******ANSWER KEY*****SI Worksheet #14 (Chapter 13)
BY 123
Meeting 10/29/2015
Chapter 13: Meiosis
1. How are the traits of parents transmitted to their offspring?
Parents pass genes to their offspring; the genes program cells to make specific enzymes and
other proteins, whose cumulative action produces an individual’s inherited traits.
2. Explain how asexually reproducing organisms produce offspring that are genetically identical to
each other and to their parents.
Such organisms reproduce by mitosis, which generates offspring whose genomes are exact
copies of the parent’s genome (in the absence of mutation)
3. What are somatic cells?
Body cells; any cell of a living organism other than the reproductive cells.
4. Homologous chromosomes or homologs are a pair of two chromosomes with the same length,
centromere position, and staining pattern in a karyotype. What is the relationship of these two
pairs of chromosomes?
They carry genes controlling the same inherited materials
5. Why are X and Y chromosomes exceptions to the general pattern of homologous chromosomes?
What is the name given to the X and Y chromosomes?
Human females have a homologous pair of X chromosomes (XX), but males have one X and one
Y chromosome (XY). Only small parts of the X and Y are homologous. Most of the genes carried
on the X chromosome do not have counterparts on the tiny Y chromosomes and the Y has genes
lacking on the X. These are called the sex chromosomes.
6. What is the name given to all other chromosomes?
Autosomes
7. A cat has a diploid chromosome number of 38. How many chromosomes does a cat inherit from
each parent? How many chromosomes are present in a cat’s gametes? How many
chromosomes are present in a cat’s somatic cells?
2n=38. So n=19. Therefore, 19 chromosomes are inherited from each parent. Gametes (haploid)
have 19 chromosomes. Somatic cells (diploid) have 38.
8. What is the female gamete? What is the male gamete? Are these cells haploid of diploid? What
does the fusion of these cells produce? Is the resulting haploid or diploid? Why?
Female gamete is egg. Male gamete is sperm. These are haploid cells. The fusion of these cells
produces a diploid zygote. It is diploid because it receives the haploid number from both the
sperm and the egg.
9. What two things happen in Prophase I that increase genetic variability in offspring? What is the
relationship of the chromosomes that participate in this phenomenon?
Synapsis and crossing over. During prophase I, replicated homologs pair up and become
physically connected along their lengths by a zipper like proteins structure called the
10.
11.
12.
13.
14.
15.
16.
17.
synaptonemal complex this is called synapsis. Genetic rearrangement between non-sister
chromatids, crossing over, then occurs.
At the end of Telophase I and cytokinesis, how many cells have formed? Are these cells haploid
or diploid?
Two haploid cells; each chromosome still consist of two sister chromatids.
At the end of Telophase II and cytokinesis, how many cells have formed? Are these cells haploid
or diploid?
4 haploid daughter cells.
How are the chromosomes in a cell at metaphase of mitosis similar to and different from the
chromosomes in a cell at metaphase II of meiosis II?
The chromosomes are similar in that each is composed of two sister chromatids and the
individual chromosomes are positioned similarly on the metaphase plate.
The chromosome differ in that in a mitotically dividing cell, sister chromatids of each
chromosome are genetically identical, but in a meiotically dividing cell, sister chromatids are
genetically distinct because of crossing over in meiosis I. Moreover, the chromosomes in
metaphase of mitosis can be a diploid set or haploid set, but the chromosomes in metaphase of
meiosis II always consist of a haploid set.
In what types of organisms do we see genetic variation?
Organisms that reproduce sexually
List and explain the three mechanisms that contribute to the genetic variation.
1) Independent Assortment: because each homologous pair of chromosomes is positioned
independently of the other pairs at metaphase I, the first meiotic division results in each
pair sorting its maternal and paternal homologs into daughter cells independently of
every other pair….. 2n possibilities
2) Crossing Over: production of recombinant chromosomes in prophase I of meiosis
3) Random Fertilization: any sperm with its many possible chromosome combination due
to independent assortment, can fuse with any egg with its many possible chromosome
combination due to independent assortment…. 2nx2n possibilities
The diploid number for fruit flies is 8, while that for grasshoppers is 46. If no crossing over took
place, would the genetic variation among offspring from a given pair of parents be greater in
fruit flies or grasshoppers? Explain.
Without crossing over, independent assortment of chromosomes during meiosis I theoretically
can generate 2n possible haploid gametes, and random fertilization can produce 2nx2n possible
diploid zygotes. Because the haploid number (n) in grasshoppers is more than that of the 4 in
fruit flies, grasshoppers would be expected to have a greater variety of zygotes than fruit flies
What is the original source of all different versions of an allele?
Mutations in a gene lead to a different versions (alleles) of that gene
Under what circumstances would crossing over during meiosis not contribute to genetic
variation among daughter cells?
Crossing over contributes to genetic variation only when it involves the rearrangement of
different alleles. If the segments of the maternal and paternal chromatids that undergo crossing
over are genetically identical and thus have the same two alleles for every gene, then the
recombinant chromosomes will be genetically equivalent to the parental chromosomes.