Meiosis and Sexual Life Cycles

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10

Mutations create different versions of genes called alleles

Reshuffling of alleles during sexual reproduction produces genetic variation

Slide 68

Origins of Genetic Variation Among Offspring

The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation

Three mechanisms contribute to genetic variation:

Independent assortment of chromosomes

Crossing over

Random fertilization

Slide 69

Independent Assortment of Chromosomes

Homologous pairs of chromosomes orient randomly at metaphase I of meiosis

In independent assortment, each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs

Slide 70

The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number

For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes

Slide 71

Fig. 13-11-1

Possibility 1

Possibility 2

Two equally probable

arrangements of

chromosomes at

metaphase I

Slide 72

Fig. 13-11-2

Possibility 1

Possibility 2

Two equally probable

arrangements of

chromosomes at

metaphase I

Metaphase II

Slide 73

Fig. 13-11-3

Possibility 1

Possibility 2

Two equally probable

arrangements of

chromosomes at

metaphase I

Metaphase II

Daughter

cells

Combination 1

Combination 2

Combination 3

Combination 4

Slide 74

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 pair up gene by gene

Slide 75

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

Slide 76

Fig. 13-12-1

Prophase I

of meiosis

Pair of

homologs

Nonsister

chromatids

held together

during synapsis

Slide 77