Figure 5.10: Butterflies receive nectar when they deposit pollen into flowers, resulting in
cross-pollination. ( 8 )
Fungi can also reproduce sexually, but instead of female and male sexes, they have (+) and
(-) strains. When the filaments of a (+) and (-) fungi meet, the zygote is formed. As with
the sexual reproduction in plants and animals, each zygote receives DNA from two parent
strains.
Meiosis and Gametes
The formation of gametes, the reproductive cells such as sperm and egg, is necessary for sex-
ual reproduction. As gametes are produced, the number of chromosomes must be reduced
to half. In humans, our cells have 23 pairs of chromosomes, and each chromosome within
a pair is called ahomologous chromosome. For each of the 23 chromosome pairs, you
received one chromosome from your father and one chromosome from your mother. The
homologous chromosomes have the same genes, although there might be alternate forms of
each gene, calledalleles, which vary between the chromosomes. These homologous chromo-
somes are separated during gamete formation, therefore gametes have only 23 chromosomes,
not 23 pairs. This separation of chromosomes is random. The probability or chance that a
particular allele will be in a gamete is 1 in 2. The gamete may receive either the paternal
allele (inherited from the father) or the maternal allele (inherited from the mother). This
random separation of chromosomes (and therefore alleles) occurs for each chromosome, re-
sulting in an widely varied combination of chromosomes in each gamete. With 23 pairs of
chromosomes, this results in over 8 million different combinations of chromosomes a gamete.