Anthropology 101: Human Biological Evolution

Lecture 3: Genetics and Genomics

Dr. Leanna WolfeLAWolfe@aol.com

Office AHS 303Drop In Hours: Thurs. 5:00-6:30 PM

Why are genetics important?

• Remember Darwin’s postulate #3:

• “Traits are passed from parent to offspring.”

• Genetics is the study of how this information is transmitted

• Fossil record has limits New strides in understanding

human evolution using genetics!• Genetics and you!

Somatic cells & Gametes both contain DNA

nucleus

mitochondria DNA in nucleus, mtDNA in mitochondria

DNA Basics

• 99.99% of all DNA in the nucleus of a cell

• DNA long strands of biochemical information (legos or beads)

• Sections of DNA form functional units = genes

• Genes are recipes for proteins

• Proteins serve functions in the body = traits

Some sections of DNA don’t code for proteins

• Structural genes DO code for proteins, affect phenotype

• Non-coding sections• Regulatory genes, affect phenotype

On/off regulators Rate of production Determine when a gene starts making its protein and

stops making its protein• Epigenetic changes, can affect phenotype

Turn genes on or off without changes to the DNA sequence

Phenotype: observable traits

The proteins that are built using the recipe.

Genotype: the alleles you carry

The recipe in your DNA.

Genotype / Phenotype

Universal Genetic Code

• All living organisms have DNA made of the same material that serves similar functions

• The universality of the genetic code implies a common ancestry for all life on the planet

• Organisms differ in the amount of DNA

• BUT the most important differences are in the arrangement of the DNA.

Different order of nucleotides different proteins

Chromosomes = packages of DNACells have 2 versions of each chromosome – we have 23 homologous pairs, 46 total

Homologous Chromosomes

Genes are segments on chromosomes• Genes on chromosomes like

beads on a string

• Each gene has a specific location = locus

Gene loci

• There can be different versions of the same kind of gene: these are called alleles

• Homologous alleles work together to produce phenotype

Cell Division: Sharing the recipe

• DNA replicates before cell division Two types of replication:

• Mitosis: makes a new somatic (body) cell

• Meiosis: makes gametes (sex cells, sperm and eggs) used in sexual reproduction

Mitosis: replication of somatic cells

• When somatic cells divide

chromosomes are doubled

• Doubles are split between two daughter cells

• Each daughter cell has an identical set of chromosomes to the original cell

Meiosis: production of gametes

• Gametes (eggs and sperm) have only 1 copy of each chromosome

• Chromosome pairs duplicate and divide into singles, distributed between 4 gametes

• When gametes fuse during sexual reproduction, they create a zygote with full set of chromosomes

To make a new organism

Parents make gametes

Gametes from two parents fuse to form offspring

For each chromosome, offspring carry two copies:

one from each parent

Mendel showed simple genetic principles• Segregation

• Traits determined jointly by pairs of alleles

• Either allele can end up in a gamete

• Zygotes get 1 allele from mom, 1 allele from dad

• Dominance• Sometimes, when two different

versions of alleles at a gene loci (heterozygous) only one is expressed

Genotype Phenotype

• Dominant does not mean better, stronger, more adaptive, or more common!!!

• Dominant means that trait gets expressed and hides the other trait

Genotype Phenotype

AA Yellow

Aa Yellow

aa Green

Complexity: Organisms inherit many genes• Genes affecting a trait occur at

particular sites on chromosome (locus)

• Homologous chromosome pairs each have one allele that work together for each gene

• Alleles for genes on same chromosome tend to stay together = linked

• Allele for seed size linked to allele for seed color

• Allele for seed color unlinked to allele for seed texture

Seed color

Seed texture

Chromosome I Homologous pairs

Chromosome II Homologous pairs

Seed size A aB b

dD

Independent Assortment

• Recombination shuffling of alleles during meiosis = unique gametes

Independent assortment of alleles at different gene loci Mixing of alleles during sexual reproduction = unique offspring

• The fate of a new mutation is unrelated to other traits New traits can spread independently in a population

• Novel combinations of traits can appear in offspring This provides new phenotypes for natural selection to act upon

Recombination is important for evolution

Mendel studied discontinuous (discrete) traits

Darwin observed continuous variation

MOST traits vary along a continuum

Continuous Traits• Height• Weight• Skin color

Discontinuous Traits• Finger number• Litter size• Rolling tongue

• Lots of traits are linked and so get inherited as a package deal Linked (same chromosome)

