Re: Some ways in which new combinations of plants could occur

Katie,

Usually when we refer to the "two ways" that genetic mixing takes place, we mean (1) the recombination of chromosomes that takes place during meiosis (like your example) and (2) the crossing over of segments of genetic material from one chromosome to its homologous chromosome during Prophase I.

During meiosis, each strand of DNA replicates itself, resulting in a single chromosome made up of two identical chromatids. Each cell has a number (depending on the species) of pairs of chromosomes, one from each parent. The pairs are referred to as homologues or homologous pairs. The pairs are alike in that each position along one of the chromosomes has the code for the same trait as that same position on the other chromosome. For example, the gene that controls flower color is on both homologues at the exact same position. Those paired genes are called alleles, and they may code for the same color or for different colors, but they both code for color, not height or pod length. Early in the process of meiosis (the "Prophase I" stage), homologous pairs of chromosomes lie directly against one another, "zippered" together, so that each gene is in contact with its partner, the allele from the other parent. While the chromosomes are arranged like this, segments of a "zippered" pair may break off and switch places with one another. The result is that the chromosome from one parent now has one or more segments of DNA from the other parent, and vice-versa. There are a number of web sites available with good descriptions of this process. Check out the following:

http://mss.scbe.on.ca/DSGENETC.htm
http://www.prenhall.com/divisions/esm/audesirk2/public_html/ch12/links/
http://wsrv.clas.virginia.edu/~rjh9u/meiosisx.html

-Brantlee