Chapter 13 Notes
Heredity Similarity and Variation
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Heredity: transmission of traits from one generation to the next o Also called inheritance
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Variation: offspring differ somewhat in appearance from parents and siblings
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Genetics: scientific study of heredity and heredity variation
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Meiosis: cell division that results in reproductive cells
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Fertilization: fusion of sperm and egg
13.1
Inheritance of Genes
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Genes: hereditary units o We inherit tens of thousands of genes
 Our genome made of these genes
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Genes are segments of DNA o DNA is a polymer of four different kinds of monomers called nucleotides
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Inherited information is passed on in the form of each gene’s specific sequence of nucleotides o Cells translate genetic sequences into characteristics that don’t look like genes
 Genes program cells to synthesize specific enzymes and other proteins o Transmission of hereditary traits comes from the precise replication of DNA
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Gametes: reproductive cells o Transmits genes from one generation to the next
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During fertilization, male and female gametes unit, thereby passing on genes from parents to their offspring
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DNA of a eukaryotic cell is subdivided into chromosomes within the nucleus
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Every living species has a unique chromosome number
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One chromosome includes several hundred to a few thousand genes
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Locus: a gene’s specific location along the length of a chromosome
Comparison of Asexual and Sexual Reproduction
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Asexual reproduction: a single individual is the sole parent and passes copies of all its genes to its offspring o Genomes of the offspring are virtually exact copies of the parent’s genome
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Clone: a group of genetically identical individuals o An individual that reproduces asexually gives rise to a clone
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Slight changes in copies of DNA are called mutations
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Sexual reproduction: two parents give rise to offspring that have unique combinations of genes inherited from the two parents o Offspring vary genetically from their siblings and both parents o Genetic variation is an important consequence of sexual reproduction

13.2
Fertilization and meiosis alternate in sexual life cycles
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Life cycle: the generation-to-generation sequence of stages in the reproductive history of an organism
Sets of Chromosomes in Human Cells
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Somatic cell: any other cell than a gamete o In humans, each somatic cells has 46 chromosomes
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During mitosis, chromosomes become condensed enough to be visible in a light microscope o Because chromosomes differ in size, in the position of their centromeres, and in the pattern of colored bands produced by certain stains, they can be distinguished from one another
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Two chromosomes of each type
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Karyotype: arrangement of chromosomes in pairs, starting with the longest
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Homologous chromosomes: two chromosomes composing a pair that have the same length, centromere position, and staining pattern o Both chromosomes of each pair carry genes controlling the same inherited characters
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Human females have a homologous pair of X chromosomes o Males have one X and one Y chromosome
 Only small parts of the X and Y are homologous
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Sex chromosomes: X and Y o Determine the individuals sex
 All other chromosomes are called autosomes
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Homologues occur because of our sexual origins o We inherit one chromosome of each pair from each parent
 46 chromosomes = 2 sets of 23 chromosomes
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One set of 23 from each parent
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Number of chromosomes in a single set is represented by n
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Any cell with two chromosome sets is called a diploid cell o Diploid cell has the diploid number of chromosomes, 2n
 In humans, diploid number = 46
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Gametes contain a single chromosome set o Gametes are haploid cells
 Haploid cells have the haploid number of chromosomes, n
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In humans, haploid number =23 o 22 autosome chromosomes and a sex chromosome
Behavior of Chromosome Sets in the Human Life Cycle
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Human life cycle begins when a haploid sperm cell from the father fuses with the haploid ovum from the mother o Fusion of gametes called fertilization
 Resulting fertilized egg called a zygote
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As a human develops from a zygote to a sexually mature adult, mitosis generates all the somatic cells of the body o The only cells of the human body not produced by mitosis are the gametes
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Meiosis: reduces the number of sets of chromosomes from two to one in the gametes o In animals, meiosis occurs only in the ovaries or testes

