Sex Linkage (Following the X and Y Chromosome)
Remember: Humans have 46 chromosomes – or 23 pairs of chromosomes.
Chromosome pairs #1-22 are autosomal chromosomes.
Chromosome pair #23 are the sex chromosomes and they determine
the sex of the individual.
So far we have looked at the inheritance of genes located on autosomal
chromosomes.
However, the _____________ chromosomes also carry genes. We use the letters X and Y
to identify these chromosomes.
FEMALE: XX
MALE: XY
Why are males male? The genes on the Y chromosome cause an organism to develop
male sexual organs. Thus, the presence of a Y chromosome makes the individual
male!
So what is the probability of having a female child?
XX
x
XY
Genotype:
Phenotype:
Therefore, the probability of having a female child is _______
NOTE: It’s the sperm/male parent that determines the sex of a child.
The sex chromosomes do more than code for _________________.
SEX LINKED TRAITS – genes located on the sex chromosomes, usually the X
chromosome.
The X chromosome is much _______________ than the Y chromosome – it carries
between 100-200 genes. Because both males and females have at least 1 X
chromosome, important genes and information are found on the X chromosome.
You can survive without a _____ chromosome, but you can’t survive without an
_______chromosome!
Thomas Hunt Morgan (1866-1945)
 Geneticist
 Studied _________________ (Drosophila melanogaster)
NOTE: Fruit fly is an ideal subject for inheritance studies because:
-reproduces rapidly
-can reproduce when they are only 10 to 15 days old
-therefore can study many generations in a short time
-small (many of them can be contained in a small vial)
-males and females are ___________________ distinguishable





Examined _______________ colour in Drosophila
Noticed the appearance of white eyed flies among many red eyed offspring
Concluded the ________________eyes must be a mutation
Mated white eyed male with purebred red eyed female (RR) and found all F1
generation had _________________ eyes
According to normal Mendelian genetics, the red eyed allele was dominant
R
R
r
r


He then decided to mate two of the F1 generation flies and got a 3:1 ratio of
red to white
Also seemed to support Mendelian genetics
R
r
R
r





Most researchers would have stopped at that point but Morgan noticed
something interesting.
All the _______________ had red eyes! Only the ______________ had white eyes (half
males were red and half were white)!
Why did the white eyed phenotype only seem to appear in males?
Figured that the pattern of inheritance must differ between males and
females!
Previous researchers found that Drosophila had 8 chromosomes (4 pairs)



Females = 4 homologous pairs (three pairs + XX)
Males = 3 homologous pairs (three pairs + XY)
XY not completely ________________________ because not the same chromosome
therefore carries different information/genes

Morgan was able to explain his results by concluding that the Y chromosome
did not carry the gene for eye colour. Only the X carried the gene

Sex linked traits – traits located on the sex chromosomes

Let’s re-examine the problem:
Purebred red eyed female is crossed with a white male
XRXR x
Xr Y
XR
(NOTE: no symbol for eye
colour on Y chromosome
because it does not carry an
eye colour gene)
XR
Xr
Y
Phenotype: all red
Phenotype: Males
red eyes
Females
red eyes
Two F1 flies are crossed:
XR
Xr
XR
Y
Phenotype: 3 red :1 white
Phenotype: Males
1 red eyes : 1 white eyes


Females
all red eyes
Can white eyed females possible occur in nature?
YES! For this to happen, the offspring would have to inherit two recessive
white eyed genes (therefore male must be white eyed (XrY) while female had
at least one white eyed gene (XR Xr)
XR
Xr
Xr
Y
Example of X Linked Genes: (Genes on located on the X chromosome)
- Hemophilia (the inability to clot blood)
- Myopia (nearsightedness)
- Night blindness
- Male Pattern Baldness
- Colour-blindness
All the above mentioned traits are recessive.
 Autosomal: You require 2 copies of the recessive allele to show the recessive
trait (Ie: Blue eyed individuals have the genotype: bb) and having at least 1
copy of the dominant allele causes the dominant trait to be expressed. (Ie:
The genotype Bb would result in a brown eyed individual)

Sex linked traits: since males have only 1 X chromosome, they only require 1
copy of the recessive allele to show a recessive trait (____________). Females
require 2 copies of the recessive allele to show a recessive trait (_____________).
Therefore, X-linked traits occur more frequently in males than females!
NOTE: When showing alleles for traits located on the X chromosome, write the allele
as a superscript. Example: XH
Example Problems:
In fruit flies, the gene for eye colour is X-linked. Red eyes are dominant to white
eyes.
a) If a heterozygous red-eyed female mated with a red-eyed male, what is the
probability of
i)
Producing a white eyed offspring?
ii)
Producing a white eyed male offspring?
iii)
Producing a white eyed female offspring?
iv)
A male being white eyed?
b) What is the probability of a homozygous red-eyed female and a white eyed
male having a white eyed female offspring?
c) What are the phenotypic and genotypic ratios when a white eyed female
mates with a red eyed male?
NOTE: CARRIERS
A carrier is an individual who carries/possess a copy of a gene for a particular trait
but doesn’t express it. Ie: The carrier is a heterozygote.
ex: The genotype Bb – would have brown eyes but would be a carrier of the blue eye
gene. The individual doesn’t have blue eyes but his/her child could have blue eyes.
ex: If XN = normal vision, and Xn = is a colour blind, a carrier would be XNXn.
For X-linked traits, only females can be carriers. Males only have one
copy of the allele, so they cannot be carriers. If they have the gene,
they will express it.