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CHAPTER 13
MEIOSIS AND SEXUAL LIFE CYCLES
OUTLINE
I. An Introduction to Heredity
A. Offspring acquire genes from parents by inheriting chromosomes
B. Like begets like, more or less: a comparison of asexual versus sexual reproduction
II. The Role of Meiosis in Sexual Life Cycles
A. Fertilization and meiosis alternate in sexual life cycles
B. Meiosis reduces chromosome number from diploid to haploid: a closer look
III. Origins of Genetic Variation
A. Sexual life cycles produce genetic variation among offspring
B. Evolutionary adaptation depends on a population’s genetic variation
OBJECTIVES
1. Explain why organisms only reproduce their own kind, and why offspring more closely
resemble their parents than unrelated individuals of the same species.
The inheritance of genes from the parent cells give the same characteristics to the offspring that the parents have.
2. Explain what makes heredity possible.
Heredity is possible because of meiosis. The contribution of traits from parent cells to the offspring make it possible
to pass o traits.
3. Distinguish between asexual and sexual reproduction.
Asexual reproduction- a single individual is the sole parent and passes copies of all its genes on to its offspring.
Sexual reproduction- two parents give rise to offspring that have unique combinations of genes inherited from both
parents.
4. 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.
5. Distinguish among the life cycle patterns of animals, fungi, and plants.
6. List the phases of meiosis I and meiosis II and describe the events characteristic of each phase.
Meiosis I
Prophase I
(1)Chromosomes condense and attach at their ends to nuclear envelope
(2)Synapsis occurs
(a)Homologous chromosomes come together in pairs
(b)Homologous chromosomes – pair of chromosomes of
(i) Same size
(ii)Same centromere position
(iii) Same staining pattern
(iv) Same gene loci (except sex chromosomes)
(3)Chromosomes condense further; appear as tetrad
(a)Appear as tetrad
(b)Tetrad
(i) 4 chromatids
(ii)2 pr. of homologous chromosomes
(c) In tetrad
(i) Sister chromatids attached by centromeres
(ii)Nonsister chromatids linked by X-shaped chiasmata
(iii) Chiasma
(a)Site of crossing over
(b)Site where homologous strand exchange occurs
(4)Cell prepares for nuclear division
(a)Centriole pairs move apart
(b)Spindle microtubules form
(c) Nuclear envelope and nucleoli disperse
(d)Chromosomes begin moving to metaphase plate
(5)Prophase I consumes more than 90% of time required for meiosis
Metaphase I
(1)Tetrads line up on metaphase plate
(2)Centromeres of homologous chromosomes point to opposite poles
(3)Each homologue attached to kinetochore microtubules
Anaphase I
(1)Homologues split & move toward opposite poles
(2)Sister chromatids remain attached at centromeres; move as unit
Telophase I & cytokinesis
(1)Homologous chromosomes reach poles
(2)Each pole has haploid set of chromosomes
(3)Chromosomes
(a)Consists of two sister chromatids
(b)Chromatids still attached by centromere
(4)Cytokinesis occurs simultaneously with telophase I
(a)Cleavage furrow in animals
(b)Cell plate in plants
(c) Produces 2 haploid cells
Interkinesis
(1)May occur before meiosis II
(2)Nuclear membrane & nucleoli reform
(3)No DNA replication occurs 3.
Meiosis II
Prophase II
(1)Nuclear envelope & nucleoli disperse
(2)Spindle forms
(3)Chromosomes move toward metaphase plate
Metaphase II
(1)Chromosomes align along metaphase plate
(2)Kinetochores of sister chromatids point toward opposite poles
Anaphase II
(1)Centromeres of sister chromatids separate
(2)Sister chromatids (now called chromosomes) move toward opposite poles
Telophase II and cytokinesis
(1)Nuclear envelopes form at each pole
(2)Cytokinesis
(3)4 haploid cells formed
7. Recognize the phases of meiosis from diagrams or micrographs.
9. Describe the process of synapsis during prophase I, and explain how genetic recombination occurs.
During prophase I of meiosis a process called synapses occurs, during this process the duplicated chromosomes pair
with their homologues. The genetic recombination occurs during the crossing over of nonsister chromatids.
10. Describe key differences between mitosis and meiosis; explain how the end result of meiosis differs from
that of mitosis.
