Download 1902: Sutton (American) and Boveri (German) Mendel`s genes

1902: Sutton (American) and Boveri (German)
Mendel’s genes during gamete formation behave
like chromosomes during meiosis.
Genes are in pairs; so are chromosomes;
Alleles of genes segregate equally into
gametes: so do homologous chromosomes;
Different genes assort independently; so do
different chromosome pairs.
Theory: Genes are located on chromosomes.
Occurs only in germ cells (precursors to gametes).
One round of replication followed by two cell divisions.
diploid (2n)
Meiosis I (homologous
chromosomes separate)
Meiosis II (sister
chromatids separate).
4 products of meiosis each (potentially) a gamete
haploid (n)
 Reduce the number of chromosomes present in
each gamete so that sexual reproduction does
not lead to an increase in the # of
chromosomes
 Meiosis allows for independent assortment of
alleles (Mixes up the genes) to create a
genetically distinct individual. Genetic diversity
allows for adaptation = evolution.
Major stages
Meiosis I Prophase I
Metaphase
I
Anaphase I
Telophase I
Meiosis II (like mitosis)
Prophase II
Metaphase II
Anaphase II
Telophase II
Prophase I
•  Condensation of
chromosomes
chromosomes can be
visualized, sister
chromatids are joined
at centromeres
•  Homologous chromosomes
pair up
• Recombination takes
place.
•  Nuclear membrane
breakdown
Don’t worry about the different stages of Prophase, Leptotene-Diakinesis
Metaphase I
•  Nuclear splindle forms
•  Paired chromosomes (4 DNA
copies) line up on equatorial plane
Anaphase I
•  Homologous chromosomes separate
Telophase I
•  Nuclear membrane reforms
•  Chromosomes decondense (in some organisms)
•  Cytokinesis generates two daughter cells
Metaphase II
Prophase II
•  Condensation of chromosomes
chromosomes can be
visualized, sister chromatids
are joined at centromeres
•  Nuclear membrane breakdown •  Nuclear spindle fibers form
•  Chromosomes line up at
equatorial plane
Anaphase II
•  Sister chromatids are separated
Telophase II
•  Nuclear membrane reforms
•  Chromosomes decondense
•  Cytokinesis generates four
daughter cells
Meiosis
The number of
homologous
chromosomes (or
the number of
centromeres) is
designated “n”.
The amount (or
concentration) of
DNA is indicated by
“c” (1c is the
amount in the
haploid genome).
C is like the number
of chromatids per
homologous
chromosome.
A
b
B
B
A
A
A
A
B
a
b
b
B a
a
b b
B
a
B
a
B
a
a
a
b
B A
A
b b
4 different types of gametes can be produced in equal numbers.
AB
Ab
aB
ab
A
B
A
A
b
a
B
b
a
B
egg
AABB
A
AABb AaBB
AaBb
A
b
a
B
b
a
9 A-B-: 3 aaB-: 3 A-bb: 1 aabb)
sperm
Validation of the Chromosome Theory Chapter 4
continued, pp 98-106.
To prove the chromosome theory of inheritance, needed to
show that specific traits were transmitted with specific
chromosomes.
Already knew that sex was correlated with the inheritance
of the X or Y, what about other traits.
Drosophila melanogaster
  Short life cycle - 12 days
  Many progeny
  Easy to grow in laboratories
Nomenclature differs from peas
 Genes are named after the first mutant allele, not
the wild type allele
a - recessive allele
a+ - wild type allele (dominant to mutant)
A - dominant allele
A+- wild type (recessive to mutant)
Sex Chromosomes
 Discovered between 1891-1909
 Males: mismatched pair of
flies have X and Y just like humans
chromosomes (XY)
 Females: matched pair (XX)
X and Y are homologs and pair and segregate
during meiosis, even though they are very different.
Genes in differential
region are hemizygous
in males (only 1 copy),
males are neither
homozygous or
heterozygous.
w+
Differential
region
Pairing
region
Genes on sex
chromosomes are not
necessarily involved in
sex functions.
{
X Y
}
Y has very few genes.
The Fly
Room
Thomas H.
Morgan’s
laboratory
Columbia
University
1910
Calvin Bridges, an undergraduate
dishwasher, discovered a white-eyed fly
(normal flies have red eyes) in one of the
fly bottles he was about to wash
P
White
X
Xw/Y (males are
hemizygous)
Xw+/Xw+
OK, red dominant.
X
F1
all daughters
had red, but
1/2 of the
sons had
white
F2
Xw+/Xw+ Xw+/Xw
Xw+/Y
Xw/Y
this is what you
would expect if eye
color gene is on the
X chromosome.
In males, what you see is what you get, genotype = phenotype.
Mutant phenotype (white eye)
was inherited in the same
manner as the X chromosome.
This provided a partial proof of
the chromosome theory.
Definitive proof was done by
Calvin Bridges (now a Ph.D
student) in Morgan’s laboratory.
white
Reciprocal
Cross
P
X
Xw/Xw
F1
X
Xw+/Xw
Xw
Y
Xw+ Xw+ Xw Xw+ Y
Xw
Xw+/Y
Xw/Y white
F2
Xw Xw Xw Y
Mendel’s reciprocal crosses gave similar results, but not these.
Bridges’
confusing
exception:
P
X
XwXw
Xw+Y
Expected F1s: Red-eyed females and White-eyed males. However, a white-eyed female or red-eyed male appeared 1/2000
times.
Bridge’s exceptions to Mendel’s Law of Segregation were
demonstrated cytologically to be exceptions to the regular
segregation of mother’s X chromosomes.
XwXwY
white
Normal
Separation
(disjoining)
of Chromosomes
Xw+O
red
Non-disjunction of chromosomes
a double negative
White females got two X’s from mother instead of 1, and red
males got no X’s from female and just the X from father.
Calvin Bridges proposed:
Sex is determined by the number of
X chromosomes, not by the presence
of the Y chromosome as was always
believed before.
2 X - female
1 X - male
3 or 0 X - lethal (never saw cases of
these)
P
X
XwXw
XwXw
Xw+
Xw+Y
Xw+
Y
Y
Xw Xw Xw+ Xw Xw Xw
Xw
Y
o
Xw+
Y
XwXwY
white
Xw+O
red
Non disjunction can also happen in male, but here phenotypes no different.
Sex Chromosomes in Humans
Also can have XYY males (XYY syndrome, very minor symptoms).
Non-disjunction in humans, can occur at all of the chromosomes at a
low frequency.
Most trisomies (three chromosomes) and monsomies (one
chromosome) die in utero, but with the smallest chromosome 21, they
can live.
Trisomy 21; Down syndrome. 47 chromosomes are present instead of 46 from
duplication of chromosome #21. Extra level of gene product causes phenotype.
Trisomy 21 appears to be related to maternal age, amniocentesis suggested.
1:100
Frequency
1:1500
35
Females gametes
form shortly
after birth, but
complete the final
stages of meiotic
segregation
during ovulation.
Do XX females have twice the
amount of gene products encoded by
the X chromosome as males?
Mammals = Humans
One of X’s is inactivated early in development in females.
Human females are mosaics, each cell makes a
random choice of which X gets inactivated.
Drosophila
Both X’s expressed in females, single X has elevated expression in
males.
Fly females are not mosaics.
Calico cats are almost always female.
X linked coat color gene
O - orange
o - black
Are there any male
Calico cats?
They are very rare and sterile (and are XXY).
black
orange