Transcript Creation/Evolution

Jeremiah 23:24
24 Can any hide himself in
secret places that I shall
not see him? saith the
LORD. Do not I fill
heaven and earth? saith
the LORD.
©2000 Timothy G. Standish
Beyond Mendel
Timothy G. Standish, Ph. D.
©2000 Timothy G. Standish
When The Ratios Are Wrong
Some traits, when they are tested using
Mendel’s techniques, do not produce a 3:1 or
9:3:3:1 ratio
Example: When disk shaped and long
summer squash are crossed they result in a
F2 phenotypic ratio of 9/16 disk, 6/16 sphere
and 1/16 long; a 9:6:1 ratio instead of the
expected 9:3:3:1 or 3:1
In such cases it is not necessary to abandon
Mendel’s basic principle of independent
assortment of genes or the chromosome
theory that genes occupy specific loci on
©2000 Timothy G. Standish
Explainations
Exceptions to Mendelian ratios may be
accounted for in the following ways:
1 Incomplete or codominance - Two or more
alleles exist, but none is dominant to the other/s
2 Multiple alleles for a single gene
3 Epistasis - In which interactions between more
than one gene result in a trait
4 X-linkage - In which the locus of a gene is on
the X chromosome
5 Sex influenced or limited genes, where
expression is influenced or limited by gender or
©2000 Timothy G. Standish
1 Incomplete Or Codominance
Incomplete or codominance - Two or more
alleles exist, but none are dominant to the
other
Incomplete dominance results in blending of
the parental traits
Example: In four O’clock flowers (and others)
red crossed with white results in pink F1
progeny
X
©2000 Timothy G. Standish
1 Incomplete Or Codominance
In the F2 generation a 1:2:1 ratio results of
red to pink to white P
F1
F2 results show
X
this is not
blended
RCR
WCW
RCW
C
C
C
inheritance
F2 Generation
CR
CW
CR CR CR CR CW
1:
2:
1
CW CRCW CWCW
©2000 Timothy G. Standish
1 Incomplete Or Codominance
Codominant traits show up clearly whether
the other allele is present or not
Example: MN blood group genes in humans
are codominant
M phenotype
MM genotype
Membrane
Cytoplasm
M
Anti M
antibodies
Erythrocyte
M antigen
©2000 Timothy G. Standish
1 Incomplete Or Codominance
Codominant traits show up clearly whether
the other allele is present or not
Example: MN blood group genes in humans
are codominant
N phenotype
NN genotype
Membrane
N
Cytoplasm
Erythrocyte
N
Anti N
antibodies
N
N antigen
©2000 Timothy G. Standish
1 Incomplete Or Codominance
Codominant traits show up clearly whether
the other allele is present or not
Example: MN blood group genes in humans
are codominant
MN phenotype
MN genotype
Membrane
Cytoplasm
Anti M
antibodies
N
Erythrocyte
M and N antigens
Anti N
antibody
©2000 Timothy G. Standish
2 Multiple Alleles
Any gene with two or more alleles is said to have
multiple alleles
Mendel worked with only two allele systems, but
variations from the kind of results he obtained
occur when more than two alleles are involved
Note that while individuals cannot have more
than two alleles for a given gene, populations can
have many different alleles
Human ABO blood types provide an excellent
example of multiple alleles in human populations
©2000 Timothy G. Standish
2 Multiple Alleles
ABO blood types are determined by the presence
of antigens on the surface of erythrocytes in
much the same way as MN blood types
The antigens are oligoscaccharides presented on
the cell surface
Almost everyone makes an oligosaccharide
called “H substance” which is a chain of sugars
joined together in the following order:
L-fructose  b galactose  Nacetylglucosamine
Individuals with O type blood only display the H
©2000 Timothy G. Standish
2 Multiple Alleles
Type A and B blood result from the
presence of enzymes which add a sugar
to the H substance
Type A individuals produce an enzyme
that adds N-acetylgalactosamine to the
galactose in the H substance
Type B individuals express a very
similar enzyme that adds galactose to
the same place
©2000 Timothy G. Standish
2 Multiple Alleles
If neither enzyme is expressed type O blood
results
If the N-acetylgalactosamine adding enzyme is
present type A blood results
If the galactose adding enzyme is present type B
blood is made
