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Unit III
Chapter 13 – Meiosis and Sexual Life Cycles
Explain why organisms only reproduce their own kind, and why offspring more
closely resemble their parents than unrelated individuals of the same species.
Parents endow their offspring with coded information in the form of hereditary
units called genes. The tens of thousands of genes we inherit from our mothers and
fathers constitute our genome. Our genetic link to our parents account for family
resemblance. Our genes program the emergence of specific traits as we develop
from fertilizes eggs to adult.
Distinguish between asexual and sexual reproduction.
In sexual reproduction, a single individual is the sole parents and passes copies of all
its genes on to its offspring. Compared to asexual reproduction, sexual reproduction
usually results in greater variation; two parents give rise to offspring that have
unique combination of genes inherited from both parents.
Diagram the human life cycle and indicate where in the human body that mitosis
and meiosis occur; which cells are the result of meiosis and mitosis; and which cells
are haploid.
Figure 12.3-12.4
List the phases of meiosis I and meiosis II and describe the events characteristic of
each phase.
Meiosis I consists of 4 stages: prophase I, metaphase I, anaphase I, and telophase I.
Prophase I begins like prophase of mitosis. The nucleolus disappears, chromatin
condenses in to chromosomes, the nuclear envelope breaks down, and the spindle
apparatus develops. However, once the chromosomes are condensed, homologues
chromosomes pair, a process called sypnasis. These pairs of homologues
chromosomes are variously referred to as tetrad or bivalents. During sypnasis,
corresponding regions along nonsister chromatids form close associations called
chiasmata, chiasmata are sites where genetic material is exchange between nonsister
homologues chromatids, a process called crossing over. A tetrad together with
chiamata and cross over events is referred to as a syptonemal complex. At
metaphase I, homologues pairs of chromosomes are spread across the metaphase
plate. Microtubules extending from other pole are attached to the kinetochore of
one of one member of each homologues pair. Microtubules from the other pole are
connected to the second member of each homologues pair. Anaphase I begins when
homologues whinthin tetrads uncouple as they are pulled to opposite poles. In
Telophase I, the chromosomes have readhed their respective poles, and a nuclear
membrane develops around them. Each pole will form a new nucleus that will have
half the number of chromosomes, but each chromosomes will contain two
chroamtids.l Since daughter nucleic will have half the number of chromosomes, cells
that they eventually form will be haploid. Beginning in the Telophase I, the cells of
many species, cytokinesis and form cleavage furrow or cell plates. In other species,
cytokinesis is delayed until after meiosis II. Also, a short interphase II may begin. In
any case, no replication of chromosomes occurs during this period. Instead, part II
of meiosis begins in both daughter nucleic. In prophase II, the nuclear envelope
disappears and the spindle develops. There are no chiasmamta and no crossing over
of genetic material as in Prophase I. In Metaphase II, the chromosomes align singly
on the metaphase plate. Single alignment of chromosomes is exactly what happens
in mitosis except that now there is only half the number of chromosomes. Anaphase
II begins as each chromosome is pulled apart into two chromatids by the
microtubules of the spindle apparatus. The chromatids (how chromosomes) migrate
to their respective poles. Again, this is exactly what happens in mitosis except that
now there is only half the number of chromosomes. In Telophase II, the nuclear
evenlope reappears at each pole and cytokinesis occurs. The end result of meiosis is
four haploid cells. Each cell conatins half the number of chromosomes and each
chromosome consists of only one chromatid. Like in interphase, a second chromatid
in each chromosome is replicated, but the cell will still have only half the number of
chromosome.
Recognize the phases of meiosis from diagrams or micrographs.
Figure 12.6
Explain how independent assortment, crossing over, and random fertilization
contribute to genetic variation in sexually reproducing organisms.
-Independent assortment of homologous- During Metaphase I, tetradsof homologues
chromosomes seperate into chromosomes that go to opposite poles. Which
chromosome goes to which pole depends upon the orientation of a tetrad at the
metaphase plate. This orientation and subsequent seperation is random for each
tetrad. For some chromosome pairs, the chromosome that is mostly maternal maygo
to one pole, but for another pair, the marternal chromosome may go to the other
pole.
-Crossing over- During Prophase I, nonsister chromatids of homologues
chromosomes exchange pieces of genetic material. As a result, each homologue no
longer entirely represents a single parents.
-Random fertilization- Which sperm fertilizes which egg is to a large degree a
random event. In many cases, howver, this event may be affected by the genetic
composition and of a gamete. For example, some sperm may be faster swimmers
and have a better chance of fertilization of the egg.
Chapter 14 – Mendel and the Gene Idea
State, in your own words, Mendel's law of segregation.
Mendel’s law of segregation mean that there is one recessive allele and one
dominant allele.
Use a Punnett square to predict the results of a monohybrid cross and state the
phenotypic and genotypic ratios of the F2 generation.
Distinguish between gemotype and phenotype; heterozygous and homozygous;
dominat and recessive.
Genotype-means the ratio of the genetic make up when two alleles are crossed
Phenotype- is the ratio of the outcome of the genes.
Heterozygous-means that there is a combination of a dominant and recessive allele.
Homozygous- means that there are either two dominants or two recessive alleles.
Dominant- the alleles that controls.
Recessive- the allele that will show later and its hidden trait.
Explain how a testcross can be used to determine if a dominant phenotype is
homozygous or heterozygous.
The testcross is designed to reveal the genotype of an organism that exhibits the
dominant trait.
Define random event, and explain why it is significant that allele segregation during
meiosis and fusion of gametes at fertilization are random events.
The random event causes genetic variation and the differentiates the traits of the
population.
State, in their own words, Mendel’s law of assortment
The law of assortment is that in a dihybrid cross each characteristics is by itself.
Use a Punnett square to predict the results of a dihybrid cross and state the
phenotypic and genotypic ratios of the F2 generation.
Give an example of incomplete dominance and explain why it is not evidence for the
blending theory of inheritance.
The crossing of snapdragons.
Explain how the phenotypic expression of the heterozygote is affected by complete
dominance, incomplete dominance and codominance.
Define and give examples of pleiotropy.
Pleiotrophy is the ability of a gene to affect an organism in many way, a good
example of this allele that are responsible for certain hereditary diseases in humans.
Explain, in their own words, what is meant by "one gene is epistatic to another."
This means that one gene alters the seconf gene at the same locus in the other
chromatid.
Describe the inheritance of the ABO blood system and explain why the IA and IB
alleles are said to be codominant.
The ABO blood system depends on the carbohydrates that is in the blood. The A
and B are codominant because they form four kinds of blood types.
Chapter 15 – The Chromosomal Basis Of Inheritance
Define linkage and explain why linkage interferes with independent assortment.
Linked genes do not assort independently because they are located on the same
chromosomes and tend to move together through meiosis and fertilization
Explain how crossing over can unlink genes.
Describe sex determination in humans.
What determines the sex is the male who carries the X and Y chromosome in the
sperm.
Describe the inheritance of a sex-linked gene such as color-blindness.
This occurs when there is a genetic disorder.
Explain why a recessive sex-linked gene is always expressed in human males.
Because females carry one of the two X-chromosomes in each randomly inactivity
during the early embryonic development.
Distinguish among nondisjunction, aneuploidy, and polyploidy; explain how these
major chromosomal changes occur and describe the consequences.
Nondisjunction- when members of a pair chromosomes don’t move properly during
meiosis I
Aneuploidy- when the offspring has an abnormal number of chromosomes.
Polypliody- when an organism has more than two complete chromosome sets.
Distinguish among deletions, duplications, translocations, and inversions.
See Figure 14.12