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UNIT THREE OBJECTIVES
CHAPTER 13
MEIOSIS AND SEXUAL LIFE CYCLES
1. Explain why organisms only reproduce their own kind, and why offspring
more closely resemble their parents than unrelated individuals of the
same species.
Like begets like due to the passing on of chromosomes; genes; traits. The
fact that offspring resemble their parents is due to heredity and genetic
variation.
2. Distinguish between asexual and sexual reproduction.
Asexual reproduction is when a single individual is the sole parent and
passes copies of all its genes to its offspring. No genetic variation occurs.
Sexual reproduction is when two parents give rise to offspring that have
unique combinations of genes inherited from both parents.
3. 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.
(See Figure 12.3) The only cells of the human body not produced by
mitosis are the gametes, which develop in the gonads (ovaries in the
females and testes in males). Each gamete has a single set of 23
chromosomes. They are haploid. Somatic cells – any other cells other than
sperm or ovum – have two sets of chromosomes and are called diploid
cells.
4. Distinguish among the life cycle patterns of animals, fungi, and plants.
Animals go through the same life cycle patterns as humans do. Gametes
are the only haploid cells. Meiosis occurs during the production of
gametes, which undergo no further cell division prior to fertilization. The
diploid zygote divides by mitosis, producing a multi-cellular organism that
is diploid.
In Fungi, after gametes fuse to forma diploid zygote, meiosis occurs
before offspring develop. This produces haploid cells, whish then divide by
mitosis to give rise to multi-cellular adult organisms that are haploid.
Plants exhibit a life cycle called alternation of generations; there are both
haploid and diploid multi-cellular stages.
5. List the phases of meiosis I and meiosis II and describe the events
characteristic of each phase.
(See pages 230-231)
6. Recognize the phases of meiosis from diagrams or micrographs.
(Already learned – refer to notes)
7. Describe the process of synapsis during prophase I, and explain how
genetic recombination occurs.
During synapsis, homologous chromosomes, each made up of 2 chromatids,
come together as pairs. This produces individual chromosomes that combine
genes inherited from both parents.
8. Describe key differences between mitosis and meiosis; explain how the
end result of meiosis differs from that of mitosis.
Mitosis produces two daughter cells that are an exact replica of the parent
cell. Meiosis, because it includes synapsis, produces genetic variation in
the 4 daughter cell it produces.
9. Explain how independent assortment, crossing over, and random
fertilization contribute to genetic variation in sexually reproducing
organisms.
All three reshuffle the various genes carried by the individual members of
a population.
CHAPTER 14
MENDEL AND THE GENE IDEA
1. State, in your own words, Mendel's law of segregation.
The two alleles (variations of a gene) are packaged into separate gametes.
For every trait a parent donates only one allele.
2. Use a Punnett square to predict the results of a monohybrid cross and
state the phenotypic and genotypic ratios of the F2 generation.
3. Distinguish between genotype and phenotype; heterozygous and
homozygous; dominant and recessive.
Genotype – genetic makeup of an organism
Phenotype – physical appearance of an organism
Homozygous – an organism having a pair of identical alleles for a
characteristic
Heterozygous – an organism having 2 different alleles for a characteristic
Dominant – characteristic is fully expressed in the organisms` appearance
Recessive – has no noticeable affect on the organisms` appearance
4. Explain how a testcross can be used to determine if a dominant phenotype is
homozygous or heterozygous.
Cross the organism with an individual expressing the recessive trait. For
example, a purple flower of an unknown genotype with a white flower of a
recessive genotype. Since the genotype of the white flower must be
homozygous, we can deduce the genotype of the purple flowered parent by
observing the phenotypes of the offspring.
5. Define random event, and explain why it is significant that allele segregation
during meiosis and fusion of gametes at fertilization are random events.
6. State, in your own words, Mendel's law of independent assortment.
Each pair of alleles segregates into gametes independently.
7. Use a Punnett square to predict the results of a dihybrid cross and state the
Phenotypic and genotypic ratios of the F2 generation.
(See Figure 13.7)
8. Give an example of incomplete dominance and explain why it is not evidence
for the blending theory of inheritance.
