Genetics and Heredity
What you must know
Mendelian genetics: monohybrid and dihybrid crosses, Punnett squares, the law of segregation and independent assortment, test crosses, and predicted ratios (3:1 phenotype from Aa x Aa; 1:1 from a test cross with a heterozygote). Beyond Mendel: incomplete dominance (blended phenotype), codominance (both alleles expressed, as in AB blood group), multiple alleles (I-A, I-B, i), sex linkage (X-linked recessive conditions appear more often in males) and polygenic inheritance. Candidates must interpret pedigrees, karyotypes and family data, and distinguish mitosis from meiosis (two divisions, four haploid cells, crossing over in prophase I, independent assortment in metaphase I, giving genetic variation).
Molecular basics expected here: DNA double helix, complementary base pairing (A with T, G with C), semi-conservative replication, transcription to mRNA, translation at ribosomes using codons, and the consequences of substitution, insertion and deletion mutations.
Traps and a worked example
Traps: applying the probability for one child to all future children; forgetting that a carrier can exist only for a recessive allele; assuming a dominant allele is more common in a population; confusing genotype ratio with phenotype ratio.
Worked example: two carrier parents (Aa x Aa). Offspring genotypes are 1 AA : 2 Aa : 1 aa, phenotypes 3 unaffected : 1 affected. The probability that the next child is affected is 1/4. The probability that two consecutive children are both affected is 1/4 x 1/4 = 1/16. Rule of thumb: multiply probabilities only across independent events such as separate conceptions.