Lec 11-2

Question 1

Sex linked genes

Answer 1

Genes on sex chromosomes that lead to a certain phenotype/disease

Question 2

Autosomal chromosomes pair with

Answer 2

Homologous counterparts

Question 3

X chromosome is

Answer 3

Similar to autosomes

Paired with another homologous X

Question 4

Homology between X and Y

Answer 4

Only small homology at the Pseudo autosomal regions

Question 5

X-linked genes

Answer 5

If gene B is located on X, gene B is X-linked

Question 6

If gene B which is located on X controls eye color,
then eye color is

Answer 6

Sex-linked (or X-linked)

Question 7

If gene B is X-linked and has a dominant and recessive allele

Male genotype/phenotypes

Answer 7

BY=> wild-type (Hemizygous)
bY=> Mutant (Hemizygous)

Question 8

If gene B is X-linked and has a dominant and recessive allele

Female genotype/phenotypes

Answer 8

BB-> Wild-type (homozygous)
Bb=> Wild-type
bb->mutant

Question 9

Hemizygous

Answer 9

Possessing only one copy of a gene

Question 10

Sex linked effects were first studied by

Answer 10

Thomas Hunt Morgan

Question 11

Reciprocal cross

Answer 11

Swapping the sex of the parental phenotypes to prove if a gene is on an autosomal chromosome

Question 12

Red-green color perception is

Answer 12

X-linked recessive

Question 13

X-Linked color blindness progeny

Answer 13

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

Question 14

Sex-linked traits can have
multi-generational effects:

Answer 14

1. Trait can alternate which sex
   has the phenotype (Mother
   then son)
2. Trait can skip a generation
   (Father does not pass it to son
   but instead to grandson)

Question 15

Not all genes on X show

Answer 15

Sex-linked inheritence

Question 16

Some regions of X and Y are

Answer 16

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

Question 17

Holandric traits

Answer 17

Traits dictated by Y chromosome genes

1. present only in males
2. always inherited from the father
3. If father has trait, then every son will
   have it

Question 18

Dosage compensation

Answer 18

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

Question 19

Without dosage compensation

Answer 19

Males will have insufficient gene dosage

Question 20

Flies dosage compensation

Answer 20

In flies,
dosage compensation consists of increasing expression of X-linked
genes only in males

Question 21

In placental mammals (humans)
Dosage compensation involves

Answer 21

increasing X-linked genes in males
AND females, BUT one X is females is inactivated

Question 22

Murray Barr

Answer 22

Found Dark bodies inside the nuclei of female cats

Known as Barr bodies

Question 23

Mary Lyon

Answer 23

made a hypothesis about Barr bodies called Lyon
hypothesis”:

Question 24

Lyon hypothesis

Answer 24

Within each female cell,
one of the two X
chromosomes is inactivated

Each Barr body is an inactivated X chromosome

Question 25

Random X-inactivation

Answer 25

The X chromosome that is inactivated is chosen at random

Question 26

Random X-inactivation example

Answer 26

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

Question 27

Result of X-inactivation

Answer 27

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

Question 28

Random X-inactivation happens

Answer 28

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

Question 29

Mechanisms for Random X inactivation

Answer 29

Step 1- Cell somehow counts how many
X chromosomes are present
Step 2- One X chromosome is selected
for inactivation

Question 30

Xist (X-inactive specific transcript)

Answer 30

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)

Question 31

Active X chromosome

Answer 31

Represses transcription of Xist and prevent the RNA coating, keeping it active

Question 32

Dosage compensation is not

Answer 32

Universal

Some species do not use it and are fine