94 ❯ STEP 4. Review the Knowledge You Need to Score Highthe diploid multicellular organism. Meiotic division later produces haploid gametes, which
continue the cycle.
The life cycle of fungi (Figure 9.8) is different from that of humans. Fungi are haploid
organisms, with the zygote being the only diploid form. Like humans, the gametes for fungi
are haploid (n), and fertilization yields a diploid zygote. But in this life cycle, instead of
dividing by mitosis, the zygote divides by meiosis to form a haploid organism. Another dif-
ference in this life cycle is that the gametes are formed by mitosis,not meiosis—the organ-
ism is alreadyhaploid, before forming the gametes.
Here is some trivia about life cycles that might come in handy on the exam. The only
diploid stage for a fungus is the zygote. The only haploid stage for a human is the gamete.
Of the plant life cycles, the moss (bryophyte) is an exception in that its prominent
generation is the gametophyte. For ferns, conifers (cone-producing plants), and angiosperms
(flowering plants), the prominent generation is the sporophyte. The dominant sporophyte
generation is considered more advanced evolutionarily than a dominant gametophyte gen-
eration. These different plant types will show up again later in Chapter 14.Sources of Cell Variation
What makes us different from our parents? Why do some people look amazingly like their
parents while others do not? The process of cell division provides ample opportunity for
variation. Remember that during meiosis, homologous chromosome pairs align together
along the metaphase plate. This alignment is a completely random process, and there is a
50 percent chance that the chromosome in the pair from the individual’s mother will go to
one side, and a 50 percent chance that the chromosome in the pair from the individual’s
father will go to that side. This is true for all the homologous pairs in an organism. This
means that 2npossible gametes can form from any given set of nchromosomes. For example,
in a 3-chromosome organism, there are 2^3 =8 possible gametes. In humans, there are
23 homologous pairs. This comes out to 2^23 (8,388,608) different ways the gametes can
separate during gametogenesis.
Another source of variation during sexual reproduction is the random determination of
which sperm meets up with which ovum. In humans, the sperm represents one of 2^23 possibil-
ities from the male gamete factory; the ovum, one of 2^23 possibilities from the female gamete
factory. All these factors combine to explain why siblings may look nothing like each other.
A third major source of variation during gamete formation is the crossover(orcrossing
over) that occurs during prophase I of meiosis. It is very important for you to remember that
this process happens onlyduring that stage of cell division. It does not occur in mitosis.Gamete (n)
Fertilization
Gamete (n) Zygote (2n)Mitosis Meiosis
Mitosis Multicellular Organism
organism (n)(n)Figure 9.8 Fungus life cycle.NYC teacher:
“Knowing the
sources of varia-
tion is
important.”BIG IDEA 3.C.2
Crossing over and
random assortment
during meiosis
increase variation.