Genetics and Heredity
Core concepts
Know Mendel's laws: segregation of alleles and independent assortment of genes on different chromosomes. Use the vocabulary precisely: gene, allele, locus, genotype, phenotype, homozygous, heterozygous, dominant, recessive, codominance, incomplete dominance, multiple alleles, pleiotropy, epistasis. Master monohybrid and dihybrid crosses, test crosses, and the Punnett square. Know sex-linked inheritance (X-linked recessive patterns), sex determination, and pedigree analysis. Understand linked genes and crossing over as a source of recombination. Know chromosomal and gene mutations: deletion, duplication, inversion, translocation, and point mutations (substitution, insertion, deletion, frameshift).
Traps
Independent assortment applies only to genes on different chromosomes or far apart on the same chromosome. A 3:1 ratio requires dominance and independent segregation; a 1:2:1 ratio appears with incomplete dominance. X-linked recessive traits affect males more because males are hemizygous. Phenotype is genotype plus environment. A test cross uses a homozygous recessive partner. Do not confuse a carrier (heterozygous) with an affected individual.
Worked example / rule of thumb
Rule of thumb: "write parental genotypes, set gametes, then square." Worked example: cross AaBb × AaBb with independent assortment gives 9:3:3:1 phenotypes. For an X-linked recessive condition, a carrier mother (X^A X^a) and unaffected father (X^A Y) give 50% of sons affected and 50% of daughters carriers. If a question gives a 1:1 ratio, suspect a test cross with a heterozygote.