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Lecture 23

BIOL 1020 Lecture 23: Lecture 23

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Biological Sciences
BIOL 1020
Joy Stacey

Lecture 23 Origins of Genetic Variation Among Offspring The behaviour of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation Why do we expend so much energy on sexual reproduction? creates tremendous amount of variation, which natural selection has selected for Three mechanisms contribute to genetic variation: Independent assortment of chromosomes Homologous pairs of chromosomes orient randomly at metaphase I of meiosis l In independent assortment, each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs Crossing over Crossing over produces recombinant chromosomes, which combine genes inherited from each parent Crossing over begins very early in prophase I, as homologous chromosomes pairs up gene by gene In crossing over, homologous portions of two nonsister chromatids trade places Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome Hair color and eye color are genes that are found very close together on a chromosome There are locations on chromosomes where they readily break and reattach the chiasma As a result of crossing over, new chromatids result that are different from both parents and did not exist before crossing over took place Random fertilization Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg) Crossing over adds even more variation Each zygote has a unique genetic identity ** Point of sexual reproduction is so that each zygote has a unique genetic identity Mendel used scientific approach to identify two laws of inheritance (monk) Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments
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