Cells, Genetics and Evolution
What you must understand
This domain connects cell structure and function to inheritance and evolutionary change. You need to know the structure and function of organelles, the difference between prokaryotic and eukaryotic cells, and the processes of mitosis and meiosis, including crossing over and independent assortment as sources of genetic variation. You must understand DNA structure and replication, the relationship between genes, proteins and phenotype, and the patterns of inheritance including dominance, codominance, incomplete dominance, sex linkage, multiple alleles and dihybrid crosses. Mutation is the ultimate source of new alleles, and natural selection, gene flow, genetic drift and reproductive isolation drive change in allele frequencies.
Traps that appear in questions
- Mitosis versus meiosis. Mitosis produces genetically identical diploid cells; meiosis produces four genetically distinct haploid cells.
- Genotype ratios versus phenotype ratios. In a cross of two heterozygotes the genotype ratio is 1:2:1 but the phenotype ratio is 3:1 only with complete dominance.
- Sex-linked crosses. Always track the sex chromosomes; a carrier mother can pass an allele to sons who then express it.
- Evolution is a population process. Individuals do not evolve; allele frequencies in populations change.
- Natural selection is not goal-directed. It does not "try" to produce an adaptation.
Worked example
Cross two heterozygous tall pea plants (Tt x Tt). The Punnett square gives TT, Tt, Tt, tt, so the genotype ratio is 1 TT : 2 Tt : 1 tt and, with complete dominance, the phenotype ratio is 3 tall : 1 short. If the cross were Tt x tt, the offspring would be 1 Tt : 1 tt, a 1:1 ratio. Always write the parental genotypes before drawing the square.