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Unit 5
8–11% of examThis unit explains how traits pass from parents to offspring. You'll see how meiosis makes egg and sperm cells with half the usual chromosomes and shuffles genes along the way, then use Punnett squares, probability and pedigrees to predict inheritance, including the many traits that don't follow Mendel's simple ratios.
Longer videos that cover the whole unit. Good for a first pass or a final review.
Meiosis is the cell division that makes gametes (eggs and sperm). One diploid cell divides twice, first pulling homologous chromosome pairs apart and then sister chromatids, to make four haploid cells, while mitosis makes two cells identical to the parent.
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Meiosis makes every gamete genetically different: crossing over swaps pieces between homologous chromosomes, each pair lines up in a random direction, and random fertilization mixes things further. When chromosomes fail to separate properly (nondisjunction), gametes end up with too many or too few chromosomes.
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Mendel's law of segregation says each gamete gets just one allele of each gene, and his law of independent assortment says genes on different chromosomes are inherited separately. You'll use Punnett squares, the multiplication and addition rules of probability, pedigrees and chi-square tests to predict offspring and check real results.
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Lots of traits don't give Mendel's ratios. Genes close together on one chromosome tend to be inherited together (their crossover rate gives map distance), and X-linked recessive traits show up more often in XY individuals. A heterozygote may show an in-between phenotype (incomplete dominance) or both phenotypes at once (codominance), one gene can affect several traits (pleiotropy), and genes in mitochondria and chloroplasts usually come only from the mother, through the egg.
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The same genotype can produce different phenotypes in different environments, which is called phenotypic plasticity. Hydrangea flower color shifts with soil pH, arctic foxes grow white fur in winter, and in some reptiles the nest temperature decides whether an egg becomes male or female.
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