Genetics, reproduction, inheritance and evolution
Genetics explains how characteristics pass from parents to offspring; evolution explains how populations change over time. The exam asks you to use DNA, chromosomes and alleles, complete a genetic cross, and explain natural selection with evidence.
From DNA to characteristics
DNA is a double helix; a gene is a length of DNA coding for a protein; chromosomes are long DNA molecules found in the nucleus. Humans have 46 chromosomes (23 pairs); gametes have 23 and are made by meiosis, which creates variation through independent assortment and crossing over. Mitosis produces identical cells for growth and repair. Alleles are versions of a gene: a dominant allele is expressed in the heterozygote, a recessive allele only when homozygous. Genotype is the alleles present; phenotype is the visible characteristic. Inherited characteristics come from genes; acquired ones (scars, language) do not.
Worked example: monohybrid cross
Both parents are Bb (B = brown eyes, dominant; b = blue). The Punnett square B b × B b gives BB, Bb, Bb, bb. Phenotype ratio 3 brown : 1 blue; genotype ratio 1 BB : 2 Bb : 1 bb. Each child has a 25% chance of being bb, regardless of earlier children. A test cross with a bb partner reveals an unknown genotype.
Evolution by natural selection
Variation exists in a population; selection pressures (predators, food, disease) favour individuals with advantageous adaptations, which survive, reproduce and pass on their alleles. Over generations allele frequencies change. Evidence includes fossils, comparative anatomy and DNA. An adaptation is a feature that increases survival; extinction follows failure to adapt or habitat loss. In humans, XX is female and XY male.
Traps
- 'Dominant' does not mean more common.
- Recessive alleles are not lost in heterozygotes.
- Stretching giraffe necks is Lamarck's idea, not natural selection.