Transcript Freeman 1e: How we got there

“Pure”
lines
Reliably
produce
progeny
like
parents.
Now often called inbred or isogenic lines.
Mendel worked with pure lines of garden peas.
Garden peas normally
reproduce by self-pollination.
Mendel
could control
parentage of
crosses.
X
P1 = parents
Pure lines in this case.
F1 = 1st generation
In this example,
one trait dominates
over the other.
F2 = 2nd generation
Produced by selfing.
The hidden trait
re-appears!
Dominance means that the phenotype will reflect the
presence of that allele equally, whether or not the other
allele is present. Short hair is dominant to long hair in trolls.
Dominance does NOT mean:
•Better (there are dominant lethal alleles)
•More common (dominant alleles can be uncommon)
Also -- for many traits the heterozygote is intermediate
between the two homozygotes, reflecting incomplete or
co-dominance. More on this later…..
Segregation=
separation of
two alleles
of parent
into gametes.
“particulate
Inheritance”
Do alleles for different
genes segregate
together?
For example, must
green peas be wrinkly?
Independent
Assortment.
Transmission
for alleles of
one trait is
independent
of transmission
for (most)
other traits.
“Dihybrid cross” between parents heterozygous for two traits.
Get famous 9:3:3:1 ratio of phenotypes.
Testcross confirms predictions of independent assortment.
Recessive parent used to reveal genotype of hypothesized RrYy
(dominant phenotype is difficult to genotype!).
Practice problems for Mendelian principles.
Next, extension of Mendel’s rules:
1)sex-linked genes.
Drosophila has same system.
red eyes
dominant
Females
have 2 copies
of allele;
Males have
only one,
Inherited from
mother.
This slide shows
meiosis to produce
sperm.
Meiosis to produce
eggs would begin
with two X-chroms,
which can cross
over (recombine).
Do sex-linkage problems from
www.biology.arizona.edu
About 10% of
human males
cannot
distinguish
long & medium
wavelengths
(red & green).
The opsin
gene for
short
wavelengths
is on an
autosome.
Which
chromosome
carries the
genes for long
& medium
wavelengths?
Pedigree analysis can reveal
sex-linkage in humans.
These two had 9 kids.
Work backwards to identify
carriers.
Extension of Mendel’s rules:
1) Sex linkage
2) Incomplete/co-dominance
Enzymes generally show co-dominance, with the
heterozygote producing both forms of the enzyme
This slide actually shows
a pcr, so reveals
genotype rather than
phenotype.
But, gels of enzymes
look very much like this,
with the heterozygote
producing two types of
enzymes that migrate
differently, and may have
different activities in
particular environments.
ADH is an example.
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Extension of Mendel’s rules:
1) Sex linkage
2) Incomplete/co-dominance
3) Multiple alleles at a locus
Distribution of ADH* alleles in human populations.
There are generally >2 common alleles in each population.
*Alcohol dehydrogenase-4 Class 1
Red blood cell membranes have stuff
sticking out….proteins, carbohydrate, etc.
The ABO molecules are carbohydrates.
cell
membrane
From http://www.blood.co.uk/hospitals/training/IntBGS/IntBGS02.htm
There are three common alleles at the ABO locus.
Blood type
Membrane
carbohydrates
Genotype
O
none
ii
A
A
IAIA or IAi
B
B
IBIB or IBi
AB
AB
IAIB
What are the dominance relationships among the alleles?
(In some cases, complete and in some co-dominance.)
Extension of Mendel’s rules:
1) Sex linkage
2) Incomplete/co-dominance
3) Multiple alleles at a locus
4) Epistasis -- interactions among alleles of
DIFFERENT loci affect phenotype.
Genes C and B both affect hair color in mice
CC or Cc
Color
cc
BB or Bb
Black Agouti
bb
Albino
Brown Agouti
What will be the F2 ratios of a cross between pure
lines of black and albino mice?
LINKAGE
Image from
http://www.jurassiccoast.com
Alleles on the same chromosome don’t assort independently.
Quantitative traits
Complex diseases influenced by multiple genes
in interaction with the environment:
Diabetes
Cardiovascular disease
Cancer
Alzheimer’s disease
Schizophrenia
Epilepsy….
Go to http://www.ncbi.nlm.nih.gov/mapview/map_search.cgi
Type in Alzheimer; you will see the locations 29 genes currently associated with the disease.
You can then click on a chromosome to see information about each gene.