males/females different
• Small
• Markedly contrasting traits
• Only 8 chromosomes (4 pairs)
Female
Male
How are they
different??
Male or Female?
Male (XY)
Non
Homologous
Chromosomes
Chromosomes in Fruit Flies
• Morgan found the following:
• Females  4 homologous pairs of
chromosomes  3 autosomal pairs,
one sex pair (XX)
• Males  3 homologous pairs:
– 3 autosomal pairs
– Sex chromosomes were not
homologous
• one X and one Y (XY)
Solving X-Linked Problems:
Eye Color In Drosophila
• Let R = red, r = white
• Assume these alleles are found on the X
chromosome only (they are sex-linked)
• Cross 1. Cross a red eyed homozygous
female with a white eyed male.
First Cross
XR
Xr
Y
XR
XR Xr XR Xr
XRY
XRY
ALL RED-EYED OFFSPRING
Second Generation (F1 Cross)
XR
XR
Y
Xr
XR XR XR Xr
XRY
XrY
- 3 RED
OFFSPRING
- 1 WHITE
MALE
What is the
probability of
these flies having
a red-eyed
daughter?
0.50 or 50 % or ½
Solving X-Linked Problems:
Eye Color In Drosophila
• Let R = red, r = white
• Assume these alleles are found on the X chromosome
only (they are sex-linked)
• Cross 2. Cross a red eyed heterozygous
female with a white eyed male.
Cross 2
XR
Xr
Xr
XR Xr Xr Xr
Y
XRY
What is the
probability of
these flies having
a red-eyed son?
0.25 or 25 % or ¼
XrY
1 Red female, 1 Red male,
1 White female, 1 White male
Morgan’s Conclusions
• Genes controlling eye color must be on X Chromosome
• Traits coded for by genes on sex chromosomes are called
SEX-LINKED TRAITS
• Y does not carry gene for eye color
• Males cannot be heterozygous for sex linked traits
Female
Male
XR XR
Xr Y
XR Xr
XRY
Xr Xr
Traits controlled
by genes carried
on Y chromosome
only affect males
Examples of Sex-Linkage in Humans
• Color Blindness
– genes for visual pigments responsible for
perception of red and green are found on the X
chromosome
• Hemophilia A
– caused by the lack of a blood protein, called
Factor VIII, that is critical for blood clotting
– Gene for Factor VIII on X Chromosome
Both known as X-linked recessive
Color Blindness
Caused by gene carried on X only
• Female Genotypes • Male Genotypes
•
C
c
•X X
• XcXc
C
C
X X
•
c
•X Y
C
X Y
• only 2 possible
phenotypes
• 3 possible genotypes • 2 possible genotypes
• 2 phenotypes
• Link to Ishihara Test for Color Blindness
• http://www.toledobend.com/colorblind/ishihara.html
Other Gender Related Inherited Traits
Sex Limited
• Eg. Milk production in
cows is controlled by
an autosomal gene
• Bulls carry the
genes...but they do not
make milk!!!!!!
• The gene is not
expressed for bulls
(because they have low
levels of female
hormones which switch
on this gene)
Sex Influenced
• Eg. Male only needs
one allele for baldness
– male hormones
stimulate the
expression of this allele
• females need two
recessive alleles to be
bald
• Gene is dominant in
males and recessive in
females
• more common in males