When crossing red snapdragons with whit snapdragons the next generation of
snapdragons is pink. But this is not evidence of the blending theory, for it was
the red and white traits may never be retrieved. Rather, the phenotype of the
following generation is a ratio of 1 red to 2 pink to I white. Further blending did
not occur.
9. Explain how the phenotypic expression of the heterozygote is affected by
complete dominance, incomplete dominance and co-dominance.
In complete dominance the phenotype of the heterozygote and dominant
homozygote are completely indistinguishable. In codominance BOTH alleles are
separately manifest in the phenotype. For example, the existence of M, N, and
MN blood types. In incomplete dominance the hybrids have the appearance
somewhere in between the phenotypes of the two parental varieties.
10. Describe the inheritance of the ABO blood system and explain why the IA
and IB alleles are said to be codominant.
The four blood groups (A, B, AB, O) result from various combinations of three
alleles of one gene, symbolized, as IA, IB, or i. Six genotypes are possible. Both
the IA and the IB alleles are dominant to the i allele. Thus, IAIA and IBi have
type A blood, and IBIB and IBi have type B. The IA and IB alleles are
codominant and produce type AB blood. Recessive homozygous, ii, have type O
blood, because neither the A nor the B substance is produced.
11. Define and give examples of pleiotropy.
Most genes affect an organisms` phenotype in more than one way – this is
called pleiotrapy. For example, pleiotrapy is responsible for certain hereditary
diseases such as sickle cell.
12. Explain, in their own words, what is meant by "one gene is epistatic to
another".
A gene at one locus alters the phenotypic expression of a gene at a second
locus.
13. Describe how environmental conditions can influence the phenotypic
expression of a character.
The product of a genotype is generally not a rigidly defined phenotype, but a
range of phenotype possibilities over which there may be a variation due to
environmental influences. This phenotypic range is called the norm of reaction
for a genotype. When factors, both genetically and environmentally, collectively
influence genotype these characters are said to be multi-factoral.
14. Given a simple family pedigree, deduce the genotypes for some of the family
members.
(Have done such examples in class)
CHAPTER 15
THE CHROMOSOMAL
BASIS OF INHERITANCE
1. Define linkage and explain why linkage interferes with independent
assortment.
Linked genes tend to be inherited together because are located on the same
chromosome. In breeding experiment the results deviate from those expected
according to the Mendelian principle of independent assortment.
2. Explain how crossing over can unlink genes.
A cross over between homologous chromosomes breaks linkages in the
parental chromosomes to form recombinant chromosomes that may bring
together alleles in new combinations. The subsequent events of meiosis
distribute the recombinant chromosomes to gametes.
3. Describe sex determination in humans.
When meiosis occurs in gonads the two sex chromosomes segregate, and
each gamete receives one. Each ovum contains one X-chromosome, while
half the sperm produced by males contains an X-chromosome, and the other
half contains a Y-chromosome. Which sperm come in contact with the ovum
determine the sex in that an XX combination produces a female, and an XY
combination produces a male.
4. Describe the inheritance of a sex-linked gene such as color-blindness.
Fathers pass sex-linked alleles to all of their daughters but to none of their
sons. In contrast, mothers ca pass sex-linked alleles to both sons and
daughters.
5. Explain why a recessive sex-linked gene is always expressed in human
males.
The term sex-linked usually refers to X-linked characters. If a sex-linked trait
is due to a recessive allele, a female will express the phenotype only if she is
a homozygote. Any male receiving the recessive allele from his mother will
express the trait. For this reason far more males than females have disorders
that are inherited as sex linked recessives.
6. Distinguish among nondisjunction, aneuploidy, and polyploidy; explain
how these major chromosomal changes occur and describe the
consequences.
Non-disjunction is a case in which members of a pair of homologous ì¥Á 7
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-disjunction of all its` chromosomes.
7. Distinguish among deletions, duplications, translocations, and inversions.
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A deletion occurs when a chromosomal fragment lacking a
centromere is lost during cell division.
The fragment may join to the homologous chromosome, producing
duplication.
It may also reattach to the original chromosome, but in the reverse
orientation, producing an inversion.
If the fragment joins a non-homologous chromosome the
rearrangement is called a translocation.