Define key terms (gene, allele, genotype, phenotype)
1. Gene
WHAT: A location on a chromosome that controls one characteristic — e.g. "the gene for flower colour" or "the gene for seed shape."
WHY it matters: Mendel didn't know about DNA, but he reasoned there must be discrete "factors" passed on whole, not blended. We now call these factors genes.
HOW to spot one: A gene is the category of trait (flower colour). It lives at a fixed address called a locus (plural: loci).
2. Allele
WHAT: If the gene is "flower colour," the alleles might be purple (P) and white (p).
WHY: Diploid organisms carry two copies of each gene (one from each parent), one on each homologous chromosome. These two copies can be the same allele or different alleles.
HOW we write them: Capital letter = dominant allele (e.g. ). Lowercase = recessive allele (e.g. ).
- Homozygous = two identical alleles ( or ). "Homo" = same.
- Heterozygous = two different alleles (). "Hetero" = different.
3. Genotype
WHAT: For flower colour the genotype is one of: , , or .
WHY it's separate from phenotype: Because a hidden (recessive) allele still exists in the genotype even when you can't see its effect. carries white but looks purple.
HOW to read it: Two letters per gene (diploid). Each letter is one allele on one chromosome.
4. Phenotype
WHAT: "Purple flowers" or "white flowers." Height, colour, blood type — anything you can observe.
WHY genotype ≠ phenotype: Dominance means one allele can mask another. So different genotypes can give the same phenotype:
Here and are two genotypes, one phenotype.
HOW environment sneaks in: Phenotype = genotype + environment. A plant with "tall" genes starved of water may stay short. So phenotype is expression, not just code.
Derivation-from-scratch: why "2 genotypes → 1 phenotype" must happen
Steel-man your mistakes
Worked examples
Recall Feynman: explain to a 12-year-old (click to reveal)
Imagine each pea plant has a tiny instruction card for flower colour. The card slot is the gene. The words you can write on it — "purple" or "white" — are the alleles. You get two cards, one from mum, one from dad. The exact pair you're holding (purple+white, or white+white) is your genotype. The colour the flower actually turns out is your phenotype. Trick: holding one "purple" card is enough to make the flower purple — purple is bossy (dominant), so it hides the white card. That's why two plants can look the same colour but secretly hold different cards!
Active recall flashcards
What is a gene?
What is an allele?
Gene vs allele in one line?
What is a genotype?
What is a phenotype?
Homozygous vs heterozygous?
Why can two organisms share a phenotype but differ in genotype?
For Pp × Pp, give genotype and phenotype ratios.
Does "dominant" mean stronger or more common?
How many genotypes arise from a single gene with 2 alleles?
Phenotype equation in words?
Connections
- Law of Segregation — why each parent passes exactly one allele per gamete
- Law of Independent Assortment — combining multiple genes
- Dominant and Recessive Alleles — how phenotype collapses genotypes
- Punnett Squares — tool to predict genotype/phenotype ratios
- Homologous Chromosomes — why diploids carry two alleles per gene
- Incomplete Dominance and Codominance — when genotype ≠ phenotype rules change
Concept Map
Hinglish (regional understanding)
Intuition Hinglish mein samjho
Dekho, yeh chaar terms confusing lagte hain par actually simple hain. Gene matlab ek "trait ka category" — jaise "phool ka colour". Yeh DNA pe ek fixed jagah (locus) hoti hai. Allele matlab us gene ki alag-alag versions — jaise purple wala allele (P) ya white wala allele (p). Simple line: gene = sawaal, allele = jawaab.
Ab har plant ke paas do copies hoti hain (ek mummy se, ek papa se). Aapke paas jo do alleles ka pair hai, usko genotype kehte hain — jaise PP, Pp, ya pp. Aur jo aap aankhon se dekhte ho — purple ya white phool — woh hai phenotype. Yaad rakho: Geno = jo genes aapke paas hain, Pheno = jo photo dikhti hai.
Sabse important point: genotype aur phenotype same nahi hote! PP aur Pp dono purple dikhte hain, kyunki P (purple) dominant hai aur p (white) ko chhupa deta hai. Iska matlab 3 genotypes (PP, Pp, pp) se sirf 2 phenotypes (purple, white) bante hain. Isiliye Pp × Pp cross mein genotype ratio 1:2:1 hota hai par phenotype ratio 3:1. Yeh gap hi pure Mendelian genetics ki jaan hai.
Ek galti se bachna: "dominant" ka matlab strong ya zyada common nahi hota — sirf itna ki agar woh allele present hai toh woh dikh jaata hai aur dusre ko hide kar deta hai. Bas yeh chaar definitions clear ho gaye toh Punnett square aur saare ratios khud-ba-khud samajh aa jayenge.