Genetics, Variation and Inheritance
What this domain tests
Genetics, Variation and Inheritance examines chromosomes, genes, DNA, cell division, monohybrid inheritance, sex determination, mutation and sources of variation.
Core rules
- Chromosomes are found in the nucleus and are made of DNA. A gene is a section of DNA that codes for a protein; alleles are alternative forms of a gene.
- Mitosis produces two genetically identical diploid cells for growth and repair. Meiosis produces four haploid gametes with genetic variation.
- In a monohybrid cross, a dominant allele is expressed in the heterozygote; a recessive allele is expressed only when homozygous.
- Genotype is the allele combination; phenotype is the observable characteristic.
- A Punnett square predicts offspring ratios. For parents Tt × Tt, gametes are T or t from each; offspring genotypes are TT:Tt:tt in 1:2:1, phenotypes 3 dominant : 1 recessive.
- Sex in humans: XX female, XY male; the father determines the sex of the child.
- Variation arises from mutation, meiosis through crossing over and independent assortment, and random fertilisation; environmental factors also affect phenotype.
Worked example
Two heterozygous tall pea plants (Tt) are crossed. Each parent produces T and t gametes. The Punnett square gives TT, Tt, Tt, tt. Three of four offspring are tall and one is short, a 3:1 phenotypic ratio. The short offspring must be tt; it cannot be Tt because T is dominant. If 200 offspring are produced, the expected number short is 1/4 × 200 = 50.
Traps
Do not write that tt is a tall genotype. Do not confuse genotype with phenotype. Meiosis halves the chromosome number; mitosis keeps it constant. A carrier has one recessive allele but does not show the recessive phenotype. Ratios are predictions, not guaranteed exact counts.