Genetics and Selection
What you must understand
Mendel's laws: uniformity of first-generation hybrids (dominance), segregation of alleles in the second generation (3:1 phenotype, 1:2:1 genotype for a monohybrid cross), independent assortment for unlinked genes (9:3:3:1 dihybrid), and the purity of gametes - each gamete carries one allele of a pair. Use a Punnett square or probability multiplication. Linked genes violate independent assortment; crossing over recombines them.
Variability: combinative (crossing over, independent assortment, random fertilization), mutational (gene, chromosomal, genomic), and modification (environmental, non-heritable). Sex genetics: in humans XX female, XY male; the father's gamete determines sex. Sex-linked traits pass on the X chromosome.
Selection: natural selection acts through differential survival and reproduction; artificial selection is human-directed and produces breeds and varieties. Stabilizing selection favors the mean, directional shifts it, disruptive favors extremes.
Traps
- 3:1 is phenotype; 1:2:1 is genotype in a monohybrid cross.
- A recessive trait can appear in offspring of unaffected parents only if both are carriers.
- Modification variation is not inherited and does not affect genes.
- Sex-linked recessive traits appear more often in males, who have one X.
Rule of thumb
Convert each parental genotype to gametes first; multiply probabilities for independent events.
Worked example: Two carriers Aa × Aa: gametes ½A and ½a each. Offspring ¼AA : ½Aa : ¼aa, so phenotype 3 dominant : 1 recessive; probability of a recessive child is ¼.