• Lots of alleles for a gene aren’t clearly dominant/recessive Codominance: Sickle-cell anemia Complex dominance: ABO blood type

• Lots of single genes controls multiple traits Pleiotropy

• Lots of genes work together to affect the same trait = Polygenic inheritance

Simple Mendelian inheritance is rare (discontinuous traits)

Linked traits are inherited together

Codominance: Two alleles, three phenotypes

Sickle-cell anemia• Normal hemoglobin (A) allele• Sickling hemoglobin (S) allele

• Three genotypes and three phenotypes:

AA = normal blood cells SS = sickled blood cells AS = slightly impaired blood

cells, greater defense against malaria

Complex Dominance: ABO Blood Type

• Three alleles: A, B & O

• O recessive to A & B

• A & B co-dominant

Genotype Phenotype Compatibility

AAType A

Rec.: A or ODon.: A or AB

AO

AB Type AB Universal recipient

BBType B

Rec.: B or ODon.: B or AB

BO

OO Type O Universal donor

Pleiotropy: One gene controls two traits

• In Darwin’s finches, beak traits are correlated

• Depth & width vary together

Deeper & wider Shallow & narrow

• Correlations arise when one gene affects multiple traits

Polygenic Traits: Many genes, continuous variation

• Many genes affect each trait• Each one has small effect• Generates a continuous

range of variation in the trait

• Height So far approx 10 genes Affect about 30% of

variation in height

All this variation! Where does it come from?

• Mutation• Meiosis

Recombination of alleles into unique gametes increases genetic variation at a faster rate than mutation

• Sexual Reproduction Recombination of alleles from unique gametes into

unique offspring New phenotypes for NS to act upon

• Complex genetic inheritance Polygenic traits Environment interacts with genotype

Polygenic Traits + Environment: Beak Depth

• Multiple genes interact to determine the actual beak phenotype

• Environmental effects blur across genetic differences producing a range of phenotypes

Polygenic Traits shaped by many genes + environment

Phenotypes are almost ALWAYS the product of genes & environment interacting

Variation is maintained in the DNA

• Intermediate types common, but genetic variation is maintained MOST individuals have some + and some – alleles for height

• Recessive alleles hidden by dominant alleles Still passed into gametes & remain in population

• Much of the variation is “hidden” from selection If a trait is affected by genes at many loci Many different genotypes may have similar selective fate Some variation is protected

• Neutral mutations can be hidden

Variation is essential for Natural Selection – without differences in traits, nothing to “select” & survival is random

Speciation: Moving beyond existing variation

How can chihuahuas be smaller than the smallest wolves?

Solution: Hidden Variation

• Normal sized wolves carry some alleles for small body size (some – alleles, and many + alleles)

• As big wolves die (or people prefer small ones), frequency of – alleles increases

• Variation is shuffled, some new combinations arise

• As – alleles become more common, more – alleles likely to be combined in a single individual

• New combinations with more – alleles will be outside initial range of variation

What is the nature of variation?

+

+

• Variation is (usually) continuous

• Continuous traits are generated via many genes and environmental effects to produce the phenotype

• Variation is generated and extended by: Mutation Meiosis Sexual reproduction Hidden alleles Polygenic inheritance + environment

Homozygous vs. Heterozygous Genotypes

• Homozygous: the same allele at the same locus on both versions of the chromosome

• Heterozygous: a different allele at a particular locus on each chromosome

Dominant & recessive alleles

• 2 different alleles (heterozygous) = Aa

• Sometimes one of the alleles “overrides” the effects of other: this is called dominance = A > a

• A dominant allele overrides the effects of a recessive allele

Aa

AA or aa

Gregor Mendel discovered the nature of inheritance

• Monk• Cultivated pea plants

• Focused on traits with 2 forms (allele variants)

Green/yellow seeds Smooth/wrinkled seeds Purple/white flowers

• Cultivated true breeding lines and then studied results of crosses

• Kept careful records

Punnett Square Method

Genotypes• 4 Aa

Phenotypes• 4 Yellow

Aa Aa

Aa Aa

A

a

a

A

Punnett Square Method

Genotypes• 1 AA• 2 Aa• 1 aa

Phenotypes• 3 Yellow• 1 Green

AA Aa

Aa aa

A

A

a

a

Heterozygous x Homozygous Dominant

Genotypes• 2 AA• 2 Aa

Phenotypes• 4 Yellow

AA AA

Aa Aa

A

A

a

A