Transcript Figure 15.1 The chromosomal basis of Mendel`s laws

The Chromosomal Basis of Mendel’s Laws
Mendel’s “principle of segregation”
a.pairs of genes on homologous
chromosomes separate
during gamete formation (meiosis)
and end up in different gametes.
b. the fusion of gametes at
fertilization pairs genes once again.
Ex: The alleles for height in Mendel’s
pea plants end up in separate
gametes. Tt X Tt could yield four
the offspring: TT, Tt, Tt, tt.
• Mendel’s “principle of independent
assortment”
a. each pair of alleles on
nonhomologous chromosomes
segregates independently of other
pairs during gamete formation
Ex: When Mendel crossed F1
plants that were heterozygous for
round yellow peas, some F2 plants
did not resemble the parent plants.
Morgan’s First Mutant
Sex-linked Inheritance
w+ = red
w = white
Drosophila Testcross
Black and Vestigial
Wild Type (Gray and Normal)
Gray and Normal
Black and Vestigial
Evidence for linked genes in Drosophila
Non-Parental
Phenotype
Recombination Due to Crossing Over
Recombination Due to Crossing Over
If the two genes were on different
chromosomes, the alleles from the F1
dihybrid would sort into gametes
independently, and we would expect to
see equal numbers of the four types of
offspring.
Since most offspring had a parental
phenotype it can be concluded that the
genes for body color and wing size are
located on the same chromosome.
Linked genes tend to be inherited
together because they are located near
each other on the same chromosome.
Recombinants have different phenotypes
than the parents.
Morgan, who discovered that genes
were located on chromosomes, proposed
that some process must occasionally
break the physical connection between
genes on the same chromosome.
This is now called crossing over, which
accounts for the recombination of linked
genes.
Sturdevant, a student of Morgan’s,
devised a way to construct genetic maps,
an ordered list of the genetic loci along a
particular chromosome.
He predicted that the further apart two
genes are on a chromosome, the higher
the probability that crossing over will
occur.
Using recombination frequencies to
construct a genetic map
a second crossover
would cancel out the
first and reduce
the observed b-vg
recomb. frequency
1 map unit is equal to 1%
recombination frequency
A genetic map based on recombination
frequencies is called a linkage map and
is based on the assumption that the
probability of a crossover between two
genetic loci is based on the distance
separating the loci.
The distances between genes are
expressed as map units, with one map
unit equivalent to a 1% recombination
frequency.
Partial Genetic Map of a Drosophila
Chromosome
The numbers represent the
distance between that locus
and the locus for aristae
length.
Investigators performed crosses that
indicated bar-eye and garnet-eye
alleles are 13 map units apart,
scallop-wing and bar-eye alleles are
6 units apart, and garnet-eye and
scallop-wing alleles are 7 units apart.
What is the order of these alleles on
the chromosome?
Given the following recombination
frequencies, what is the correct order
of the genes on the chromosome?
A-B, 8 map units; A-C, 28 map units;
A-D, 25 map units; B-C, 20 map units;
B-D, 33 map units
a)A-B-C-D
d) B-A-D-C
b)D-C-A-B
e) D-A-B-C
c)A-D-C-B
The frequency of crossing over between
linked genes A and B is 35 percent; between
B and C, 10 percent; between C and D, 15
percent; between C and A, 25 percent;
between D and B, 25 percent. The sequence
of the genes on the chromosome is:
a. ACDB
c. ABDC
e. ADCB
b. ADBC
d. ABCD
Suppose alleles M and n are linked on one
chromosome, and m and N are linked on the
homologous chromosome. Individuals
homozygous for M and n are mated with
individuals homozygous
for m and N. Their offspring are crossed with
homozygous recessive individuals, and the
following results are recorded:
Mn/mn
15
mN/mn 13
MN/mn 232
mn/mn 240
How many units apart are these genes on the
chromosome?
Some Chromosomal Systems of Sex Determination
Males have 1 X
Sex Chromosome present in ovum
Unfertilized males; fertilized
females
Although female mammals,
including humans, inherit two X
chromosomes, one X chromosome
in each cell becomes almost
completely inactivated during
embryonic development. The
inactive X in each cell of a female
condenses into a compact object
called a Barr body, which lies along
the inside of the nuclear envelope.
If a female is heterozygous for a
sex-linked trait, about half her cells
will express one allele, while the
others will express the alternate
allele.
X inactivation and the Tortoiseshell Cat
Calico Cat
The white areas are
determined by yet another
gene.
Meiotic Nondisjunction
Translocation
Alterations of Chromosome
Structure
Down Syndrome/Trisomy 21
Klinefelter Syndrome
Male with at least 1 extra X chromosome
XYY Karyotype or Jacob
Syndrome
Testing a Fetus for Genetic Disorders