If both the N-acetylgalactosamine and galactose
adding enzymes are present, type AB blood
results
As the enzymes are coded for by genes, blood
type is under direct genetic control
©2000 Timothy G. Standish
Epistasis
When a single trait is controlled by more than
one gene epistasis may result
The squash example we started with is an
example of epistasis
Understanding biochemical pathways helps
us understand epistasis
A
1
B
2
A
C
C
1
2
B
D
3
D
©2000 Timothy G. Standish
Epistasis
1
2
A
B
C
Imagine that this pathway produces a red
pigment, C, in flowers and that A is a colorless
precursor and B is a yellow intermediate
A
X
1
B
2
C
If the gene for enzyme 1 was knocked out, the
flower would be colorless
©2000 Timothy G. Standish
Epistasis
1
2
A
B
C
Imagine that this pathway produces a red
pigment, C, in flowers and that A is a colorless
precursor and B is a yellow intermediate
A
1
B
X
2
C
If the gene for enzyme 1 was knocked out, the
flower would be colorless
If the gene for enzyme 2 was knocked out, the
flowers would be yellow
©2000 Timothy G. Standish
Epistasis
1
2
A
B
C
If both genes were knocked out, the flowers
would be colorless
X
1
X
2
A
B
C
Because enzymes can catalyze many
reactions in a short period of time, the
presence of just one copy of a gene is
typically enough to mask the absence of a
bad copy
Thus an individual heterozygous for enzyme 1
©2000 Timothy G. Standish
Epistasis
Consider a cross between a two individuals
heterozygous for both enzyme coding genes
Lets call the functional enzyme 1 gene 1F and
the mutated gene producing nonfunctional
enzyme 1, 1n
We will use the same convention for enzyme
2 with genes 2F and 2n
Our cross would look like this:
F
n
F
n
1 12 2
X
F
n
F
n
1 12 2
©2000 Timothy G. Standish
F
n
F
n
1 12 2
X
F
n
F
n
1 12 2
1F2F
1F2n
1n2F
1n2n
F1F2F2F 1F1F2F2n 1F1n2F2F 1F1n2F2n
F
F
1
1 2
1F2n 1F1F2F2n 1F1F2n2n 1F1n2F2n 1F1n2n2n
1n2F 1F1n2F2F 1F1n2F2n 1n1n2F2F 1n1n2F2n
n
n
12
1F1n2F2n 1F1n2n2n 1n1n2F2n 1n1n2n2n
©2000 Timothy G. Standish
F
n
F
n
1 12 2
X
F
n
F
n
1 12 2
1F2F
1F2n
1n2F
1n2n
F1F2F2F 1F1F2F2n 1F1n2F2F 1F1n2F2n
F
F
1
1 2
1F2n 1F1F2F2n 1F1F2n2n 1F1n2F2n 1F1n2n2n
1n2F 1F1n2F2F 1F1n2F2n 1n1n2F2F 1n1n2F2n
n
n
12
1F1n2F2n 1F1n2n2n 1n1n2F2n 1n1n2n2n
©2000 Timothy G. Standish
A 9:4:3 Ratio
A biochemical pathway like the one discussed
will result in a 9:4:3 ratio as long as there are
two alleles each of which behaves in a simple
dominant/recessive way
The 9:4:3 ratio is really a 9:(3+1):3 ratio
Other possible phenotypic ratios for a dihybrid
cross involving epistasis include:
– 9:7 = 9:(3+3+1)
– 12:3:1 = (9+3):3:1
– 12:4 =(9+3):(3+1)
– 10:3:3 = (9+1):3:3
– 10:6 = (9+1):(3+3)
– 13:3 = (9+1+3):3
A ratio made up of some combination of
9:3:3:1 is generally a good hint that epistasis
©2000 Timothy G. Standish
Agouti Mice - A 9:4:3 Ratio
Brown mice actually exhibit agouti coloration, a
mix of yellow and black with hair strands
alternating yellow and black melanin pigment
Agouti
Yellow
Black
Albino
Mutating a gene coding for an enzyme necessary
to make black pigment results in yellow mice
Mutating an enzyme for yellow results in black
Mice lacking yellow or black are albino
©2000 Timothy G. Standish
Agouti Mice - A 9:4:3 Ratio
Colorless Y
Yellow
Precursor
Colorless B
Precursor
Black
YB
Yb
Agouti
Two possible
explanations
of agouti
Colorless B Black Y Agouti/
Precursor
Yellow
yB
yb
YB
YYBB YYBb
YyBB
YyBb
Yb
YYBb
YYbb
YyBb
Yybb
yB
YyBB
YyBb
yyBB
yyBb
yb
YyBb
Yybb
yyBb
yybb
Cross two agouti
individuals who
are both
heterozygous
YyBb X YyBb
©2000 Timothy G. Standish
Agouti Mice - A 9:4:3 Ratio
Colorless Y
Yellow
Precursor
Colorless B
Precursor
Black
YB
Yb
Agouti
Two possible
explanations
of agouti
Colorless B Black Y Agouti/
Precursor
Yellow
yB
yb
YB
YYBB YYBb
YyBB
YyBb
Yb
YYBb
YYbb
YyBb
Yybb
yB
YyBB
YyBb
yyBB
yyBb
yb
YyBb
Yybb
yyBb
yybb
Cross two agouti
individuals who
are both
heterozygous
YyBb X YyBb
©2000 Timothy G. Standish
Another Example Of Epistasis:
Fruit Shape In Squash, A 9:6:1 Ratio
As mentioned earlier, the F2 generation of a
cross between a disk shaped and a long
squash has a disk:spherical:long ratio of 9:6:1
How can this ratio be explained?