(See figure 12.7)
11. Explain how independent assortment, crossing over, and random fertilization contribute to genetic
variation in sexually reproducing organisms
Independent Assortment: Independent assortments of chromosomes during meiosis, makes each of us produce
gametes containing diverse combinations of the chromosomes inherited from our two parents.
Crossing Over: Crossing Over produces individual chromosomes that combine genes inherited from the two
parents. This occurs during prophase of meiosis I.
Random Fertilization: the random nature of fertilization adds to the genetic variation arising from meiosis.
12. Explain why inheritable variation was crucial to Darwin's theory of evolution.
If there was no inheritable variation the population would turn out to look like each other.
13. List the sources of genetic variation.
 Independent assortment of homologous chromosome pairs during meiosis.
 Crossing over between homologous chromosomes during prophase of meiosis I.
 Random fertilization of an ovum by a sperm.
CHAPTER 14
MENDEL AND THE GENE IDEA
OUTLINE
I. Gregor Mendel’s Discoveries
A. Mendel brought an experimental and quantitative approach to genetics: science as a process
B. By the law of segregation, the two alleles for a character are packaged into separate gametes
C. By the law of independent assortment, each pair of alleles segregates into gametes independently
D. Mendelian inheritance reflects rules of probability
E. Mendel discovered the particulate behavior of genes: a review
II. Extending Mendelian Genetics
A. The relationship between genotype and phenotype is rarely simple
III. Mendelian Inheritance in Humans
A. Pedigree analysis reveals Mendelian patterns in human inheritance
B. Many human disorders follow Mendelian patterns of inheritance
C. Technology is providing new tools for genetic testing and counseling
OBJECTIVES
5. State, in their own words, Mendel's law of segregation.
Mendel’s law of segregation means that there is one recessive allele and one dominant allele.
6. Use a Punnett square to predict the results of a monohybrid cross and state the phenotypic and genotypic
ratios of the F2 generation.
7. Distinguish between genotype and phenotype; heterozygous and homozygous; dominant 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 a recessive allele
Homozygous- means that there are either two dominants or two recessive alleles
Dominant- the allele that controls
Recessive- the allele that will show later on and it’s a hidden trait
8. 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.
9. 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.
13. State, in their own words, Mendel's law of independent assortment.
The law of independent assortment is that in a dyhibrid cross each characteristic is by its self.
14. Use a Punnett square to predict the results of a dihybrid cross and state the phenotypic and genotypic
ratios of the F2 generation.
16. Give an example of incomplete dominance and explain why it is not evidence for the blending theory of
inheritance.
The crossing of snapdragons.
17. Explain how the phenotypic expression of the heterozygote is affected by complete dominance, incomplete
dominance and codominance.
18. 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.
19. Define and give examples of pleiotropy.
Pleiotropy is the ability of a gene to affect an organism in many way, a good example of this are alleles that are
responsible for certain hereditary diseases in humans.
20. Explain, in their own words, what is meant by "one gene is epistatic to another."
This means that one gene alters the second gene at the same locus in the other chromatid.
23. Describe how environmental conditions can influence the phenotypic expression of a character.
For example the color of ones’ skin can be altered by the sun light and the different climate conditions. Because of
these environmental changes the skin can deviate from its original shade.
24. Given a simple family pedigree, deduce the genotypes for some of the family members.
CHAPTER 15
THE CHROMOSOMAL
BASIS OF INHERITANCE
OUTLINE
I. Relating Mendelism to Chromosomes
A. Mendelian inheritance has its physical basis in the behavior of chromosomes during sexual life cycles
C. Linked genes tend to be inherited together because they are located on the same chromosome
D. Independent assortment of chromosomes and crossing over produce genetic recombinants
E. Geneticists can use recombination data to map a chromosome’s genetic loci
II. Sex Chromosomes
A. The chromosomal basis of sex varies with the organism
B. Sex-linked genes have unique patterns of inheritance
III. Errors and Exceptions to Chromosomal Inheritance
A. Alterations of chromosome number or structure cause some genetic disorders
OBJECTIVES
4. 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.
6. Explain how crossing over can unlink genes.
10. Describe sex determination in humans.
What determines our sex it’s always the Y chromosome.
11. Describe the inheritance of a sex-linked gene such as color-blindness.
This occurs when there is a genetic disorder
12. 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.
14. 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
Polyploidy- when an organism has more than two complete chromosome sets
16. Distinguish among deletions, duplications, translocations, and inversions.
See figure 14.12