• Tasks. The related task is to further consider Figure 1. This
resource shows that a human body cell contains two of each kind
of chromosome, which is said to contain a diploid, or 2n, number
of chromosomes. A cell containing one of each kind of chromosome is said to contain a haploid, or n, number of chromosomes.
Why Is Meiosis Necessary for Organisms?
• Expectations. Students will realize that mitosis is not the only
means of cell division; there must be another form of cell division that produces gametes containing half the number of chromosomes as a parent’s body cell.
• Resources. The corresponding resources are two studies in the
history of biology. (1) In 1876, Oscar Hertwig observed sea
urchin fertilization and found pronuclear fusion of sperms
and eggs (Briggs & Wessel, 2006). (2) In 1884, Eduard Strasburger found a similar phenomenon in the process of angiosperm fertilization (Volkmann et al., 2012).
• Tasks. The corresponding task is for students to read the above
How Is the Chromosome Number in the Daughter
materials and imagine whether reproductive cells are produced
by mitosis. They should be asked, “What will happen in the off-
spring cell?” and guided to the following answer: “Each parent
has 46 chromosomes and would produce gametes containing a
complete set of 46 chromosomes. This means that each offspring
formed by fertilization would have twice the number of chromo-
somes as each of its parents.” Next, the following question should
be posed: “Can you make a guess at how to ensure that offspring
have the same number of chromosomes as their parents?”
Cells Reduced by Half?
• Expectations. Students will understand that a reproductive cell
possesses half the number of chromosomes of a body cell and
propose their own model for the process of producing gametes;
this model will be guided by an analogy with mitosis.
• Resources. The first resource consists of two studies in the history
of biology. (1) In 1883, Edouard Van Beneden discovered that the
number of chromosomes in a gamete of the horse ascarid is half
the amount in a body cell, and a fertilized egg possesses the same
number of chromosomes as a body cell (Hamoir, 1992). (2) In
1891, August Weismann proposed that gametes are produced
through a process of reducing the chromosome number by half
(Churchill, 2010). The second resource is enough colored clay
for students to construct their own model demonstrating the process of producing gametes using an analogy with mitosis.
Figure 1. Human karyotypes. The 46 human chromosomes can be arranged into 23 pairs. In order of decreasing size, the pairs of
chromosomes are numbered 1–22. The X and Y sex chromosomes are the 23rd pair. The sex chromosomes from a female and a
male are shown in the insets (Belk & Borden, 2004; used with permission of the publisher).