Standard Set 2. Genetics (Meiosis and Fertilization)
Students should know that organisms reproduce offspring of their own kind and that organisms of the
same species resemble each other. Students have been introduced to the idea that some characteristics
can be passed from parents to offspring and that individual variations appear among offspring and in the
broader population. Understanding genetic variation requires mastery of the fundaÂ¬mentals of sex cell
formation and the steps to reorganize and redistribute genetic material during defined stages in the cell
Students should understand the difference between asexual cell reproduction (mitosis) and the formation
of male or female gamete cells (meiosis). Sexual reproÂ¬duction initially requires the production of
haploid eggs and haploid sperm, a proÂ¬cess occurring in humans within the female ovary and the male
testis. These haploid cells unite in fertilization and produce the diploid zygote, or fertilized cell.
The mechanisms involved in synapsis and movement of chromosomes during meiosis bring about the
halving of the chromosome numbers for the production of the haploid male or female gamete cells from
the original diploid parent cell and different combinations of parental genes. The exchange of
chromosomal segments between homologous chromosomes (crossing over) revises the association of
genes on the chromosomes and contributes to increased diversity. Any change in genetic constitution
through mutation, crossing over, or chromosome assortment during meiosis promotes genetic variation in
2. Mutation and sexual reproduction lead to genetic variation in a population.
As a basis for understanding this concept:
2. a. Students know meiosis is an early step in sexual reproduction in which the pairs of
chromosomes separate and segregate randomly during cell division to produce gametes
containing one chromosome of each type.
Haploid gamete production through meiosis involves two cell divisions. During meiosis prophase I, the
homologous chromosomes are paired, a process that abets the exchange of chromosome parts through
breakage and reunion. The second meiotic division parallels the mechanics of mitosis except that this
division is not preceded by a round of DNA replication; therefore, the cells end up with the hapÂ¬loid
number of chromosomes. (The nucleus in a haploid cell contains one set of chromosomes.) Four haploid
nuclei are produced from the two divisions that charÂ¬acterize meiosis, and each of the four resulting
cells has different chromosomal conÂ¬stituents (components). In the male all four become sperm cells.
In the female only one becomes an egg, while the other three remain small degenerate polar bodies and
cannot be fertilized.
Chromosome models can be constructed and used to illustrate the segregation taking place during the
phases of mitosis (covered initially in Standard 1.e for grade seven in Chapter 4) and meiosis.
Commercially available optical microscope slides also show cells captured in mitosis (onion root tip) or
meiosis (Ascaris blastocyst cells), and computer and video animations are also available.
2. b. Students know only certain cells in a multicellular organism undergo meiosis.
Only special diploid cells, called spermatogonia in the testis of the male and oogonia in the female ovary,
undergo meiotic divisions to produce the haploid sperm and haploid eggs.
2. c. Students know how random chromosome segregation explains the probability that a
particular allele will be in a gamete.
The steps in meiosis involve random chromosome segregation, a process that accounts for the
probability that a particular allele will be packaged in any given gamete. This process allows for genetic
predictions based on laws of probability that pertains to genetic sortings. Students can create a genetic
chart and mark alterÂ¬nate traits on chromosomes, one expression coming from the mother and
another expression coming from the father. Students can be shown that partitions of the chromosomes
are controlled by chance (are random) and that separation (segregaÂ¬tion) of chromosomes during
karyokinesis (division of the nucleus) leads to the random sequestering of different combinations of
2. d. Students know new combinations of alleles may be generated in a zygote through the
fusion of male and female gametes (fertilization).
Once gametes are formed, the second half of sexual reproduction can take place. In this process a
diploid organism is reconstituted from two haploid parts. When a sperm is coupled with an egg, a
fertilized egg (zygote) is produced that contains the combined genotypes of the parents to produce a
new allelic composiÂ¬tion for the progeny. Genetic charts can be used to illustrate how new
combinaÂ¬tions of alleles may be present in a zygote through the events of meiosis and the chance
union of gametes. Students should be able to read the genetic diploid karyotype, or chromosomal
makeup, of a fertilized egg and compare the allelic composition of progeny with the genotypes and
phenotypes of the parents.
2. e. Students know why approximately half of an individualâ€™s DNA sequence comes from
each parent. Chromosomes are composed of a single, very long molecule of double-stranded DNA
and proteins. Genes are defined as segments of DNA that code for polypepÂ¬tides (proteins). During
fertilization half the DNA of the progeny comes from the gamete of one parent, and the other half comes
from the gamete of the other parÂ¬ent.
2. f. Students know the role of chromosomes in determining an individualâ€™s sex.
The normal human somatic cell contains 46 chromosomes, of which 44 are pairs of homologous
chromosomes and 2 are sex chromosomes. Females usually carry two X chromosomes, and males
possess one X and a smaller Y chromosome. Combinations of these two sex chromosomes determine
the sex of the progeny.
2. g. Students know how to predict possible combinations of alleles in a zygote from the
genetic makeup of the parents.
When the genetic makeupâ€™s of potential parents are known, the possible assortÂ¬ments of alleles in
their gametes can be determined for each genetic locus. Two parental gametes will fuse during
fertilization, and with all pair-wise combinations of their gametes considered, the possible genetic
makeups of progeny can then be predicted.