Download Topic 10: Inheritance/Genetics, or Why do we resemble our

Inheritance:
Offspring are of the same species as their parents
Example #5
Inheritance:
Offspring are of the same species as parents
Offspring inherit particular traits, such as skin, hair and
eye colors, hair pattern, earlobe attachment, …
Example #1
Some inherited human traits
Example #11
Inheritance is essential in …
Plant and animal breeding (artificial selection)
and
Evolution by the mechanism of natural selection
Trait can skip a generation
In family histories, a trait sometimes “skips a
generation”:
Present in parent generation (P generation)
Absent in children (F1 generation)
Reappears in grandchildren (F2 generation)
Source of new variability
Occasionally brand new traits appear, providing a
source of variability upon which natural or artificial
selection can act.
General questions:
What explanations did people propose in the past for
the facts of inheritance?
How did we arrive at our present understanding if
inheritance?
History of our understanding of
inheritance
Some long-known “facts of life”
about reproduction
1. We grow within, and come out of, our mother’s
body. The mother contributes at least most of
the matter to a baby.
Some long-known “facts of life”
about reproduction
2. The father has something to do with
reproduction. The father seems to be necessary
to get the baby started, but contributes little, if
any, matter to a baby (semen; pollen).
Some old questions about
parents’ contributions:
Does the mother contribute anything more than just
matter to the offspring? Are mother’s traits passed to
offspring?
What does father contribute? Any matter? His traits?
One early answer: Both parents
contribute to offspring’s traits
Hippocrates (460-377 BC): Greek atomist
philosopher
Both parents contribute to traits of offspring via
particles of “seed matter” from all parts of their
bodies that are combined in the offspring.
One early answer: Both parents
contribute to offspring’s traits
Hippocrates (460-377 BC): Greek atomist philosopher
Both parents contribute to traits of offspring, via
particles of “seed matter” from all parts of their bodies
that are combined in the offspring.
Evidence:
“Hybrids” produced by mating dissimilar parents
are often intermediate, showing traits of both
parents.
A defect in either parent can be inherited by
offspring.
Early answer: Only father
contributes to offspring’s traits,
but not in material “seed”
Aristotle (384-322 BC): Father’s entirely liquid
semen acts upon the substance provided by the
mother (blood) and gives it form , much as a
sculptor gives form to stone to create a statue.
“Spermist” hypothesis
In 1677, van
Leeuwenhoek
observed animallike sperm
(spermatozoa)
swimming in semen
of humans and
other animals, and
proposed that they
grew into children.
Preformationism
Structures of the offspring are present in miniature
form in sperm (spermists) or in egg (ovists), and
merely grow larger during development.
Perhaps a tiny
“humunculus” is
present within
sperm?
Problems with preformationism
1. Preformed structures are not visible, even with
microscope
2. What about homunculi of grandchildren, greatgrandchildren, etc., like nested matreshka dolls?
Two distinct garden pea traits
Height (tall or dwarf)
Seed color (yellow or green)
Garden pea flower structure
Work before Mendel : Crosses by
plant breeders
Results of crossing true-breeding pea varieties:
Some traits are “dominant” over others (“recessive”)
When F1’s self pollinate, recessive trait reappears in
F2 generation
P generation
tall x dwarf 
F1 generation
all tall
“tall is dominant over dwarf”
F2 generation
 mix of tall and dwarf
dwarf trait reappears
Work before Mendel II: Crosses
by plant breeders
Plant breeders before Mendel …
Did not quantify their results (count how many
plants showed each trait)
Work before Mendel II: Crosses
by plant breeders
Plant breeders before Mendel …
Did not quantify their results (count how many plants
showed each trait)
and
Did not formulate any hypotheses to explain their
results
Gregor Mendel (1822-1884)
Austrian monk (Brunn, AUS)
Trained at U. of Vienna to
become science teacher under
some of the top scientists of
his day (physics, chemistry,
biology)
From physicists, gained
appreciation for quantification
From botanists, gained
appreciation for variation of
specific traits among
individuals (rather than
fixation on species “essence”).
