Name____________________________________PD__________________Date_______________
Mr. Orend Honors Biology
Meiosis and Karyotypes Reading Guide
Campbell Sec 8.12 – 8.23 (pgs 138-150)
Section 8.12. Chromosomes are matched in homologous pairs
1. In humans, a typical body cell, called a _________________ _____________ has 46 chromosomes.
2. Altogether we see __________ matched pairs of duplicated chromosomes.
3. Moreover, when treated with special dyes, the chromosomes of a pair display matching
_____________________ ____________________.
4. Notice that each chromosome consists of two _______________ _______________ joined at the
centromere.
5. The two chromosomes of such a pair are called _______________ _________________ because they
carry genes controlling the same inherited characteristics.
6. For example, if a gene for eye color is located at a particular place, or ___________, on one
chromosome, then the other chromosome of the homologous pair also has a gene for eye color at that
locus.
Section 8.13. Gametes have a single set of chromosomes
1. Having two sets of chromosomes, one inherited from each parent, is a key factor in the human life
cycle, and in the life cycles of all other species that reproduce ____________________.
2. Cells whose nuclei contain two homologous sets of chromosomes are called _____________
_________, and the total number of chromosomes is called the diploid number (abbreviated 2n).
3. In humans, the diploid number is _________ ()that is, 2n = 46).
4. Humans are said to be diploid organisms because almost all our cells are diploid. The exceptions are
the egg and sperm cells, collectively known as gametes.
5. Each gamete has a single set of chromosomes: 22 ____________ plus a single ________ chromosome.
6. A cell with a single chromosome set is called a _________________ _____________.
7. For humans, the haploid number is ____________.
8. In the human life cycle, sexual intercourse allows a haploid __________ cell from the father to reach
and fuse with a haploid ____________ cell of the mother in the process of ____________________.
9. The resulting fertilized egg, called a ___________________, is diploid (46 chromosomes).
10. Gametes are made by a special sort of cell division called ___________, which occurs in the
reproductive organs (_____________ and __________ in animals).
Section 8.14. Meiosis reduces the chromosome number from diploid to haploid.
1. In meiosis, a cell undergoes _________ _____________ _______________, called Meiosis I and
Meiosis II.
2. Four _________________ cells result from these divisions.
3. However, the two divisions of meiosis are preceded by only one _____________ of chromosomes.
4. As a result, each of the four daughter cells resulting from meiosis has only ____________ as many
chromosomes as the starting cell—a single haploid set of chromosomes.
5. The actual halving of chromosome number occurs during _______________ ______.
Section 8.15. Review: A comparison of mitosis and meiosis (READ THIS!!! BUT WE WILL LEARN THIS LATER)
Section 8.16. Independent orientation of chromosomes in meiosis and random fertilization lead to varied
offspring
1. Offspring that result from sexual re production are highly varied; they are genetically _____________
from their parents and from one another.
2. Figure 8.16 illustrates on way in which the process of meiosis contributes to genetic differences in
gametes.
3. The figure shows how the _____________________ of homologous chromosome pairs at metaphase
of meiosis I affects the resulting genetics.
4. The orientation of the homologous pairs of chromosomes (____________________) at metaphase I is
a matter of _______________, like the flip of a coin.
5. So we see that for this example, a total of ____________ chromosome combinations is possible in the
gametes, and in fact the organism will produce gametes of all four types!!!
6. This variety in gametes arises because each homologous pair of chromosomes orients itself on the
metaphase I plate _______________ of one another.
a. THIS DEMONSTRATES WHY THE LAW OF INDEPENDENT ASSORTMENT MAKES SENSE!!!!
7. For a human (n = 23) there are about 8 million possible _____________ __________________!!!
8. A human egg cell, representing one of about 8 million possibilities, will be fertilized at random by one
sperm cell, representing one of about 8 million other possibilities.
9. By multiplying 8 million times 8 million, we find that a man and a woman can produce a diploid zygote
with any of ______ ______________________ combinations of chromosomes!!!!
10. Actually, these two events are only part of the picture…….
8.17. Homologous chromosomes carry different versions of genes.
1. So far, we have focused on genetic variability in games and zygotes at ________- ________________
level.
2. WE have yet to discuss the actual genetic information—_________ ___________-- contained in the
chromosomes of gametes and zygotes.
3. The question we need to answer now is this: What is the significance of the independent orientation of
metaphase chromosomes at the level of ______________?
4. The letters on homologous chromosomes represent gens. Recall that homologous chromosomes have
genes for the same characteristic at _________________ _________.
5. The fact that homologous chromosomes can bear two different kinds of genetic information for the
same characteristic (for instance, coat color) is what really makes gametes—and therefore offspring—
different from one another.
8.18. Crossing over further increases genetic variability.
1. ___________________ ___________ is the exchange of corresponding segments between two
homologous chromosomes.
2. The chromosomes are a tetrad—four ________________, with each pair of sister chromatids joined at
their centromeres.
3. The sites of crossing over appear as X-shaped regions; each is called a ________________.
4. The exchange of segments by homologous chromatids adds to the _____________ _____________
that will result from the independent orientation of chromosomes at metaphase I.
5. The chromosomes carried by these gametes are called “recombinant” because they result from
___________________ _____________________, the production of gene combinations different from
those carried by the original chromosomes.
6. We have now explained three sources of genetic variability in sexually reproducing organisms:
____________________________________________________,
_______________________________________________, and random fertilization.