Genetics and Heredity
Genetics questions test the vocabulary of inheritance, the difference between mitosis and meiosis, and your ability to carry out and interpret a monohybrid cross.
Core vocabulary
A gene is a section of DNA coding for a protein; alleles are the different versions of a gene. Genotype is the allele pair, phenotype the visible trait. Homozygous means two identical alleles, heterozygous two different ones. A dominant allele shows its effect even when paired with a recessive allele; the recessive phenotype appears only when both alleles are recessive.
Cell division
Mitosis produces two genetically identical diploid cells - growth and repair. Meiosis produces four genetically different haploid gametes, with halving of chromosome number and crossing over, so it is the source of new allele combinations.
Variation
Variation is continuous (height, mass - many genes plus environment) or discontinuous (blood group, earlobes - single genes). Inherited variation comes from alleles; environmental variation from diet, climate and lifestyle; most traits are a mixture. Mutations are random changes in DNA and are the ultimate source of new alleles.
Worked example
Two parents who can both roll their tongue but are heterozygous (Rr) have a child. Set out the cross Rr × Rr: gametes R, r and R, r, giving RR, Rr, Rr, rr. Genotype ratio 1 RR : 2 Rr : 1 rr; phenotype ratio 3 tongue-rollers : 1 non-roller; the probability of a non-rolling child is 1 in 4, or 25%. Note that "3 : 1" is a phenotype ratio, not a genotype ratio - state which one you are writing.
Traps
Using "gene" and "allele" interchangeably; saying meiosis makes two cells; giving a phenotype ratio when the genotype ratio was asked; assuming a dominant allele is the more common one; writing probabilities as "1 : 4" instead of "1 in 4" or "25 %"; forgetting that sex cells are haploid.