Mendel’s research program
Mendel decided to study inheritance of specific traits
in crosses of related plant varieties
He knew he must work with a species that …
Had true-breeding varieties with distinct trait
differences
Permitted artificial pollination, while excluding
foreign pollen
Had normal fertility in successive generations (F1, F2,
…)
Mendel wisely chose to work with garden peas
Mendel’s garden plot today
The 7 pea traits that Mendel
selected for study
Mendel’s inheritance hypothesis:
Each trait in an individual is determined by two, and
only two, genetic “elements” [= “factors” = “genes”]
related to that trait - one received from male gamete
(pollen; sperm), and one received from female gamete
(egg; ovum).
When gametes are produced, each contains only
one element (gene). Fertilization (pollen/sperm +
egg) restores the two-element condition.
Mendel’s inheritance hypothesis:
Each trait in an individual is determined by two, and
only two, genetic “elements” [= “factors” = “genes”]
related to that trait - one received from male gamete
(pollen; sperm), and one received from female gamete
(egg; ovum).
When gametes are produced, each contains only one
element (gene). Fertilization (pollen/sperm + egg)
restores the two-element condition.
These genes come in two types (=two “alleles”), of
which one type is dominant and the other is
recessive. Pure-breeding varieties contain two
copies of the same type of element.
Prediction I: Children (F1)
Yellow allele (Y) dominant; green allele (y) recessive
Prediction II: F1 gametes
2 kinds of eggs
2 kinds of pollen/sperm
Pred. III: Grandchildren (F2)
1/4 of F2 are YY = yellow
2/4 of F2 are Yy = yellow
1/4 of F2 are yy = green
Predicted ratio of yellow
to green is 3/4 to 1/4,
or 3 to 1
Predictions IV:
All of the green F2’s (they are all yy) will be true-breeding
(only green descendents)
1/3 of the yellow F2’s (the YY’s) will be true-breeding
(only yellow descendents)
2/3 of the yellow F2’s (the Yy’s) will have both yellow and
green children, in 3 to 1 ratio.
Results of 8 years work
Dom x Rec
yellow x green
F1
all yellow
F2
Ratio
6022 yellow; 2001 green
3.01 to 1
smooth x wrinkle all smooth
5474 smooth; 1850 wrinkle 2.96 to 1
tall x dwarf
787 tall; 277 dwarf
all tall
2.84 to 1
Etc, etc.
Also: F2 recessives found to be true-breeding
1/3 of F2 dominants found to be true-breeding
2/3 of F2 dominants  F3 with 3 to 1 ratio of dominant to
recessive
ALL RESULTS SUPPORT HYPOTHESIS THAT PAIRED
ELEMENTS DETERMINE THE TRAITS INVESTIGATED
Reaction to Mendel’s 1866
publication: None!
Mendel published in obscure journal (Proceedings of
the Natural History Society of Brunn)
Few were aware of his publication.
Those who were aware were not interested – didn’t fit
then-current paradigm of species essences, etc.
Completely forgotten for next 34 years, until 1900!
Stop
Human chromosomes – 23 pairs
Advances in cell structure and
inheritance 1866 - 1900
Gametes contain half the normal number of
chromosomes – one chromosome from each pair
(division by “meiosis” reduces chromosome number)
In fertilization, normal number of chromosomes is
restored
(Note similarity to behavior of Mendel’s pairs of
“elements”!)
Nucleus determines the traits of the organism
Acetabularia
1900 - Mendel’s paper discovered
Independently discovered within 3 month period by
three biologists: de Vries, Correns, and Tschermak
Each recognized the parallel behavior of Mendel’s
“elements” and chromosomes in the nucleus (in pairs –
one from each parent; pairs separate when gametes
formed; pairs re-established in fertilization)
Could the hereditary “elements” be physically in the
chromosomes?