In the F2 generation the following genotypes
must result in the indicated phenotypes
9
3
3
1
A_B_
A_bb
aaB_
aabb
Disk
Sphere
Long
©2000 Timothy G. Standish
4 X-Linkage
Thomas Hunt Morgan was the first to
associate a trait (gene) with a chromosome.
Worked with fruit flies (Drosophila
melanogaster)
In humans and Drosophila, males are XY
Thus males are haploid for the X
chromosome
Because of this, recessive genes on the X
chromosome show up far more commonly in
male than female phenotypes
©2000 Timothy G. Standish
Drosophila Nomenclature
+ = Wild type, phenotype in nature (i.e. red
eyes and round wings)
Mutants are alternatives to the wild type
Fruit fly genes are named after the mutant
Dominant mutations are capitalized (i.e.
Hairless or H and Bar or B)
Recessive mutants are named using lower
case letters (i.e. black or b and white or w)
©2000 Timothy G. Standish
Sex Determination
Two ways in which sex can be determined:
Environment:
Turtles - Temperature of development
Some fish - Social structure
Chromosomes - Three methods:
XO - Haploid/diploid ie bees, haploid males
diploid females
ZW - Heterogametic (ZW) females, homogametic
(ZZ) males, ie birds
XY - Heterogametic (XY) males, homogametic
(XX) females, ie humans and Drosophila
©2000 Timothy G. Standish
Morgan’s Discovery Of An XLinked Drosophila Gene
X+ X+
A white-eyed
male was
discovered
P
X
1/4
Xw X+ Xw X+
Y
X+Y X+Y
F1
X
1/4
Xw
1/2
X+ Xw
X+
X+ X+ Xw X+
Y
X+Y XwY
F2
©2000 Timothy G. Standish
The Key To Morgan’s Discovery
The key to Morgan’s discovery was the
observation that all the white eyed individuals in
the F2 generation were males
Without this vital data on the association of white
eyes with being male, the gene for white eyes
could have been seen as a simple recessive trait on
an autosome
This illustrates the importance of recording all the
data possible and being alert to the possibility of
interesting things being present in the data
“Fate favors the prepared mind” (Louis Pasture)
©2000 Timothy G. Standish
Human X-linked Recessive
Genes
Brown enamel - Tooth enamel appears
brown rather than white
Hemophilia - Two types:
– A - Classic hemophilia, deficiency of
blood clotting factor VIII
– B - Christmas disease, deficiency of
blood clotting factor IX
©2000 Timothy G. Standish
X-linked Recessive Genes
Related to sight
Coloboma iridis - A fissure in the eye’s iris
Color Blindness - Two types:
– Deutan - Decreased sensitivity to green light
– Protan - Decreased sensitivity to red light
Congenital night blindness - Not due to a
deficency of vitimin A
Mocrophthalmia - Eyes fail to develop
Optic atrophy - Degeneration of the optic
nerves
©2000 Timothy G. Standish
Royal Pedigree
Edward Duke of Kent (1767-1820)
Victoria Princess of Saxe-Coburg (1786-1861)
Albert of Saxe-Coburg (18XX-18XX)
Victoria Queen of England (1819-1910)
Victoria
(1840-1901)
Leopold Duke
of Albany
(1853-1884)
Alice
(1843-1878)
Alix (Alexandra)
(1872-1918)
King Edward VII
of England (1841-1910)
Tsar Nicholas II
of Russia (1868-1918)
Olga
Marie
(1895-1918) (1899-1918)
Tatiana
(1897-1918)
Emperor Frederick III of
Germany (1831-1888)
Beatrice
King Alfonso XIII
(1857-1944) of Spain (1841-1910)
Irene
(1866-1953)
Victoria
(1866-1953)
Alexis
(1904-1918)
Anastasia
(1901-1918)
©2000 Timothy G. Standish
5 Sex-Influenced Or Limited Genes
Expression of genes that are not necessarily
on the X chromosome may be influenced by
the gender of the individual
One major reason for this is the impact that
steroid sex hormones have on the expression
of genes
Male pattern baldness is the classic example
of a sex influenced gene in humans
Genotype
BB
Bb
bb
Phenotype
Female
Male
Bald
Bald
Flocculent
Bald
Flocculent
Flocculent
©2000 Timothy G. Standish
Environmental Effects
Genes do not work in
isolation, but their
expression is influenced
Being Himalayan
by their environment
gives a whole new
Just
meaning to the
termas expression of
“brown nosing.”
sex influenced genes
are influenced by the
hormones in their
environment other
environmental variables
impact expresson of
most genes
©2000 Timothy G. Standish