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Crossing Over and Linkage
Prophase I lasts such a long time because the chromosomes deliberately tie themselves
in knots and then untangle (using topoisomerase II, page 74). Figure 19.4 shows what
happens. At the top we see the paternal and maternal chromosomes, each composed of
two chromatids, lined up side by side. As before we show the chromosome that originated
from the father in light green and the maternal one in dark green. During prophase 1 the
chromosomes are cut and resealed at points called chiasmata (singular chiasma) so that
lengths of paternal chromosome are transferred to a maternal one and vice versa. The rest
of meiosis I proceeds, followed by meiosis II, and the end result is that some gametes
contain chromosomes that are neither completely paternal nor completely maternal but are
a recombination of the two.
The biological advantage of sexual reproduction is that it allows organisms to possess a
random selection of the genes from their ancestors. Those individuals with a complement of
genes that makes them better suited to their environment tend to do better, allowing evolution
by natural selection of the individuals posessing the better genes. Without crossing over
this could not happen: Those genes that are located on the same chromosome would remain
linked down the generations, greatly reducing the number of gene permutations possible
at each generation. Crossing over allows a child to inherit, for example, his grandmother’s
green eyes without also inheriting her defective sodium channel gene (page 331), although
both genes are on chromosome 19. Even with crossing over, genes on the same chromosome
are inherited together more than they would be if they were on different chromosoomes.
The closer the genes, the less likely it is that a chisma will form between them, and therefore
the greater the probability that they will be inherited together. This phenomenon is used to
help identify the genes responsible for specific diseases such as cystic fibrosis (page 426).
Fertilization and Inheritance
At fertilization the sperm cell fuses with the much larger egg cell. The nuclei that
originated
in sperm and egg, now called pronuclei, move together and eventually fuse to restore the
normal somatic cell chromosome number. This diploid cell then divides many times by
mitosis to generate the multicellular organism.