错误做法:

Forgetting to circle gametes in genetic diagrams

原因:

Circled gametes are a mandatory mark point in all Edexcel inheritance questions, missing them loses marks even if the rest of the diagram is correct.

正确做法:

Draw a clear circle around every gamete allele in your genetic diagram.

错误做法:

Stating that the mother determines the sex of offspring

原因:

Females only produce X gametes, while males produce X and Y gametes, so the father’s gamete determines the sex of the child.

正确做法:

Always explicitly state that the male parent determines offspring sex, with a 50% chance of each sex.

错误做法:

Confusing mitosis and meiosis functions, stating meiosis is used for growth

原因:

Meiosis only produces haploid gametes for sexual reproduction, growth is a function of mitosis.

正确做法:

Use the GRAC mnemonic for mitosis uses: Growth, Repair, Asexual reproduction, Cloning.

错误做法:

Writing that bacteria adapt to antibiotics by mutating on purpose

原因:

Mutations are random, and occur before exposure to antibiotics. Antibiotics only select for pre-existing resistant bacteria.

正确做法:

Explain that random pre-existing mutations give some bacteria resistance, which is selected for when antibiotics are used.

错误做法:

Using thymine (T) in RNA sequences (Biology-only)

原因:

RNA uses uracil (U) instead of thymine, so writing T in an RNA sequence is a mark-loss error.

正确做法:

Double check base pairing questions: replace T with U for all RNA sequences.

错误做法:

Giving a 9:3:3:1 ratio for monohybrid crosses

原因:

Dihybrid crosses are out of scope for this specification, so 9:3:3:1 ratios are never correct for monohybrid questions.

正确做法:

Use 3:1 for heterozygous × heterozygous monohybrid crosses, and 1:1 for heterozygous × homozygous recessive crosses.