Genetics and Inheritance
This domain tests inheritance, DNA, cell division and the interpretation of genetic crosses. You need to use correct symbols, distinguish cell division types, and explain how variation arises.
DNA and cell division
DNA is a double helix. A gene is a section of DNA that codes for a protein; chromosomes are coiled DNA. Mitosis produces two genetically identical diploid cells for growth and repair. Meiosis produces haploid cells: four sperm in males, and one ovum plus polar bodies in females. Meiosis introduces variation by crossing over and independent assortment.
Inheritance
Genotype is the allele combination; phenotype is the observable characteristic. Dominant alleles are expressed when present; recessive alleles only when two are present. A monohybrid cross uses a Punnett square. Sex in humans is determined by XX (female) and XY (male); the father's sperm determines sex.
Worked example
Two heterozygous brown-eyed parents (Bb × Bb) have a child. The gametes are B or b from each parent. The Punnett square gives BB, Bb, Bb, bb. Three of four combinations are brown-eyed, one is blue-eyed, so the expected ratio is 3:1 and the probability of blue eyes is 25%.
Traps
- 'Dominant' does not mean more common in the population.
- Meiosis halves the chromosome number; mitosis keeps it constant.
- Genotype letters should be clearly defined, e.g. B = brown, b = blue.