Inheritance, Variation and Evolution

4.6.1.1 Sexual and Asexual Reproduction

Sexual reproduction:

• Involves fusion of gametes (sperm + egg in animals, pollen + egg in plants)

• Leads to genetic variation

• Involves meiosis → gametes are non-identical

Asexual reproduction:

• Only one parent

• No gametes / no fusion

• No genetic variation → offspring are clones (genetically identical)

• Involves mitosis

4.6.1.2 Meiosis

Purpose: Forms gametes with half the number of chromosomes

Process:

• DNA is copied

• Cell divides twice → 4 gametes, each with single set of chromosomes

• Gametes are all genetically different

• At fertilisation: chromosome number is restored → embryo forms → grows by mitosis

4.6.1.3 DNA and the Genome

DNA:

• Polymer of 2 strands forming a double helix

• Stored in chromosomes

Gene:

• Section of DNA coding for a protein (via amino acid sequence)

Genome:

• Entire genetic material of an organism

• Human genome helps:

  • Search for disease genes

  • Understand/treat inherited disorders

  • Trace human migration history

4.6.1.4 Genetic Inheritance

Key terms:

• Gamete = sex cell

• Chromosome = DNA structure in nucleus

• Gene = section of DNA coding for protein

• Allele = different forms of the same gene

• Dominant = expressed if one copy present

• Recessive = expressed if two copies present

• Homozygous = two same alleles

• Heterozygous = two different alleles

• Genotype = combination of alleles

• Phenotype = physical expression of alleles

Examples:

• Single gene traits: fur colour (mice), red-green colour blindness (humans)

Punnett squares:

• Show inheritance and probabilities

• Use ratios and direct proportion

• (HT) Use to predict outcomes in crosses

4.6.1.5 Inherited Disorders

Polydactyly:

• Extra fingers/toes

• Caused by dominant allele

Cystic fibrosis:

• Affects cell membranes

• Caused by recessive allele

Embryo screening:

• Identify carriers of disorders

• Consider economic, social, ethical issues

4.6.1.6 Sex Determination

• 23 pairs of chromosomes in body cells

• 22 control characteristics

• 1 pair = sex chromosomes

  • Female: XX

  • Male: XY

• Use genetic crosses to show inheritance of sex

4.6.2.1 Variation

Variation:

• Differences between individuals in a population

• Caused by:

  • Genetic factors

  • Environmental factors

  • Both combined

Mutations:

• Continuous random changes in DNA

• Most have no effect

• Some influence phenotype

• Rarely create new phenotype → can lead to rapid species change

4.6.2.2 Evolution

Definition:

• Change in inherited characteristics of a population over time via natural selection

Darwin’s theory:

• All species evolved from simple life forms over 3 billion years ago

• Best adapted individuals survive + reproduce → pass on genes

Speciation:

• If 2 populations become too different to breed → new species

4.6.2.3 Selective Breeding

Definition:

• Humans breed organisms for desirable traits

Uses:

• Disease-resistant crops

• Animals producing more meat/milk

• Gentle-natured dogs

• Unusual/large flowers

Risks:

• Inbreeding → increased disease or defects

4.6.2.4 Genetic Engineering

Definition:

• Altering an organism’s genome by inserting a gene from another organism

Examples:

• GM crops: resistant to insects/herbicides, higher yield

• Bacteria producing insulin

Concerns:

• Wildflower/insect population effects

• Uncertain long-term effects on humans

(HT only) Process:

• Enzymes cut out required gene

• Gene inserted into vector (plasmid/virus)

• Vector inserts gene into target cells early in development

4.6.3.1 Evidence for Evolution

Main evidence sources:

• Fossil record

• Antibiotic resistance in bacteria

• DNA shows traits passed in genes

4.6.3.2 Fossils

Types:

• Hard parts preserved (no decay)

• Parts replaced by minerals

• Traces (footprints, burrows)

Limitations:

• Soft-bodied organisms left few traces

• Geological activity destroyed early evidence

Use:

• Show how organisms evolved over time

• Used in evolutionary trees

4.6.3.3 Extinction

Definition:

• No living individuals of a species remain

Causes:

• New predators

• New diseases

• Habitat changes

• Competition

• Catastrophic events (e.g. asteroid impact)

4.6.3.4 Resistant Bacteria

Process:

• Mutations create resistant strains

• Non-resistant bacteria die → resistant survive + reproduce

• MRSA = resistant to many antibiotics

Prevention:

• Don’t overuse antibiotics

• Complete full courses

• Limit use in farming

Challenge:

• New antibiotics take time and money to develop

• Resistance spreads faster than discovery of new antibiotics

4.6.4 Classification of Living Organisms

Linnaean system (Carl Linnaeus):

• Kingdom → Phylum → Class → Order → Family → Genus → Species

• Binomial name = Genus + species

Modern classification (Carl Woese):

• Based on DNA analysis

• Three-domain system:

  • Archaea (extremophiles)

  • Bacteria

  • Eukaryota (plants, animals, fungi, protists)

Evolutionary trees:

• Show relationships based on fossils and DNA evidence