Genetics and Heredity
What to know
Genetics begins with the relationship between DNA, genes, chromosomes, and traits. A gene is a segment of DNA encoding a product; alleles are variants. Know the vocabulary of genotype (the allele combination) and phenotype (the observable trait), and distinguish dominant, recessive, homozygous, and heterozygous conditions. Master Mendelian patterns: segregation of alleles, independent assortment of genes on different chromosomes, and the predicted ratios from monohybrid (3:1) and dihybrid (9:3:3:1) crosses using Punnett squares.
Move beyond simple dominance to incomplete dominance (blended phenotype), codominance (both traits expressed), multiple alleles (as in ABO blood type), sex-linked inheritance (X-linked traits appear more often in males), and polygenic traits (continuous variation). Understand meiosis as the source of genetic variation through crossing over and independent assortment, and contrast it with mitosis. Know the structure of DNA (double helix, complementary base pairing: A-T, G-C) and the processes of replication, transcription, and translation. Mutations change sequence and may alter phenotype.
Traps
- Phenotype vs. genotype. Two organisms with the same phenotype can carry different alleles.
- Probability confusion. Each offspring is an independent event; past outcomes do not change the next cross's ratio.
- Carrier questions. A recessive carrier shows the dominant phenotype but can pass the recessive allele.
- Sex-linked reasoning. Males have one X, so a single recessive X-linked allele is expressed.
Rule of thumb
Write parental genotypes first, then set up the square. For a recessive disorder, an affected child means each parent contributed a recessive allele.
Worked example
Two heterozygous parents (Aa x Aa) for a recessive trait. The Punnett square gives AA, Aa, Aa, aa. Probability of an affected (aa) child is 1/4, and 3/4 show the dominant phenotype; two-thirds of those showing the dominant phenotype are carriers.