Evidence that chromosomes
contain the hereditary material:
1905: Nettie
Stevens discovers
that the trait of
gender
(male/female) is
determined by
one particular
chromosome pair
– the XX / XY
pair
Human female and male chromosomes
Female (XX)
Male (XY)
Evidence that chromosomes
contain the hereditary material:
Working with the fruit fly (Drosophila), Thomas Hunt
Morgan and co-workers map the location of genes for
many traits on the chromosomes
Evidence that chromosomes contain
the hereditary material:
Which chromosome component
is the genetic material?
Chromosomes contain two components – deoxyribonucleic acid (DNA) and protein
Which of these is the genetic material?
1944 – Avery, Macleod and McCarthy demonstrate that
DNA is the genetic material of bacteria
1952 – Hershey and Chase demonstrate that DNA is
the genetic material of viruses
About proteins
Protein structure: Chains of 20 kinds of amino acids
Protein functions:
Structure of organisms (structural proteins)
Catalyze reactions within organisms (enzymes)
Our traits result from the properties of our
proteins
About proteins
Protein structure: Chains of 20 kinds of amino acids
Structure of proteins
1. Amino acid sequence = the particular sequence of
different kinds of amino acids in the protein.
Amino acid sequence determines
how proteins fold
local folds =
secondary
structure
Overall folding = tertiary structure
Folded protein shape determines
protein function
Mechanism of inheritance =
Mechanism of specifying and
passing on protein amino acid
sequences
Each species has many thousands of different
kinds of proteins – e.g., about 40,000 different
proteins in a human.
About nucleic acids (DNA and
RNA)
Nucleic acids are chains of 4 kinds of nucleotides
In DNA, sugar is
deoxy-ribose, and
bases can be A, G,
T, or C
In RNA, sugar is
ribose, and bases
can be A, G, U, or
C (U replaces T)
Nucleic acid
chain (DNA)
How can DNA be copied
(replicated)?
How can DNA specify protein
structure?
Chargaff’s Rule
1950: Chargaff analyzed DNA bases from many
species
In all cases, %A = %T and %G = %C
%A
%T
%G
%C
Human
30.3
30.3
19.5
19.9
Frog
26.3
26.4
23.5
23.8
X-ray diffraction by DNA
In 1953, Rosalind Franklin obtained additional
information about the structure of DNA by
photographing the pattern of diffracted x-rays
X-ray diffraction by DNA
Franklin’s interference pattern revealed that:
DNA is a multi-stranded helix 2 nm (billionths of a
meter) in diameter,
Some structural feature repeats every 0.34 nm
Some other structural feature repeats every 3.4 nm
Watson/Crick model of DNA
1953: James Watson and Francis Crick figured out the
structure of DNA based on:
Chargaff’s Rule (%A=%T; %G=%C)
Franklin’s x-ray photo (obtained without her
permission)
Fiddling with precise molecular models
Inspiration – “complementary” pairing of base A with
base T, and of base G with base C
Complementary base pairing
A = T (also A = U)
Lines represent
“hydrogen bonds”
between bases (+/attraction)
G=C
Watson and Crick with their
model of the DNA double helix
DNA
double
helix
DNA
double
helix with
dimensions
DNA replication
DNA
replication
Transcription
(DNAmessenger
RNA) and
Translation
(mRNAProtein)
Translation using mRNA and
transfer RNA’s (tRNA)
Translation using mRNA and
tRNA’s
The genetic code
AUG = “start” codon as well as methionine codon
Transcription and Translation
Mutations create new variability
An error in replication creates a new inheritable
version of the gene (= a “mutation”), yielding a
protein with a different amino acid sequence.
DNA:
CTC

CAC
mRNA codon:
GAG

GUG
Glutamic acid
Valine
Amino acid:
The new protein may be advantageous or
dis-advantageous
Some current research areas:
1. Development – Understanding how cells with the
same genes develop into different kinds of cells,
with different proteins
2. Genetic engineering – introducing new genes into a
species, such as to obtain a better plant, or to
produce a drug, or to cure an inherited disease
3. Human Genome Project – learning the entire
human DNA nucleotide sequence (about 3 billion
bases)
4. Bio-informatics – efficiently using the results of the
HGP (recent large grant to University of Buffalo)
Polar bear mom and cub