anthro 101: human biological evolution lecture 3: genetics...
TRANSCRIPT
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Anthro 101: Human Biological Evolution Lecture 3: Genetics & Inheritance Prof. Kenneth Feldmeier [email protected] feldmekj.weebly.com
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What is Genetics???
○ Spend a few minutes discussing…
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Genetics..
○ Genetics is the scientific study of heredity.
○ Heredity is what makes each species unique.
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Who was the man when it came to genetics?
○ https://www.youtube.com/watch?v=CBezq1fFUEA
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Mendel’s Work ○ Mendel used true-breeding plants which means if
they were left to breed with themselves they would produce offspring identical to themselves.
○ Mendel studied 7 different traits in pea plants. ○ A trait is a specific characteristic that varies from
one individual to another.
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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, two different alleles at a gene loci (heterozygous) only one is expressed
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More on Mendel’s Work
○ Hybrids are offspring from parents with different traits.
○ Genes are the chemical factors that determine a trait.
○ The different forms of a gene are called alleles.
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Lets get down to the basics Somatic cells & Gametes both contain DNA
nucleus
mitochondria DNA in nucleus, mtDNA in mitochondria
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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
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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 à different proteins
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Reminders
• Every Thursday AHS 143 ◆ Before Class- I’m at the tables
◆ First Quiz, Tuesday????
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Phenotype: observable traits
The proteins that are built using the recipe.
Genotype: the alleles you carry
The recipe in your DNA.
Genotype / Phenotype
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Questions?
• What are Phenotypes? • What are Genotypes? • Why is the Universal genetic code important?
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What the heck is DNA? http://www.youtube.com/watch?v=8kK2zwjRV0M
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Chromosomes = packages of DNA Cells have 2 versions of each chromosome – we have 23 homologous pairs, 46 total
Homologous Chromosomes
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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
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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
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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
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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
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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
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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
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• 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
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Mendel studied discontinuous (discrete) traits
Darwin observed continuous variation
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MOST traits vary along a continuum
Continuous Traits • Height • Weight • Skin color
Discontinuous Traits • Finger number • Litter size • Rolling tongue
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• 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)
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Linked traits are inherited together
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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
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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
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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 ◆ Over 700 genes ◆ variation in height
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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
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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
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More Questions
• What are some of the causes of variation? • What is Pleiotropy? • Why is recombination important for evolution?
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Reminder
• Quiz 1 next Thursday