Heritable variation among individuals

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Heritable variation among individuals

• Read Chapter 5 of your text

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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.

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Discovery of genes

• Heredity was a big problem for Darwin because he
  didnt know how it worked.
• Darwin knew offspring resembled their parents,
  but it was widely believed that heritability was
  a sort of blending process akin to the way
  different paints can be mixed to produce a new
  shade.
• The problem with blending inheritance is that a
  new trait would be diluted in a large population
  and disappear.

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Discovery of genes inheritance is particulate

• Gregor Mendel (1822-1884) proved that inheritance
  is not a blending process.
• Instead he showed that discrete particles (we now
  call them genes) which remain intact through many
  generations carry the hereditary information.
• An individual allele may sometimes be hidden in a
  generation (e.g. a recessive allele as a
  heterozygote), but later reappear intact in a
  later generation when present as a homozygote.
• Demonstrated this with his famous experiments
  using pea plants (see box 5.2 pages 142-143 of
  the text or any introductory biology text for a
  description of Mendels work)

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Gene-centered thinking

• Different versions of genes, which we call
  alleles, are the ultimate target of natural
  selection because they can last for generations
  passing from body to body.
• Changes in population allele frequencies result
  in evolution.
• Important to remember that individual bodies
  built by genes are temporary assemblages of sets
  of genes.

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Gene-centered thinking

• Individual organisms live and die. Each body
  (survival machine in Dawkins term from his book
  the Selfish Gene) is built by a temporary
  collection of genes working together.
• Alleles that work well with others and help to
  build well adapted bodies will become more common
  and those that dont will be disappear.

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Gene-centered thinking

• To illustrate the idea of selection judging
  individual genes 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.
• By randomly making crews and racing boats 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.
• 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 would be favored by
  selection and spread.

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Genes

• Mendel did not know what genes were, but we know
  today that they are made of DNA and that they
  work by coding the structure of proteins.
• Proteins are made of chains of amino acids joined
  together and DNA dictates the identity and order
  in which amino acids are joined together.

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Structure of DNA

• DNA made up of sequence of nucleotides. Each
  nucleotide includes a sugar, phosphate and one of
  four possible nitrogenous bases (adenine and
  guanine both purines, and thymine and cytosine
  both pyrimidines).

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4.1a
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4.
4.1b 4.1d
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Structure of DNA

• The opposite strands of the DNA molecule are
  complementary because the strands are held
  together by bonds between the opposing bases and
  adenine bonds only with thymine and cytosine only
  with guanine.
• Thus, knowing the sequence on one strand enables
  one to construct the sequence on the other
  strand.

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4.2
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Structure of DNA

• The sequence of nucleotides in a gene codes for
  the protein structure as each three nucleotide
  sequence codes for one amino acid in the protein
  chain.

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4.3a
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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.

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Production of protein from DNA requires
transcription and translation
Gene expression process by which information
from a gene is transformed into product
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Ribosomes translate mRNA into protein
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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.

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RNA splicing can create multiple proteins from a
single gene
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Mutations creating variation

• A change in the structure of DNA, which may
  perhaps result in a change in the protein coded
  for, is called a mutation.
• Mutations are the ultimate source of all genetic
  variation.
• A change to a gene can result in a new allele
  (version of a gene) being produced.

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Where do new alleles come from?

• When DNA is synthesized, 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.

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Mutation and genetic variation

• Mutations are raw material of evolution.
• No variation means no evolution and mutations are
  the ultimate source of variation.

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Types of mutations

• A mistake that changes one base on a DNA molecule
  is called a point mutation.

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Examples of point mutations
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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.

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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.

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Types of mutations

• Another type of mutation 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
  non-functional protein.

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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.

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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.

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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

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4.7
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Gene duplication

• Extra sections of DNA are duplicates and 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.

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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.

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Globin genes

• Hemoglobin (the oxygen-carrying molecule in red
  corpuscles) consists of an iron-binding heme
  group and four surrounding protein chains (two
  coded for by genes in the Alpha cluster and two
  in the Beta cluster).

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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.

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From Campbell and Reese Biology 7th ed.
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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.

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Exons (blue), introns (white), number in box is
number of nucleotides.
4.9
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Globin genes

• Different genes in alpha and beta families are
  expressed at different times in development.
• For example, 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.

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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.
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Chromosomal alterations

• Two major forms important in evolution
  inversions and polyploidy.

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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.

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4.10
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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

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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.

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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.

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Polyploidy

• Polyploidy is common in plants, rare in animals.
• Half of all angiosperms (flowering plants) and
  almost all ferns are polyploid.

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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.

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FIG 4.12
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Polyploidy

• If a sterile plant undergoes polyploidy and
  self-fertilization a new species can develop
  essentially immediately.

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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.

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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 isnt sterile. It has
  two copies of each chromosome (e.g. two 1A
  chromosomes, two 3B chromosomes, etc.).

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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 3
  ancestral species.

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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.

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Mutation rates

• Some mutations cause readily identified
  phenotypic changes.
• E.g. Achrondoplastic dwarfism is a dominant
  disorder. An Achrondoplastic individuals
  condition must be the result of a mutation, if
  his parents do not have the condition.

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Mutation rates

• Human estimate is 1.6 loss-of-function
  mutations/genome/generation.
• A comparison on the entire genomes of two human
  children with their parents resulted in an
  estimate of 70 mutations per child.

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Other sources of genetic variation

• A very important source of variation in offspring
  results from sexual reproduction.
• During sexual reproduction new chromosomes are
  produced during the process of meiosis (gamete
  formation) in which homologous chromosome
  exchange segments of DNA.
• In addition, homologous chromosomes independently
  assort into gametes so unique combinations of
  chromosomes occur in each gamete
• Finally, the merger of sperm and egg brings
  together new combinations of chromosomes.

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Independent assortmentensures novel
combinations of alleles
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The link between genotype and phenotype

• The genetic makeup of an individual is its
  genotype.
• The physical appearance of an individual is its
  phenotype.

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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

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Simple polymorphisms can produce differences in
phenotype
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Simple genetic polymorphisms

• Similar simple genetic polymorphisms result in
  various diseases of humans.
• Sickle cell anemia, Tay-Sachs disease and
  Huntingtons 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.)

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Quantitative genetic traits

• Most traits however are not under such simple
  direct control of one or a few genes.
• Traits, such as height, do not exhibit discrete
  catagories. 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.

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Quantitative traits influenced by multiple genes
Quantitative traits influenced by multiple genes
generate a normal distribution
Francis Galton (1822-1911)
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Environmental influences on phenotype

• The environment also plays a role in quantutative
  values of traits.
• Environmental influences can be factors such as
  food, but a genes environment includes the
  activity of other genes, which may influence how
  much or even whether a 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.