Lec 11-2 Flashcards
Sex linked genes
Genes on sex chromosomes that lead to a certain phenotype/disease
Autosomal chromosomes pair with
Homologous counterparts
X chromosome is
Similar to autosomes
Paired with another homologous X
Homology between X and Y
Only small homology at the Pseudo autosomal regions
X-linked genes
If gene B is located on X, gene B is X-linked
If gene B which is located on X controls eye color,
then eye color is
Sex-linked (or X-linked)
If gene B is X-linked and has a dominant and recessive allele
Male genotype/phenotypes
BY=> wild-type (Hemizygous)
bY=> Mutant (Hemizygous)
If gene B is X-linked and has a dominant and recessive allele
Female genotype/phenotypes
BB-> Wild-type (homozygous)
Bb=> Wild-type
bb->mutant
Hemizygous
Possessing only one copy of a gene
Sex linked effects were first studied by
Thomas Hunt Morgan
Reciprocal cross
Swapping the sex of the parental phenotypes to prove if a gene is on an autosomal chromosome
Red-green color perception is
X-linked recessive
X-Linked color blindness progeny
Female has to inherit mutant X alleles from both parents to be affected
Males only need to inherit mutant X from mother to be affected
Thus, more common in males
Sex-linked traits can have
multi-generational effects:
- Trait can alternate which sex
has the phenotype (Mother
then son) - Trait can skip a generation
(Father does not pass it to son
but instead to grandson)
Not all genes on X show
Sex-linked inheritence
Some regions of X and Y are
Homologous
Genes in this region of X will have two copies in both males and females
Thus, this region will not display sex-linked inheritance
Holandric traits
Traits dictated by Y chromosome genes
- present only in males
- always inherited from the father
- If father has trait, then every son will
have it
Dosage compensation
Mechanisms to
equalize the amount
of protein produced
by X-linked genes in
males with respect to
autosomal genes and
X-linked genes in
females
Without dosage compensation
Males will have insufficient gene dosage
Flies dosage compensation
In flies,
dosage compensation consists of increasing expression of X-linked
genes only in males
In placental mammals (humans)
Dosage compensation involves
increasing X-linked genes in males
AND females, BUT one X is females is inactivated
Murray Barr
Found Dark bodies inside the nuclei of female cats
Known as Barr bodies
Mary Lyon
made a hypothesis about Barr bodies called Lyon
hypothesis”:
Lyon hypothesis
Within each female cell,
one of the two X
chromosomes is inactivated
Each Barr body is an inactivated X chromosome
Random X-inactivation
The X chromosome that is inactivated is chosen at random
Random X-inactivation example
If cell is heterozygous
for a gene B on X, then
each cell within a
tissue will randomly
choose whether the X
with the B or b allele is
inactivated
Result of X-inactivation
Female placental
mammals are functionally
hemizygous at the
cellular level (only one
copy of gene is produced)
for X–linked genes
This means that females are mosaics for the expression of X-linked genes
Can be seen in calico cats
Random X-inactivation happens
Much earlier in development when there are much less cells
All cells produced by mitosis from an X inactivated cell will also have X inactivated
Mechanisms for Random X inactivation
Step 1- Cell somehow counts how many
X chromosomes are present
Step 2- One X chromosome is selected
for inactivation
Xist (X-inactive specific transcript)
Becomes active in the X chromosomes chosen to be inactive
Makes a very long RNA molecule that coats inactive chromosome
RNA then recruits protein complexes that alter the chromatic structure
This turns the X into a Barr Body (inactivating transcription)
Active X chromosome
Represses transcription of Xist and prevent the RNA coating, keeping it active
Dosage compensation is not
Universal
Some species do not use it and are fine
