Transcript Lecture #3 Sex Linked Traits

Lecture #3 Sex Linked Traits
Unit: Mendelian Genetics
Refresher from last Quarter
The complete human genome consists of 46
chromosomes.
23 of these chromosomes come from ‘mom’
(egg) and the other 23 chromosomes come
from ‘dad’(sperm).
Only 2 of those 46 chromosomes determine
an individuals sex and are referred to as the
SEX CHROMSOMES.
The other 44 are referred to as AUTOSOMES.
Sex Chromosomes
Two varieties of SEX CHROMSOMES: X & Y
– Females have two copies of the X
chromosome. (XX)
– Males have one X chromosome and one Y
chromosome. (XY)
** Males determine the sex of a baby
because females have only an ‘X’ to give to
each egg. Sperm contains either an ‘X’ or
a ‘Y’ (50:50 chance of either)
Sex Chromosomes
– This Punnett
square illustrates
why males and
females are born in
a roughly 50 : 50
ratio.
Sex-Linked Inheritance
– A sex-linked gene is a gene located on a sex
chromosome
– Genes on the Y chromosome are found only
in males and are passed directly from father
to son.
– Genes located on the X chromosome are
found in both sexes, but the fact that men
have just one X chromosome leads to some
interesting consequences.
Sex Linked Traits
– More than 1200 genes are
found on the X chromosome
– The human Y chromosome
is much smaller than the X &
contains only about 140
genes, most of which are
associated with male sex
determination and sperm
development.
Examples of sex-linked disorders
•
•
•
•
Hemophilia ( bleeders disease)
Color blindness
Muscular dystrophy
Spotted coat coloring in cats
Sex-Linked Inheritance
Most sex-linked traits are recessive
Males exhibit sex-linked disorders more often.
Why?
• The sex-linked trait is on the X chromosome.
Males only have one copy of the X chromosome.
If they have 1 copy of the recessive allele that
causes the disorder, they will have the disorder
Red Green Color Blindness is the
most common type
Male Red-Green Color
Blindness: 1 in 12
Female Red-Green
Color Blindness: 1in 200
Females: need 2 copies of the recessive
allele in order to exhibit (have) the condition.
This makes the chances of the female
exhibiting the condition significantly less.
Carrier- an individual who is heterozygous for an
inherited disorder, but does not show symptoms of the
disorder.
• Female can be carriers for sex-linked disorders and
pass on the recessive allele to her offspring however.
• If she were to have a colorblind son, where did the
recessive allele come from? Mom? Dad? Is it possible
to tell?
Answer: Its mom’s fault!
Explanation- Dad gave the son his “y” (that
is why the child is a male!), mom gave the X,
which contained the recessive allele for
colorblindness.
Take 2 minutes to summarize what
you have learned regarding the
inheritance patterns of sex linked
traits.