Transcript Genetics: Sex-Linked Inheritance

Bio 12
Review: Sex Chromosomes
 Humans have 23 pairs of chromosomes.
 22 of them are called autosomal.
 The 23rd pair are the sex chromosomes.
 The sex chromosomes determine whether an offspring
will be male or female.
o XX = Female
o XY = Male
Review: Females
Have two x chromosomes (one from
mother and one from father)
Review: Males
Have one x chromosomes (from mother)
and one y (from father)
What is sex-linked inheritance?

 Genes that are carried by either sex chromosome are
said to be sex linked.
 Sex linked genes have inheritance patterns that differ
from autosomal genes
o Since males and females differ in their sex
chromosomes, inheritance patterns are different for
males and females.
History…..

 The study of inheritance of
genes located on sex
chromosomes was pioneered by
T. H. Morgan and his students at
the beginning of the 20th
century.
 He used Drosophila melanogaster
(fruit flies) to study genetics
 Although Morgan studied fruit
flies, the same genetic principles
apply to humans.
More history…..

 Morgan noticed that
chromosomes of male and
female fruit flies were slightly
different
 He is credited as the first person
to see the physical difference in
the x and y chromosomes!
More history…..

 Knowing red eyes are
dominant and white eyes are
recessive, Morgan crossed
whited eyed with red eyed
o F1’s were what he expected
3 red: 1 white
 But in F2, he found all the
white eyed flies where male!
o He predicted that the eye
colour was connected with
the sex of the fruit fly!
Genotype notation:

 Use X or Y to represent sex chromosome and capital
or lowercase letters to represent dominant or
recessive allele
 Examples:
•
•
•
•
•
XG XG
XG Xg
Xg Xg
XG Y
Xg Y
X-linked

X-linked diseases are those for which the
gene is present on the X chromosome.
Because of this, males and females show
different patterns of inheritance and severity
of disease.
There are both dominant and recessive Xlinked diseases
X-linked Characteristics:

 X-linked genes are never passed from father to son.
 Males are never carriers – if they have a mutated
gene on the X chromosome, it will be expressed ….
Why?
o The Y chromosome is the only sex chromosome that
passes from father to son……does not pass on an X!
X-linked dominant

 X-linked dominant diseases are those that are
expressed in females when only a single copy of the
mutated gene is present.
 Very few X-linked dominant diseases have been
identified (e.g. hypophosphatemic rickets, Alport
syndrome, diabetes insipidus)
X-linked dominant diseases:

 Affected females produce 50% normal and 50%
affected offspring…….. heterozygous
 Females are more likely to be affected. Since
females have 2 X chromosomes, they have 2
“chances” to inherit the mutated allele.
The pattern for the pedigree of X-linked
dominant inheritance:

Note: If father is affected, all daughters will be affected
X-linked recessive

X-linked recessive diseases are those in which
a female must have two copies of the mutant
allele in order for the mutant phenotype to
develop.
o Trait skips generations
o Affected fathers DO NOT pass to their sons
o Males are more often affected than females
Includes: color blindness, hemophilia, and
Duchenne muscular dystrophy.
The pattern for the pedigree of Xlinked recessive inheritance:

Y….

In humans there are very few traits that are
sex linked on the “Y” chromosomes
 These traits are only expressed in males
 Transmitted from father to son
 Example: gene SRY (triggers testis development )
Note: We will not be doing any y-linked genetics
problems in Bio 12!
To do:

 X-linked Genes WS
 Page 167 #6, 7