Crossing Over and Gene Mapping WS
Ever wonder why, except for identical twins, no two people look
exactly alike? While genes determine most of our physical
characteristics, the exact combination of genes we inherit, and
thus our physical traits, is in part due to a process our
chromosomes undergo, known as genetic recombination. Genetic
recombination happens during meiosis. Inside the cells that
produce sperm and eggs, chromosomes become paired. While
they are pressed together during Prophase I, the chromosomes
may break, and each may swap a portion of its genetic material
for the matching portion from its mate. This form of recombination is called crossing-over. When the
chromosomes glue themselves back together and separate, each has picked up new genetic material
from the other. The distance apart that genes are on a chromosomes are affects their crossing over
rate. The further apart genes are from each other increases their chance of cross over. The closer
genes are, the less likely they are to cross over so they remain on the same chromosome. The genes
and the physical characteristics are now different than before crossing-over. This process is another
major source of genetic diversity.
1. In what kinds of cells does crossing-over take place? _______________________________
2. During what cell process does crossing-over occur? BE SPECIFIC! ______________________
3. What happens during crossing-over? ____________________________________________
4. What is the benefit of crossing-over? ___________________________________________
5. What determines how often crossing over occurs between any 2 particular genes?
What happens to genes on chromosomes during crossing over?
1. Homologous chromosomes pair up
2.
Adjacent chromatids cross over.
3. Homologous chromosomes separate.
Now you can try it! Use the diagram on the previous page as a guide.
What is the genetic significance of crossing over?
Gene Mapping: