Heritable variation among
individuals
 Read Chapter 5 of your text
Heritable variation among
individuals
 Variation provides the raw material of evolution.
 Without variation there could be no selection because
there would be no differences to select for or against.
Discovery of genes
 Heredity was a big problem for Darwin.
 Darwin thought heritability was a blending process.
 The problem with blending inheritance is that a new
trait would be diluted in a large population and
disappear. Natural selection could not work
Discovery of genes – inheritance is
particulate
 Gregor Mendel (1822-1884) proved inheritance is not a
blending process.
 Mendel showed that discrete particles (genes) which
remain intact through many generations carry the
hereditary information.
 Recessive alleles sometimes appear to disappear, but
really are just hidden (as heterozygotes) and can
reappear in later generations.
Gene-centered thinking
 Different versions of genes, are called alleles and
natural selection chooses among them.
 Changes in allele frequencies result in evolution.
Gene-centered thinking
 Individual organisms live and die. Each body
(“survival machine” is Richard Dawkins term from his
book the Selfish Gene”) is built by a temporary
collection of alleles working together.
 Alleles that work well with others to
build well adapted bodies become
more common. Those that don’t, disappear.
Gene-centered thinking
 To illustrate the idea of selection judging individual
alleles from the products they build, imagine trying to
select the best crew of rowers for an 8-man boat from a
large pool of potential rowers.
 How could we do this?
Gene-centered thinking
 By randomly making crews and racing them against
each other and repeating the practice many time you
would eventually realize that certain rowers tended to
be found more often in winning boats and others in
losing boats.
Gene-centered thinking
 Even though strong rowers would sometimes be in
losing boats, on average, they would win more often
than weaker rowers. Using the information on wins
you could then build a very strong crew.
 Similarly, genes that tend to build more successful
bodies on average are favored by selection and spread.
Genes
 A gene is a piece of DNA that codes for a protein.
 Proteins are made of chains of amino acids joined
together.
 A gene determines the identity and order in which
amino acids are joined together to make a protein.
Structure of DNA
 DNA made up of sequence of 4 nucleotides.
 Each nucleotide includes a sugar, phosphate and one
of four possible nitrogenous bases (A,T, G or C).
4.1a
4 + 4.1d
4.1b
.
Structure of DNA
 Opposite strands of the DNA molecule are
complementary.
 Knowing the sequence on one strand enables one
to construct the sequence on the other strand.
4.2
Structure of DNA
 The sequence of nucleotides in a gene codes for the
protein structure as each three nucleotide sequence
(or codon) codes for one amino acid in the protein
chain.
4.3a
Transcription and translation
 To make a protein the DNA must first be transcribed
into an RNA copy (called mRNA for messenger RNA)
and that mRNA translated into a protein or
polypeptide.
Production of protein from DNA requires
transcription and translation
Gene expression: process by which information from a gene is
transformed into product
Ribosomes translate mRNA into protein
One gene one protein
 The expression “one gene one protein” is widely used,
but most genes actually code for multiple proteins
because they join different “exons” the executable or
coding portions of a gene together to make different
proteins. This process is called alternative splicing.
RNA splicing can create multiple
proteins from a single gene
Mutations: creating variation
 A change in the structure of DNA is called a mutation.
 Mutations are the ultimate source of all genetic
variation.
Where do new alleles come from?
 When DNA is copied, an enzyme called DNA
polymerase reads one strand of the DNA molecule and
constructs a complementary strand.
 If DNA polymerase makes a mistake and it is not
repaired, a mutation has occurred.
Types of mutations
 A mistake that changes one base on a DNA
molecule is called a point mutation.
Examples of point mutations
Type of mutations
 A point mutation in a gene coding for the structure of
one of the protein chains in a hemoglobin molecule is
responsible for the condition sickle cell anemia.
Types of mutations
 Not all mutations cause a change in amino acid coded
for. These are called silent mutations.
 Mutations that do cause a change in amino acid are
called replacement mutations.
Types of mutations
 Another type of mutation (insertion/deletion)
occurs when bases are inserted or deleted from the
DNA molecule.
 This causes a change in how the whole DNA strand is
read (a frame shift mutation) and produces a nonfunctional protein.
Types of mutations
 There are multiple other forms of mutations that
involve larger quantities of DNA.
 Genes may be duplicated as may entire chromosomes
or even entire genomes.
 Genes may also be inverted.
Where do new genes come from?
 Mutation can produce new alleles, but new genes are
also produced and gene duplication appears to be most
important source of new genes.
Gene duplication
 Duplication results from unequal crossing over when
chromosomes align incorrectly during meiosis.
 Result is a chromosome with an extra section of DNA
that contains duplicated genes
4.7
Gene duplication
 Extra sections of DNA are duplicates.
 They can accumulate mutations without being
selected against because the other copies of the
gene produce normal proteins.
 Gene may completely change over time so gene
duplication creates new possibilities for gene
function.
Globin genes
 Human globin genes are examples of products of gene
duplication.
 Globin gene family contains two major gene clusters
(alpha and beta) that code for the protein subunits of
hemoglobin.
http://easypediatrics.com/physiology-ofhemoglobin
Globin genes
 Ancestral globin gene duplicated and diverged into
alpha and beta ancestral genes about 450-500 mya.
 Later transposed to different chromosomes and
followed by further subsequent duplications and
mutations.
From Campbell and Reese Biology 7th ed.
Globin genes
 Lengths and positions of exons and introns in the
globin genes are very similar. Very unlikely such
similarities could be due to chance.
Exons (blue), introns (white), number in box is number of nucleotides.
4.9
Globin genes
 Different genes in alpha and beta families are
expressed at different times in development.
