heritable variation among individuals

Heritable variation among individualsRead Chapter 5 of your text

Heritable variation among individualsVariation 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 genesHeredity was a big problem for Darwin

because he didn’t 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.

Discovery of genes – inheritance is particulateGregor 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 Mendel’s work)

Gene-centered thinkingDifferent 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.

Gene-centered thinkingIndividual organisms live and die. Each body

(“survival machine in Dawkin’s 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 don’t will be disappear.

Gene-centered thinkingTo 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.

GenesMendel 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.

Structure of DNADNA 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]).

4.1a

4.

4.1b + 4.1d

Structure of DNAThe 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.

4.2

Structure of DNAThe 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.

4.3a

Transcription and translationTo 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 proteinThe 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 variationA 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.

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.

Mutation and genetic variationMutations are raw material of evolution.

No variation means no evolution and mutations are the ultimate source of variation.

Types of mutationsA mistake that changes one base on a

DNA molecule is called a point mutation.

Examples of point mutations

Type of mutationsA 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 mutationsNot 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 mutationsAnother 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.

Types of mutationsThere 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 duplicationDuplication 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 duplicationExtra 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.

Globin genesHuman 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.

Globin genesHemoglobin (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).

Globin genesAncestral 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.

4.9

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

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.

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 alterationsTwo major forms important in evolution:

inversions and polyploidy.

InversionsA 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

InversionsInversion 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”

InversionsInversions 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.

PolyploidyPolyploidy 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].

PolyploidyPolyploidy is common in plants, rare in

animals.Half of all angiosperms (flowering plants) and

almost all ferns are polyploid.

PolyploidyPolyploidy 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

PolyploidyIf a sterile plant undergoes polyploidy and

self-fertilization a new species can develop essentially immediately.

PolyploidyCross-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.

PolyploidyTriticum 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.).

PolyploidyFurther 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.

Mutation ratesMost 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 ratesSome 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 ratesHuman 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.

Other sources of genetic variationA 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.

Independent assortmentensures novel combinations of alleles

The link between genotype and phenotypeThe genetic makeup of an individual is its

genotype.

The physical appearance of an individual is its phenotype.

Simple genetic polymorphismsThe 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 polymorphismsSimilar simple genetic polymorphisms result

in various diseases of humans.

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 traitsMost 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.

Quantitative traits influenced by multiple genes

Francis Galton (1822-1911)

Quantitative traits influenced by multiple genes; generate a normal distribution

Environmental influences on phenotypeThe 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.