Transcript The Big Picture: an outline of the concepts covered to date

The Big Picture: an outline of the concepts covered to date
1.
Genes are physical units of hereditary that carry information
from one generation to the next
2. Mendel elucidated the following principles regarding the
inheritance patterns of genes
A. Each individual contains two copies of a given gene
B. Genes have different forms called alleles.
The form that is expressed phenotypically in the
heterozygote is known as the dominant allele. It is an
operational definition
C. These copies (alleles) segregate from one another to form
gametes (There is a single copy of each gene in a gamete)
D. Pairs of genes assort independently from one another during
gamete formation
3. The inheritance pattern of genes parallels the behavior of
chromosomes at meiosis. This generated the hypothesis that
genes reside on chromosomes
A
B
a
A
a
OR
b
b
B
1
The Big Picture
4. Exceptional patterns of chromosome segregation
Non-disjunction: homologous chromosomes migrate to the
same pole during meiosis
The X/X and X/Y sex chromosomal system produces
exceptional segregation patterns because males contain only one
copy of X-linked genes
5. Exceptional expression patterns:
Incomplete dominance,
Co-dominance,
Lethal alleles
No exceptions to Mendellian laws
at the level of the gene, but
Phenotype ratios are modified
6. Genes that reside close to one another on the same
chromosome do not assort independently- linkage
7. Occasionally recombination occurs between these linked
genes. The higher the frequency of recombination between any
two genes, the greater the distance is between them.
Recombination frequencies serve as a useful method of mapping
genes along a chromosome.
2
Ratios
A
a
A
a
A
a
b
B
A
a
b
B
x
A
a
3:1
x
a
a
1:1
x
A
a
b
B
9:3:3:1
x
a
a
b
b
1:1:1:1
3
a
10
b
20
c
20
d
15
e
f
15
5
g
This map means that there is a 20% recombination frequency
between the genes b and c and a 5% recombination frequency
between the genes f and g
Genes very far apart on the same chromosome will appear to
assort independently
How many map units between a and f?
A-f = 80 cM
Recombination freq is not 80%
Recombinant/total =
4
Mendel studied 7 traits that assorted independently.
The only explanation for this behavior is that the genes
controlling these traits are located on different chromosomes.
True
False
Seed color
Flower color
Pod shape
Flower position
Stem length
Pod color
Seed shape
5
The largest distance that can be measured by this technique
is 50MU.
50% also indicates NO LINKAGE
If two genes are very far apart on the same chromosome,
use markers between these genes to more accurately
map the genes
Therefore when you obtain a recombination frequency of 50%
this means that either:
__________________
___________________
6
Interference
Interference: this is a phenomenon in which the occurrence of
one crossover in a region influences the probability of another
crossover occurring in that region.
Interference is readily detected genetically. For example, we
determined the following map for the genes v ct and cv.
Expected double crossovers = product of single crossovers
The expected frequency of a double crossover is the product of
the two frequencies of single crossovers:
DCO=
Total progeny =
Expected number of DCO is
Observed number of DCO =
Reduction is because of interference
7
Factors affecting MU
In most cases the order of genes revealed by mapping techniques
correspond to the order of genes determined by sequencing.
In contrast, actual physical distance between genes does not show
direct correspondence to map units.
-for genes far apart, double, triple etc crossovers affect MU
-hotspots of recombination and recombination deserts
-Species specific differences
Humans 1MU is ~ 1 million bp
Yeast 1MU is ~ 2500 bp
-Sex specific differences
For example markers D12s7 and Pah
males recombination frequencies of 9%
females recombination frequencies of 22%
8
Extreme example: In Drosophila males, there is no recombination
during meiosis
Each chromosome consists of one long strand of DNA (about
30 million bp per chromosome arm).
The most complete physical map would be a description of
sequence of these base pairs (ACTGCCCCGTTTAAATGCGC....)
and a description of where each gene resides in this sequence.