 In a very young human fetus, zeta (from alpha
cluster) and epsilon (from beta cluster) chains are
present initially then replaced.
 Similarly G-gamma and A-gamma chains present
in older fetuses are replaced by beta chains after
birth.
4.8
Gestation (weeks)
Post-birth(weeks)
Fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin.
Enhances oxygen transfer from mother to offspring.
Chromosomal alterations
 Two major forms important in evolution: inversions
and polyploidy.
Inversions
 A chromosome inversion occurs when a section of
chromosome is broken at both ends, detaches, and
flips.
 Inversion alters the ordering of genes along the
chromosome.
4.10
Inversions
 Inversion affects linkage (linkage is the likelihood that
genes on a chromosome are inherited together i.e., not split
up during meiosis).
 Inverted sections cannot align properly with another
chromosome during meiosis and crossing-over within
inversion produces non-functional gametes.
 Genes contained within inversion are inherited as a set of
genes also called a “supergene”
Inversions
 Inversions are common in Drosophila (fruit flies)
 Frequency of inversions shows clinal pattern and
increases with latitude.
 Inversions are believed to contain combinations of
genes that work well in particular climatic
conditions.
Polyploidy
 Polyploidy is the duplication of entire sets of
chromosomes.
 A polyploid organism has more than two sets of
chromosomes.
 E.g. A diploid (2n chromosomes) organism can
become tetraploid (4n), [where n refers to one set of
chromosomes].
Polyploidy
 Polyploidy is common in plants, rare in animals.
 Half of all angiosperms (flowering plants) and almost
all ferns are polyploid.
Polyploidy
 Polyploidy can occur if an individual produces diploid
gametes and self-fertilizes generating tetraploid
offspring.
 If an offspring later self fertilizes or crosses with its
parent, a population of tetraploids may develop.
FIG 4.12
Polyploidy
 If a sterile plant undergoes polyploidy and self-
fertilization a new species can develop essentially
immediately.
Polyploidy
Polyploidy
 Cross-fertilization of different species, followed by
polyploidy, was responsible for the development of many
crop plants e.g. wheat.
 Initial cross-fertilization produces sterile offspring, because
chromosomes cannot pair up during meiosis.
Polyploidy
 Triticum monococcum (AA) X wild Triticum (BB) cross
produced sterile hybrid with 14 chromosmomes (AB; 1-7A
and 1-7B). {capitalized letters symbolize species source of
chromosomes, number denotes individual chromosome
e.g. 1A, 3B}
 Polyploidy of first sterile hybrid produced Emmer Wheat T.
turgidum (AABB) which has 28 chromosomes. Emmer
Wheat isn’t sterile. It has two copies of each chromosome
(e.g. two 1A chromosomes, two 3B chromosomes, etc.).
Polyploidy
 Further cross between Emmer Wheat and T.
tauschii which has a total of 14 chromosomes (DD)
produced a sterile hybrid with 21 chromosomes
(ABD).
 Further polyploid error in meiosis produced T.
aestivum Bread Wheat with 42 chromosomes
(AABBDD). Those chromosomes are derived from
three ancestral species.
Mutation rates
 Most data on mutations comes from analysis of loss-
of-function mutations.
 Loss-of-function mutations cause gene to produce a
non-working protein.
 Examples of loss-of-function mutations include:
insertions and deletions, mutation to a stop codon and
insertion of jumping genes.
Mutation rates
 Some mutations cause readily identified phenotypic
changes.
 E.g. Achrondoplastic dwarfism is a dominant disorder.
An achrondoplastic individual’s condition must be the
result of a mutation, if his parents do not have the
condition.
Mutation rates
 Human estimate is 1.6 loss-of-function
mutations/genome/generation.
 A comparison of the entire genomes of two
human children with their parents resulted in an
estimate of 70 mutations per child.
Other sources of genetic variation
 A lot of variation in offspring results from sexual
reproduction.
 New chromosomes are produced during meiosis (gamete
formation). Homologous chromosomes exchange segments
of DNA.
 Homologous chromosomes independently assort into
gametes so unique combinations of chromosomes occur in
each gamete
 Finally, fusion of sperm and egg brings together new
combinations of chromosomes.
Independent
assortment of
chromosomes
ensures novel
combinations of alleles
The link between genotype and
phenotype
 The genetic makeup of an individual is its genotype.
 The physical appearance of an individual is its
phenotype.
Simple genetic polymorphisms
 The traits Mendel studied (fortunately for him) were
simple, discrete traits that were controlled by single
genes.
 When the link between genotype and phenotype is so
simple and direct it is easy to see how genotype affects
phenotype.
 For example, alleles of a single gene controls leaf shape
in the ivy-leaf morning glory
Simple polymorphisms can
produce differences in phenotype
Simple genetic polymorphisms
 Similar simple genetic polymorphisms cause various
human diseases.
 Sickle cell anemia, Tay-Sachs disease and Huntington’s
Disease are all homozygous recessive disorders.
 (Someone with two copies of the disease-causing
allele develops the disorder, heterozygotes and
homozygotes for the “normal” allele do not.)
Quantitative genetic traits
 Most traits however are not under such simple direct
control of one or a few genes.
 Traits, such as height and skin color, do not exhibit
discrete categories. Instead variation is continuous.
 The continuous variation is the result of differences in
genotypes where there many genes contribute to the
value of a trait.
Quantitative traits influenced by
multiple genes
Francis Galton (1822-1911)
Quantitative traits influenced by multiple
genes; generate a normal distribution
Environmental influences on
phenotype
 The environment also affects quantitative values of
traits.
 Environmental influences include the activity of other
genes, which may influence how much or even
whether a different gene is expressed.
 Traits differ in their degree of phenotypic plasticity.
Height can be strongly influenced by diet, but our
number of eyes is not.