Iii

IIIm n

IIIp n

Gametes

Im IIm III m

Ip IIp IIIp

Im IIm IIIp

Ip IIp IIIm

Im IIp IIIp

Ip IIm IIIm

Im IIp IIIm

Ip IIm IIIp

Im IIm IIIm

Ip IIp IIIp

Im IIm IIIp

Ip IIp IIIm

Im IIp IIIp

Ip IIm IIIm

Im IIp IIIm

Ip IIm IIIp

Conclusion: Eight different combinations of chromosomes in the gametes are possible, depending on how the chromosomes align and separate in meiosis I and II.

combinations of chromosomes in the gametes. In general, the number of possible combinations is 2", where n equals the number of homologous pairs. As the number of chromosome pairs increases, the number of combinations quickly becomes very large. In humans, who have 23 pairs of chromosomes, there are 8,388,608 different combinations of chromosomes possible from the random separation of homologous chromosomes. Through the random distribution of chromosomes in anaphase I, alleles located on different chromosomes are sorted into different combinations. The genetic consequences of this process, termed independent assortment, will be explored in more detail in Chapter 3.

In summary, crossing over shuffles alleles on the same homologous chromosomes into new combinations, whereas the random distribution of maternal and paternal chromosomes shuffles alleles on different chromosomes into new combinations. Together, these two processes are capable of producing tremendous amounts of genetic variation among the cells resulting from meiosis.

Concepts]"

Meiosis consists of two distinct divisions: meiosis I and meiosis II. Meiosis (usually) produces four haploid cells that are genetically variable. The two processes responsible for genetic variation are crossing over and the random distribution of maternal and paternal chromosomes.

www.whfreeman.com/pierce A tutorial and animations of

Connecting Concepts 9

Comparison of Mitosis and Meiosis

Now that we have examined the details of mitosis and meiosis, let's compare the two processes ( FIGURE 2.19). In both mitosis and meiosis, the chromosomes contract and m p

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