Download 2-HumanGen SexLinked

Sex Ratio in Humans:
• In the early 1900’s Thomas
Hunt Morgan began
breeding a type of fruit fly
called Drosophila
melanogaster.
Morgan observed the Drosophila’s four pairs of
homologous chromosomes
– one pair was different in males and females.
– In females the chromosomes were identical.
– In males the chromosomes were different. One
chromosomes looked like those of the female but the
other was shorter and hook-shaped.
• Morgan called the large chromosome X
CHROMOSOME.
• He called the short, hooked shaped
chromosome the Y CHROMOSOME.
Morgan correctly identified these
chromosomes as the sex chromosomes.
You may want to jot this in the
margin of your notes!
• Why do scientists like
to use fruit flies for
genetic studies?
• They have short life
spans, many babies,
they have just a few
chromosomes that
are very large!
The presence of HETEROMORPHIC
(different shaped) sex chromosomes in one sex
of a species, but not the other, provides a
potential mechanism for producing equal
proportions of male and female offspring.
Determining Birth Rates:
• The actual proportion of male and female
offspring is called the SEX RATIO .
• It can be assessed in two ways.
• The PRIMARY SEX RATIO reflects the
proportion of males and females conceived in a
population.
The SECONDARY SEX RATIO reflects the
proportion that are born.
– The secondary sex ratio in the human
population is determined using worldwide
census data, it does not equal 1.0.
•Sex ratio by country for total population.
• Blue represents more women, red more men
•than the world average of 1.01 males/female.
Sex Ratios and Race:
• For example in the CAUCASIAN
population in the UNITED STATES, the
secondary ratio is 1.06 reflecting 106
males born for each 100 females born.
• In the black population the ratio is 1.0025 .
• In other countries the excess of male
births is even higher. In Korea the
secondary sex ratio is 1.15.
WHY?
• There is no experimental evidence that
suggest why there are more male births
than females.
– There has been a speculation that the
human Y chromosome is smaller than the
X chromosome.
•
The Y bearing sperm are of less
mass and therefore more motile.
– These sperm may have a greater chance
of reaching the egg.
X-Linked:
• The traits that are determined by genes on
the X chromosome are referred to as
X- LINKED Traits.
• Scientists have identified 161 identified
loci on the human X chromosome.
• One example is the inheritance of the
common type of color blindness.
• This condition is caused by a recessive
allele located on the X chromosome.
• We indicate X chromosome having the
two alternative alleles as XC and X c
represents the normal and color-blind
alleles, respectively.
Linkage cont.
• In females there are three different
genotypes XCXC , XCXc, and XcXc ) in
which only the last is color-blind
because the normal allele is dominant in
heterozygotes.
– Males only have one X and a Y. Because
the Y chromosome do not carry the color
blindness gene .
– The males have two possible
combinations: XC Y and XcY normal and
colored blind respectively. There is a test
for color blindness.
Other Sex-linked traits
• Hemophilia is another type of sex linked
disease in which the blood does not clot
normally.
• Most individuals with X-linked forms of
hemophilia are males who have received
the hemophilia allele form their unaffected
carrier mothers.
History of Hemophilia:
• It has been noted
that hemophilia
has afflicted a
number of the
descendants of
Queen Victoria of
England.
Czar Nicholas the II’s wife was named Alexandra and was
a granddaughter of Queen Victoria.
Their son Alexis had hemophilia and was executed with
the rest of his family when he was 14 years of age.
X INACTIVATION
• Females have two alleles for every gene
on the X chromosome, while males have
only one.
• There is an inequality in the expression of
the genes on the X chromosome.
• Early in development of a female one X
chromosome in each cell is
inactivated.
• Which X chromosome is turned off in
each cell ( the one inherited by mother
or the one inherited by the father) is a
matter of chance.
– As a result a female mammal expresses
the X chromosome genes from her father
in some cells and those from her mother in
others.
– Once the X chromosome is inactivated in
one cell all the cells that form when that
cell divides have the same inactivated X
chromosome.
• Because the inactivation occurs early in
development the adult female has
patches of tissue that are phenotypically
different in their expression of sexlinked genes.
• X inactivation alters phenotype NOT
GENOTYPE.
• It is not permanent, because the
inactivation reverses in germ-line cells.
Fertilization does not have an
inactivated X.
• X inactivation can be observed at the cellular
level because the turned off X chromosome
absorbs a stain much faster than the active X
chromosome.
• The nucleus of a female cell in interphase has
one dark-staining X chromosome called a BARR
BODY ( named after Canadian researcher, Murray Barr).
Why does a normal male cell
have no bar body?
• Only one X chromosome,
• and is NOT TURNED OFF.
A striking and familiar
example of X inactivation
is seen in the coat colors
of “calico cats”.
A sex linked gene
confers black, brown,
and orange coat color.
A male cat with the
recessive orange allele
has orange fur.
– Coat color in cats is an X-linked gene, with
alleles for black and orange-brown, so
XBXB and XBY cats will have a black coat,
while XOXO and XOY will have an orangebrown coat. Another possible combination
for female cats would be XBXO. Both of the
color alleles would be expressed, so the
cat would end up being partially brown and
partially black.
• A female who is heterozygous for this
gene has patches of orange and either
brown or black forming a tortoiseshell
pattern that reflects different cells
expressing the two different alleles.
• The earlier the X inactivation the larger the
patches of color, because more cell divisions
can occur afterward. If white patches also appear,
the cat is a calico. Tortoiseshell and calico cats
are almost always female.
Pleiotropy
A gene can sometimes affect another characteristic.
This ability of having multiple effects is called pleiotropy.
Genes that control fur pigmentation in cats may
have an influence on the cats eyes and brain.
Epistasis
• One gene may interfere with the expression of
another gene that is independently inherited.
This is called epistasis. In flower color a P is
required for it to exhibit purple color. PP and Pp
= purple colored flowers. This can only happen
if a dominant allele is present for another
characteristic. PPCc =purple Ppcc = white. The
C characteristic has an effect on the color of the
flower.
Polygenic Traits
• Polygenic traits have 3 sets of alleles for a
characteristic. Skin color is polygenic. There
are 6 genes responsible for this characteristic.
BBBBBB= Very dark pigmentation where as
bbbbbb = the opposite very light pigmentation.
All the other genotypes are intermediates of
these combinations.