Genetics vocabulary ladder: genome to phenotype
Genome = the entire DNA of an organism, packaged into chromosomes in the nucleus. A gene is a section of DNA coding for a specific protein; an allele is an alternative form (version) of a gene. Genotype is the alleles present (e.g. ); phenotype is the observable result. Homozygous = two identical alleles (/); heterozygous = two different (). A dominant allele shows with one copy, a recessive allele needs two. Most human features are polygenic — several genes, not one.
Monohybrid cross: parents to gametes to Punnett to ratio
A monohybrid cross tracks a single gene. Write the parental genotypes (), take one allele per gamete, then combine every pairing in a Punnett square. gives in a 3 : 1 phenotype ratio, so a recessive offspring has probability . Heterozygous homozygous recessive () gives 1 : 1. Fertilisations are independent, so two recessive children is . Give a ratio, fraction, decimal or percentage as asked.
Mitosis, meiosis, mutation and natural selection
Mitosis divides a diploid cell into two genetically identical, diploid cells (growth, repair, cloning, asexual reproduction). Meiosis makes four genetically different, haploid gametes; random fertilisation then creates variation. Humans have diploid number 46, haploid 23. Variation is genetic, environmental or both. A mutation is a rare, random inheritable change in DNA — the source of new alleles. Natural selection: an advantage-giving allele lets those individuals reproduce more, so it spreads.
Drawn from real examiner reports.
Inherit the allele, not the trait
In natural-selection answers the ALLELE (the gene version) is inherited, not the "characteristic" or "trait" — and selection works because advantaged individuals survive and REPRODUCE MORE, not because "the numbers increase". Avoid Lamarckian wording ("it changes to adapt"): the advantageous allele already exists from a random mutation before selection acts.
Genetic-cross technique drops easy marks
Put ONE allele per gamete — a gamete is haploid and carries only one allele of each gene, never two. Use distinct upper/lower-case letters (/, not /). Deduce genotypes from the data — do not assume both parents are heterozygous when a pedigree shows one is homozygous. Label the offspring PHENOTYPES, not just genotypes, and state the ratio asked.
Mitosis is not meiosis
Mitosis makes TWO genetically identical, DIPLOID cells; meiosis makes FOUR genetically different, HAPLOID gametes. Vague "cell division" scores zero. A classic error is saying meiosis produces the embryo — really the zygote grows into an embryo by MITOSIS. Nail three contrasts: cell number (2 vs 4), genetic content (identical vs different), chromosome number (diploid vs haploid).
A gene is not an allele
A gene is a section of DNA coding for one protein/characteristic; an allele is one particular VERSION of that gene. The gene for eye colour has a brown allele and a blue allele. The two alleles at a locus are different forms of the SAME gene, so a heterozygote carries one of each version of that one gene, not two different genes.
Genotype vs phenotype
The genotype is the alleles an organism carries (); the phenotype is what you actually observe (brown eyes). Two different genotypes can share a phenotype — and both give the dominant phenotype. When a question asks for the phenotype, give the visible feature (e.g. "black fur"), not the letters; when it asks for the genotype, give the allele pair.
Dominant does not mean most common
A dominant allele is simply the one expressed in a heterozygote — it can be rare in a population, and a recessive allele can be very common. Dominance is about which allele shows in the phenotype, not about frequency. Likewise "recessive" does not mean weak or dying out: a recessive allele is only expressed when two copies are present (homozygous recessive).
Environmental features are not inherited
Variation can be genetic (from alleles, e.g. blood group), environmental (from surroundings, e.g. a scar, a sun-tan, a learned language) or a combination of both (e.g. height, body mass). Only genetic variation is passed to offspring — do not call an environmental feature "inherited". A trait caused only by the environment cannot be selected for or shown in a genetic cross.
Use the fixed genetic-diagram scaffold
Set out every cross the same way: parental genotypes (from the data), gametes with one allele each, a 2x2 Punnett square, then offspring genotypes with PHENOTYPE labels and the ratio asked. Show the working even if you can see the answer — method marks reward it.
Multiply independent probabilities
Each fertilisation is independent, so the chance of two events together is the PRODUCT, not the sum: two affected children from is . An earlier affected child does not change the next one's risk. Give the form asked — fraction, decimal or percentage.
Read a pedigree for recessive carriers
Two unaffected parents with an affected child means the condition is RECESSIVE and both parents are heterozygous carriers (). This fixes the parental genotypes before you draw the cross — the step later marks depend on. An affected child of unaffected parents is never dominant.
Explain an observed ratio that is not 3:1
A real cross rarely gives an exact 3 : 1 because fertilisation is random and the offspring sample is small — chance makes observed numbers deviate from the expected ratio. Explain a mismatch this way, not by calling the cross wrong; a larger sample gets closer to the prediction.
DNA (deoxyribonucleic acid) is stored on chromosomes inside the nucleus of a cell. All the definitions below are the two-element, mark-scheme forms — learn them exactly.
| Term | Precise definition |
|---|---|
| Genome | The entire DNA of an organism |
| Gene | A section of a DNA molecule that codes for a specific protein |
| Allele | An alternative form (version) of a gene |
| Genotype | The combination of alleles an organism has (e.g. ) |
| Phenotype | The observable characteristic(s) that result from the genotype |
| Dominant allele | An allele that is expressed even when only one copy is present |
| Recessive allele | An allele only expressed when two copies are present (homozygous) |
| Homozygous | Having two identical alleles of a gene ( or ) |
| Heterozygous | Having two different alleles of a gene () |
| Mitosis | Division of a diploid cell producing two genetically identical, diploid cells |
| Meiosis | Division producing four genetically different, haploid gametes |
| Mutation | A rare, random change in the genetic material (DNA) that can be inherited |
Most phenotypic features (height, skin colour, mass) are polygenic — controlled by several genes together, not a single gene.
Define the term gene.
In rabbits, the allele for black fur () is dominant to the allele for brown fur ().
A rabbit has the genotype .
(a) State the phenotype of this rabbit. (1) (b) State whether this rabbit is homozygous or heterozygous. (1) (c) Explain what is meant by a recessive allele. (1)
