Inheritance, Variation and Evolution
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Sexual and asexual reproduction
Sexual reproduction involves fusion of gametes from two parents, producing variation. Asexual reproduction involves one parent and produces genetically identical offspring.
The key distinction is variation — sexual reproduction shuffles genetic material, asexual reproduction copies it.
Sexual and asexual reproduction — Key Knowledge
- Sexual reproduction fusion of male and female gametes — offspring genetically different
- Asexual reproduction one parent, no gamete fusion — offspring are clones via mitosis
- Meiosis cell division producing four non-identical haploid gametes
- Advantage of sexual variation helps survival if environment changes
- Advantage of asexual faster, only one parent needed
DNA and the genome
DNA is a double helix polymer that codes for the order of amino acids in proteins. The genome is the entire genetic material of an organism.
Understanding the human genome opened up personalised medicine and tracing human ancestry.
DNA and the genome — Key Knowledge
- DNA deoxyribonucleic acid — two strands forming a double helix
- Codes for proteins determines the order of amino acids
- Genome entire genetic material of an organism
- Genome uses identifying genes linked to disease, understanding migration patterns, understanding evolutionary relationships
Alleles and inheritance
Some characteristics are controlled by a single gene, which may have different forms called alleles. Alleles can be dominant or recessive.
A Punnett square uses these terms to predict the outcomes of a genetic cross.
Alleles and inheritance — Key Knowledge
- Alleles different forms of the same gene
- Dominant allele expressed when one or two copies present — capital letter
- Recessive allele only expressed when two copies present — lowercase letter
- Homozygous two identical alleles — BB or bb
- Heterozygous two different alleles — Bb
- Genotype the alleles an organism has
- Phenotype the physical characteristic expressed
Inherited disorders and sex determination
Some disorders are caused by inheriting particular alleles. Sex in humans is determined by the X and Y chromosomes.
Two carrier parents (Ff) have a 1 in 4 chance of having a child with cystic fibrosis.
Inherited disorders and sex determination — Key Knowledge
- Polydactyly extra fingers/toes — caused by a dominant allele
- Cystic fibrosis thick sticky mucus — caused by a recessive allele
- Sex determination XX = female, XY = male
- Y chromosome carries the gene for male development
Variation and mutation
Variation within a species is caused by genetics, the environment, or both. Mutations are changes in DNA sequence.
Without variation, a population could be wiped out by a single disease or environmental change.
Variation and mutation — Key Knowledge
- Genetic variation differences caused by different alleles
- Environmental variation differences caused by conditions an organism lives in
- Combined variation most characteristics influenced by both genes and environment
- Mutations changes in DNA sequence — most have no effect, some change the protein produced, very rarely beneficial
Natural selection and evolution
Evolution is the gradual change in inherited characteristics of a population over many generations, driven by natural selection.
Natural selection acts on random variation — organisms do not choose to evolve.
Natural selection and evolution — Key Knowledge
- Natural selection individuals best suited to the environment survive and reproduce, passing on their alleles
- Evolution change in inherited characteristics over many generations
- Variation within a population provides the raw material for natural selection
Selective breeding and genetic engineering
Selective breeding is humans choosing organisms with desirable traits to breed together. Genetic engineering directly transfers genes between organisms.
Both methods produce organisms with desired characteristics, but genetic engineering is faster and can cross species barriers.
Selective breeding and genetic engineering — Key Knowledge
- Selective breeding choose desirable characteristics, breed over many generations — e.g. higher crop yields, dog temperaments
- Risk of selective breeding reduces genetic variation — populations vulnerable to disease
- Genetic engineering cutting a gene from one organism and inserting into another — e.g. human insulin gene into bacteria
- Restriction enzymes cut DNA at specific points
- Ligases join DNA sections together
Darwin, Wallace and Lamarck
Darwin and Wallace independently proposed evolution by natural selection. Lamarck's earlier theory of acquired characteristics is now rejected.
Darwin's theory became accepted as the evidence base grew and genetics explained the mechanism.
Darwin, Wallace and Lamarck — Key Knowledge
- Darwin's theory evolution by natural selection — initially controversial because it challenged religious beliefs and lacked evidence for the mechanism of inheritance
- Wallace independently proposed similar theory, worked with Darwin — contributions often underappreciated
- Lamarck proposed acquired characteristics could be inherited — now rejected
Evidence for evolution
Fossils and antibiotic-resistant bacteria provide key evidence for evolution. The fossil record is incomplete.
Antibiotic resistance is evolution happening in real time — reducing overuse of antibiotics slows the process.
Evidence for evolution — Key Knowledge
- Fossils preserved remains or traces in rock — show how organisms changed over time
- Fossil formation mineralisation, casts/impressions, preservation in amber/ice/peat
- Incomplete fossil record soft tissue decays, many organisms didn't fossilise, geological activity destroys fossils
- Antibiotic-resistant bacteria observable natural selection — bacteria evolve resistance when exposed to antibiotics
- Extinction causes new predators, new diseases, environmental change, catastrophic events, new competitors
Speciation
Speciation occurs when populations of the same species become so different they can no longer interbreed.
Speciation explains how the diversity of life on Earth arose from common ancestors.
Speciation — Key Knowledge
- Speciation formation of a new species
- Geographic isolation populations separated by a physical barrier
- Different selection pressures each population adapts to its own environment
- Cannot interbreed the populations eventually become too different to produce fertile offspring
Classification
Living organisms are classified into groups based on shared characteristics. The three-domain system is based on genetic analysis.
The three-domain system recognised that archaea are fundamentally different from bacteria, despite both being prokaryotes.
Classification — Key Knowledge
- Linnaean system Kingdom, Phylum, Class, Order, Family, Genus, Species
- Binomial naming two-part Latin name — genus + species, e.g. Homo sapiens
- Three-domain system proposed by Carl Woese based on ribosomal RNA analysis — Archaea, Bacteria, Eukaryota
- Evolutionary trees show relationships between organisms based on common ancestors
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Inheritance, Variation and Evolution
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