Transcript Ch 15: Sex Determination & Sex Linkage

Ch 15: Sex
Determination
& Sex Linkage
• Human somatic cells
contain 23 pairs of
chromosomes
– 22 pairs of autosomes
(same in males &
females)
– 1 pair of sex
chromosomes (XX or XY)
• Females have 2 matching
sex chromosomes
• Males have 1 X and 1 Y
• Sex chromosomes carry genes that
affect other traits
• a gene on one of the sex
chromosomes is called: SEX-LINKED
• When a gene is located on the X
chromosome, females receive 2 copies
of the gene, and males receive only 1
copy
– Example: Color-blindness (c) is recessive
to normal vision (C), and it is located on
the X chromosome; hemophilia
LE 15-4b
P
Generation
Ova
(eggs)
Sperm
F1
Generation
Ova
(eggs)
F2
Generation
Sperm
EXAMPLE PROBLEM:
• A female heterozygous for normal
vision: (we say she has normal vision,
but is a carrier of the colorblindness
allele)
XC Xc
• A male who is colorblind:
Xc Y
What is the probability that:
a) they will have a son who is colorblind?
b) they will have a daughter who is colorblind?
c) their first son will be colorblind?
d) their first daughter will be carrier?
XC Xc
a) 1:4 (25%)
c
X
XC Xc Xc Xc
b) 1:4 (25%)
Y
XC Y
d) 1:2 (50%)
Xc Y
c) 1:2 (50%)
EXAMPLE PROBLEM:
• Hemophilia is a hereditary disease in which the
blood clotting process if defective. Classic
hemophilia results from an abnormal or missing
clotting factor VIII; it is inherited as an Xlinked recessive disorder (h).
• If a man without hemophilia and a woman who
is a carrier of the hemophilia allele have
children, what is the probability that…
H
X Y
x
H
X
h
X
what is the probability that:
a) they will have a daughter with hemophilia?
b) they will have a son with hemophilia?
c) their first son will have hemophilia?
d) their first daughter will be a carrier?
XH
XH
Y
Xh
XH XH XH Xh
XH Y Xh Y
a) 0:4 (0%)
b) 1:4 (25%)
c) 1:2 (50%)
d) 1:2 (50%)
Pedigree Charts
The Big Question…
• It may be easy to see that genes located on
different chromosomes assort independently
but what about genes located on the same
chromosome?
Thomas Morgan’s Research
• Morgan identified more than 50 genes on
Drosophila’s 4 chromosomes.
• He discovered that many seemed to be
“linked” together
– They are almost always inherited together &
only rarely become separated
• Grouped genes into 4 linkage groups
Morgan’s Conclusion
• Each chromosome is actually
a group of linked genes
• BUT Mendel’s principle of
independent assortment still
holds true
• It is the chromosome that
assorts independently!!
– Mendel missed this because 6
of the 7 traits he studied were
on different chromosomes.
So…
• If 2 genes are found on the same
chromosome are they linked forever?
– NO!!
• Crossing-Over during Meiosis can separate
genes
– Increases genetic diversity
Gene Maps
• Alfred Sturtevant was a graduate
student working in Morgan’s lab
part-time in 1911
• He hypothesized that the farther
apart 2 genes are on a
chromosome the more likely they
are to be separated by crossingover
• The rate of at which linked genes
are separated can be used to
produce a “map” of distances
between genes
Alfred
Sturtevant
1891-1970
17%
LE 15-6a
Testcross
parents
Gray body,
normal wings
(F1 dihybrid)
Black body,
vestigial wings
(double mutant)
Replication of
chromosomes
Replication of
chromosomes
Meiosis I: Crossing
over between b and vg
loci produces new allele
combinations.
Meiosis I and II:
No new allele
combinations are
produced.
Meiosis II: Separation
of chromatids produces
recombinant gametes
with the new allele
combinations.
Ova
Gametes
Recombinant
chromosomes
Sperm
LE 15-6b
Sperm
Ova
Gametes
Ova
Testcross
offspring
Sperm
965
Wild type
(gray-normal)
944
Blackvestigial
Parental-type offspring
206
Grayvestigial
185
Blacknormal
Recombinant offspring
Recombination
391 recombinants
 100 = 17%
=
frequency
2,300 total offspring
Gene Maps
• This map shows the
relative locations of
each known gene on a
chromosome
– We have used this
technique to help
construct maps of the
Human Genome
LE 15-7
Recombination
frequencies
9%
9.5%
17%
b
cn
vg
Chromosome
Recombination frequency between b and cn: 9%
Recombination frequency between vg and cn: 9.5%
Recombination frequency between b and vg: 17%
IB Linkage Symbolism
• Instead of writing TtBb for a two-gene cross
(indicating the genes are on different
chromosomes), you will see the following
TB
notation indicating the genes are linked:
– Imagine each line is a chromosome and the 2
lines are the homologous pair
– What would be an example of a recombinant?
– What process led to this recombinant?
tb
LE 15-11
Barr bodies: during embryological development, 1 of
the 2 “X” sex-chromosomes in each cell randomly
condenses and is inactivated
Two cell populations
in adult cat:
Active X
Early embryo:
Orange
fur
X chromosomes
Cell division
Inactive X
and X
chromosome Inactive X
inactivation
Black
fur
Allele for
orange fur
Allele for
black fur
Active X
-1 gene for fur color is
located on the “X”
chromosome
-orange and black are
alleles
-which embryological cell
in the female inactivates,
determines the coat color
LE 15-12
Meiosis I
Nondisjunction
Meiosis II
Nondisjunction
Gametes
n+1
n+1
n–1
n–1
n+1
n–1
n
Number of chromosomes
Nondisjunction of homologous
chromosomes in meiosis I
Nondisjunction of sister
chromatids in meiosis I
n
If a condition is sex linked…
A.
B.
C.
D.
It is inherited on the “X” sex chromosome only
It is inherited on the “Y” sex chromosome only
It is inherited on either the “X” or the “Y” sex chromosome
and you need to inherit 2 copies
It is inherited on either the “X” or the “Y” sex chromosome
How is a gene map produced?
A.
B.
C.
D.
Using known gene positions and then hypothesizing the
locations of nearby genes
Using electron microscopes to see the locations of specific
genes that have been radioactively stained
Using recombination frequencies which show that genes
with low percents of recombination are closer together
Using mutation frequencies which show that genes with
low percents of mutation are closer together
If two genes are linked, what does this mean?
A.
B.
C.
D.
They are part of the same haploid gamete
They can only be inherited together
They are located on the same chromosome
They are located on either the “X” or “Y” chromosome
In a testcross of T b what
tB
would be an example of a phenotypic recombinant?
(T=
tall;
B=
black)
A. Tall-white
B.
C.
D.
Tall-black
Short-white
All of the above are recombinants
If Genes A and B recombine at a 12% frequency and
Genes A and C recombine at a 7% frequency, which
Genes are closest together?
A.
B.
C.
D.
Genes A and B
Genes B and C
Genes A and C
You can’t tell from the information provided.