L2 Genetic Variability PDF

11.2 Mechanisms That Generate Genetic Variability Genetic variability is the main evolutionary advantage of sexual reproduction Variability increases the chance that at least some offspring will survive and reproduce in changing environments Sources of Variability During meiosis and...

11.2 Mechanisms That Generate Genetic Variability Genetic variability is the main evolutionary advantage of sexual reproduction Variability increases the chance that at least some offspring will survive and reproduce in changing environments Sources of Variability During meiosis and fertilization, genetic variability arises from three sources: Crossing-over between paired homologous chromosomes (genetic recombination) Independent assortment of chromosomes segregated to the poles during anaphase I The particular sets of male and female gametes that unite in fertilization Crossing-Over between Homologous Chromosomes Exchange of chromosome segments between homologous chromosomes (crossing-over) occurs during prophase I If there are genetic differences between the homologs, crossing-over can produce new allele combinations in a chromatid After the second meiotic division, four nuclei are produced, each receiving one of the four chromatids (chromosomes) Crossing-Over (cont.) Two nuclei receive unchanged chromosomes (parental chromosomes) and two receive chromosomes with new combinations of alleles (recombinant chromosomes) Therefore, crossing-over is a mechanism for genetic recombination – it produces genetic recombinants One or more crossing-over events may occur in the same chromosome pair, involving the same or different chromatids Homologous pair Sister chromatids ParentalsRecombina nts Crosso ver (chias ma) 1 2 3 4 Stepped Art Fig. 11-5, p. 23 Independent Assortment A pair of homologous chromosomes consists of one maternal chromosome and one paternal chromosome Independent assortment of chromosomes is the second major source of genetic variability in meiosis In metaphase I, one of each homologous pair is randomly attached to the spindle of each pole The 23 chromosome pairs of humans allow 223 different combinations of maternal and paternal chromosomes Independent Assortment Maternal Combinations possible in chromosomes A B C gametes 1 Paternal 2 chromosomes or 3 4 or 5 6 or 7 8 Fig. 11-6, p. 23 Random Joining of Male and Female Gametes The chance union of gametes increases the variability of sexual reproduction The possible combinations of human chromosomes is 223 (eg, if 3 chromosomes – 2 3 = 8 combinations) The only exception is identical twins, which arise from mitotic division of a single fertilized egg