Drosophila X
Recomb
Freq
Cen
Tel
Fw
Wy
Fa
Pa
Real distance
Fa-Pa = 106 bp
Fw-Wy = 0.5x106 bp
9
2m.u lies between pn and fa- these two genes are located near
the telomere
2m.u lie between fw and wy- these two genes are located in
the middle of the chromosome
What can you conclude about the physical distance between
these two sets of genes?
Gene order, but not gene distance, is usually consistent between
genetic and physical maps.
One major reason for this is that the recombination rates are
not equal through the length of the chromosome.
10
Linkage maps in males and females: human chromosome 12.
11
MU to bp
Genetic maps are based on recombination frequencies and
describe the relative order and relative distance between
linked genes.
Remember genes reside on chromosomes.
So what we would like to know is where are the genes located
on the chromosomes
22% Rf = 22MU
What does this mean in terms of chromosomes and DNA?
12
Physical maps
Physical maps provide information concerning the location of
genes on chromosomes
Cytological studies have been successfully used to map genes to
specific regions of a chromosome.
For example in Drosophila in some cells the chromosomes
become highly replicated and exhibit very characteristic
banding patterns:
13
In situ hybridization
Salivary glands
Squash on slide
Denature/Stain polytene chromosomes
label gene probe
Hybridize probe to polytene chromosomes
Autoradiography
14
Polytene chromosomes
Drosophila X chr tip
This map is actually very crude. The Drosophila genome
consists of about 165 Mega base pairs (165 million bp). This
region represents a small fraction (5 to 10 million base
pairs).
15
Mitotic recombination
Although not as frequent, recombination between homologous
chromosomes occurs in mitosis as well as meiosis.
This was first discovered in Drosophila by Kurt Stern and now
has important implications for the origins of some human
cancers
Stern made the following cross:
ｷ
y = yellow body
ｷ
sn = singed bristles
ｷ
y+ = normal body
ｷ
sn+ = normal bristles
The F1 females
These should be phenotypically normal
16
Mitotic recombination
However, some females in a background of normal bristle
and normal body color had
sectors of singed bristle, normal colored tissue
next to sectors of yellow body color normal bristle length.
y+ sn
y sn+
Because these sectors were adjacent to one another, Stern
thought that the two spots must be reciprocal products of the
same event.
That event may be a rare crossover between homologs during
the mitotic divisions.
17
A normal mitotic division would occur as follows:
Replication
Segregation
Genotypically identical
Daughter cells
18
If you have recombination in mitosis
Replication
y+ sn
y sn+
y+ sn
y sn+
Segregation
Genotypically different
Daughter cells
19
Mitotic recombination
20
Rb
While mitotic recombination is rare, it does occur giving rise to
recombinant and wild-type cells.
Formation of homozygosity in cells can be carcinogenic if a
mutated gene becomes homozygous in somatic cells!!!
Retinoblastoma (Rb) is a human tumor that sometimes results
from a mitotic recombination event.
Rb is a childhood cancer of the eye
It occurs from birth to 4 years of age
If discovered early enough, treatment is 90% effective
21
Rb
There are two forms of Rb:
sporadic and hereditary.
Hereditary Rb: patients typically develop multiple eye tumors
involving both eyes.
These tumors develop at an early age.
Siblings often develop the same sort of tumors.
Sporadic Rb (60% of the cases): The development of the eye
tumor is a spontaneous event in the patient with no history of
the disease.
Tumors develop only in one eye.
Occurs later than hereditary Rb
22
Sporadic Retinoblastoma
Rb+/Rb+
(phenotypically normal)
First point mutation
Rb+/Rb- cell
(phenotypically normal)
Second mutation
Rb-/Rb- cell
(eye tumor)
Hereditary Retinoblastoma
Rb+/Rb(phenotypically normal)
One mutation
Rb-/Rb- cell
(eye tumor)
23