Transcript LECTURE #30: Sex Linkage

Lecture #30
Introduction to Sex Linkage
Honors Biology
Ms. Gaynor
Gene Linkage

Linked genes
 Usually inherited together
because located near each other
on the SAME chromosome

Genes closer together on the same chromosome
are more often inherited together
Each chromosome
 Has 100’s or 1000’s of genes
 Sex-linked genes exhibit unique
patterns of inheritance; genes on
the X or Y chromosome

Morgan’s Experimental
Evidence
 Thomas
Hunt Morgan
 Provided convincing
evidence that
chromosomes are
the location of
Mendel’s heritable
alleles
Sex linkage
explained
http://nobelprize.org/nobel_prizes/medicine/articles/lewis/index.html


Thomas Hunt Morgan
(Columbia University 1910)
Fruit Flies (Drosophila)
melanogaster)
© 2007 Paul Billiet ODWS
Morgan’s Choice of
Experimental Organism
 Morgan
worked with
fruit flies
 Lots of offspring
 A new generation
can be bred every
two weeks
 They have only 5
pairs of
chromosomes
Morgan and Fruit Flies
Morgan first observed and noted
 Wild type (most common)
phenotypes that were common in
the fly populations
 Traits alternative to the wild type are
called mutant phenotypes

w+
WILDTYPE
w
MUTANT
The case of the whiteeyed mutant
Character
Eye color
type)
Traits
Red eye (wild
White eye
(mutant)
P Phenotypes
Wild type (red-eyed) female x White-eyed male
F1 Phenotypes All red-eyed
Red eye is dominant to white eye
Hypothesis
A cross between the F1 flies
should give us: 3 red eye : 1
white eye
F2
Phenotypes
Numbers
So far so good
Red eye
White eye
3470
82%
782
18%
An interesting
observation
The F2 generation showed the 3:1
red: white eye ratio, but only
males had white eyes
Phenotypes
F2
Numbers
Redeyed
males
Redeyed
females
Whiteeyed
males
Whiteeyed
1011
2459
782
0
24%
58%
18%
0%
females
A reciprocal cross
Morgan tried the cross the other way
round
white-eyed female x red-eyed
male
Result
All red-eyed females and all whiteeyed males
This confirmed what Morgan
suspected
The gene for eye color is linked to the
Morgan’s Discovery:
Sex Linked Traits
 Eye
color is linked on X
Chromosome
 Females carry 2 copies of gene;
males have only 1 copy
 If mutant allele is recessive,
white eyed female has the trait
on both X’s
 White eyed male can not hide
the trait since he has only one X.
The Chromosomal Basis
of Sex
 An
organism’s sex
 Is
an inherited phenotype
determined by the
presence or absence of
certain chromosomes
 XX = girl
 XY = boy
Inheritance of SexLinked Genes
sex chromosomes
 Have genes for many
characters unrelated to sex
(especially the X
chromosome)
 A gene located on either sex
chromosome
 Is called a sex-linked gene
 The
(Usually on X chromosome)
What genes are on the X
chromosome?
carries a couple
thousand genes but
few, if any, of these
have anything to do
directly with sex
determination
 Larger and more
active than Y
chromosome

What genes are on the Y
chromosome?



Gene called SRY
triggers testis
development, which
determines male sex
characteristics
This gene is turned “on”
~6 weeks into the
development of a male
embryo
Y-Chromosome-linked
diseases are rare
Sex-linked genes follow
specific patterns of inheritance
Fathers  pass sex-linked alleles
to ALL their daughters but NONE
to their sons
 XY (Father)  XX (daughter)
 XY (Father)  XY (son)
 Mothers  can pass sex-linked
alleles to BOTH sons and
daughters
 XX (Mother)  XX (daughter)
 XX (Mother)  XY (son)

Sex Linkage

If sex-linked recessive on Xn
 Females have to be Xn Xn to show
sex-linked trait
 Xn X Females do NOT show sexlinked trait
 Males have to be Xn Y to show sexlinked trait
**Most sex-linked disorders affect
males; sometimes females
Sex-Linked
Disorders
 Some
recessive alleles found
on the X chromosome in
humans cause certain types of
disorders
 Color blindness
 Duchenne muscular
dystrophy
 Hemophilia
 Male pattern baldness
X-Linked Trait =
Male Pattern Baldness
Baldness
Another X-Linked Trait =
Hemophilia

About 85% of
hemophiliacs suffer from
classic hemophilia





1 male in 10 000
cannot produce factor VIII
The rest show Christmas
disease where they can’t
make factor IX
The genes for both forms
of hemophilia are sex
linked
Hemophiliacs have trouble
clotting their blood
Another X-Linked Trait =
Red-Green Colorblindness
Normal vision
Color blind simulation
http://www.onset.unsw.edu.au/issue1/colourblindness/colourblindness_print.htm
Another X-Linked Trait =
Duchenne Muscular Dystrophy