MYP Biology: DNA, Genes & Chromosomes FAQ

DNA is the molecule itself — a double helix of nucleotides. A gene is a specific section of that DNA coding for one protein or trait. A chromosome is a tightly coiled DNA molecule wrapped around proteins, visible under a microscope during cell division. Think: molecule, instruction, package.

Humans have 46 chromosomes arranged in 23 pairs — one set of 23 from each parent. It matters because this pairing is exactly how MYP Biology explains inheritance: one allele per gene comes from each parent, which is why offspring share traits with both.

A gene is the section of DNA controlling a particular trait, like eye colour. An allele is one specific version of that gene — brown or blue, for example. Every gene can have two or more alleles; you inherit one allele per gene from each parent.

Genotype is the genetic makeup — the actual alleles an organism carries, like Tt. Phenotype is the observable result — what you actually see, like a tall plant. Two organisms can share a phenotype (tall) while having different genotypes (TT or Tt).

Cross two parents' alleles in a 2x2 grid, one parent's alleles along the top, the other's down the side, then fill in each box with the combined pair. Count the resulting genotypes and phenotypes to get your ratio — usually asked as 3:1 or 1:2:1.

It's assessed mainly under MYP Sciences Criterion A (Knowing and Understanding) and Criterion D (Reflecting on the Impacts of Science), using command terms like define, explain and outline. You'll typically meet it through unit tests, a genetics investigation, or an inquiry into inherited disorders, marked against the official MYP achievement level descriptors 1-8.

The three you'll see constantly are define, describe and explain — each demands a different depth of answer. Define wants a precise, concise statement of meaning. Describe wants detail without reasoning. Explain wants you to give reasons or a cause-and-effect chain, which is where most marks get lost.

Expect a mix: define/describe/explain short-answer questions on terminology, a Punnett square calculation with a genotype/phenotype ratio to interpret, and a data-response or extended question linking genetics to a real scenario like inherited disorders or selective breeding.

The biggest one, hands down: confusing dominant with 'more common' or 'stronger'. Dominant just means it's expressed when present — it has nothing to do with frequency in a population. Close behind: mixing up genotype and phenotype, and miscounting chromosome pairs.

Start with the vocabulary hierarchy — DNA, gene, allele, chromosome — before touching Punnett squares; genetics falls apart fast if the terms are shaky. Then practise crosses until the 3:1 ratio feels automatic, and finish with a past-paper-style Criterion D question on a real genetic issue.

Genetics is more abstract than cells or ecosystems — your child can't observe a gene directly, only its effects, so it demands stronger reasoning rather than memorisation. It's also the first MYP topic combining vocabulary, a mathematical skill (Punnett squares) and ethical evaluation in one unit, which is a lot to juggle at once.

MYP genetics is the groundwork for DP Biology's genetics topic, where Mendelian crosses expand into dihybrid crosses, pedigree analysis and molecular detail like meiosis and mutation. According to the IB, DP Biology's current syllabus (first exams 2025) builds directly on the allele/genotype/phenotype vocabulary introduced at MYP level.

Look for materials that separate terminology practice from calculation practice from evaluation practice, rather than mixing all three at once — that's how the unit is actually assessed. Topical worksheets with mark schemes let a student self-check against real achievement-level descriptors, which is more useful than a textbook summary alone.