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Fertilization and meiosis alternate in sexual life cycles, offsetting each other’s effects on the chromosome number and thus perpetuating a species’ chromosome count
The Variety of Sexual Life Cycles
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Timing of fertilization and meiosis events in the life cycle varies, depending on the species
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Three main types of life cycles: o In animals, gametes are the only haploid cells o In plants, alternation of generations
 Includes both diploid and haploid multicellular stages
 Sporophyte: multicellular diploid stage
 Spores: haploid stage
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Spore gives rise to a multicellular individual without fusing with another cell
 Gametophyte: multicellular haploid stage
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Makes gametes by mitosis
 Sporophyte generation produces a gametophyte, and the gametophyte generation produces the next sporophyte generation o In fungi, gametes fuse and form an diploid zygote
 Then, meiosis occurs without a diploid offspring developing
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Meiosis produces haploid cells that then divide by mitosis and give rise to a haploid multicellular adult organism
 Only diploid stage is the single-celled zygote
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Each cycle of chromosome halving and doubling contributes to genetic variation among offspring
13.3
Meiosis reduces the number of chromosome sets from diploid to haploid
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Meiosis preceded by replication of chromosomes o Single replication followed by two cell divisions
 Called meiosis I and meiosis II
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Result in four daughter cells
The Stages of Meiosis
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Both members of a single homologous pair of chromosomes in a diploid cell are replicated and the copies are then sorted into four haploid daughter cells
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Remember: sister chromatids are two copies of one chromosome o Two chromosomes of a homologous pair are individual chromosomes that were inherited from different parents
 Not usually connected to each other
 Look alike in a microscope, but carry different versions of the same gene at the same loci on the chromosome
Meiosis halves the total number of chromosomes, reducing the number of sets from two to one
Interphase o Chromosomes replicate o Centrosome replicates

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Prophase I o Occupies more than 90% of the time required for meiosis o Chromosomes begin to condense o Homologous chromosomes loosely pair along their lengths o In crossing over, the DNA molecules in nonsister chromatids break and corresponding places and then rejoin to the other’s DNA o In synapsis, a protein structure called the synaptonemal complex forms between homolgoues, holding them tightly together o Synaptonemal complex disassembles o Each chromosome pair becomes visible as a tetrad o Movement of centrosomes, formation of spindle microtubules, breakdown of the nuclear envelope
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Metaphase I o Pairs of homologous chromosomes now in pairs on metaphase plate o Both chromosomes of homologous pair attach to kinetochore fibers
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Anaphase I o Chromosomes move toward the poles o Sister chromatids remain attached
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Telophase I and Cytokinesis o Cytokinesis occurs simultaneously with telophase I o Two haploid daughter cells form o No chromosome replication occurs
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Prophase II o Spindle forms
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Metaphase II o Chromosomes positioned on metaphase plate o Sister chromatids not genetically identical
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Anaphase II o Sister chromatids separate o Move toward opposite poles
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Telophase II and Cytokinesis o Nuclei form o Chromosomes begin decondensing o Cytokinesis occurs o Four haploid daughter cells form
A Comparison of Mitosis and Meiosis
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Meiosis o Reduces the number of chromosome sets from two to one o Produces cells that are genetically different from their parent cell and each other
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Mitosis o Conserves the number of chromosome sets o Produces daughter cells that are genetically identical to their parent cell and to each other
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Synapsis and crossing over o During prophase I, duplicated homologous chromosomes line up and become physically connected by a zipper-like protein structure called the synaptonemal complex
 Process is called synapsis

13.4 o Genetic rearrangement between nonsister chromatids is called crossing over
 Occurs during prophase I o Tetrad: four chromatids in a homologous pair
 Normally contains at least one X-shaped region called a chiasma
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Tetrads on the metaphase plate o Tetrads are positioned on the metaphase plate, rather than individual replicated chromosomes
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Separation of homologues o Duplicated chromosomes of each homologous pair move toward opposite poles, but sister chromatids remain attached
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Meiosis I called the reductional division because it halves the number of chromosome sets per cell
Genetic variation produced in sexual life cycles contributes to evolution
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Mutations are the original source of genetic diversity o These changes in an organism’s DNA create different versions of genes
Origins of Genetic Variation Among Offspring
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In species that reproduce sexually, the behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises each generation
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Three mechanisms that contribute to genetic variation: o Independent assortment o Crossing over o Random fertilization
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Independent Assortment of Chromosomes o One aspect of sexual reproduction that generates genetic variation is the random orientation of homologous pairs of chromosomes at metaphase of meiosis I o Homologous pairs lined up at metaphase plate o Each pair may orient with either its maternal or paternal homologue closer to a given pole
 Its orientation is a 50/50 chance o Each homologous pair is positioned independently of the other pairs at metaphase I
 Each daughter cell represents one outcome of all possible combinations of maternal and paternal chromosomes
 The number of combinations possible for daughter cells formed by meiosis of a diploid cell with two homologous pairs of chromosomes if 4
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Only two of the four chromosomes would result from meiosis of a single diploid cell because a single parent cell would have one or the other possible chromosomal arrangement at metaphase I, but not both o The number of possible combinations when chromosomes sort independently during meiosis is 2 n , where n is the haploid number of the organism
 In humans, the number of possible combinations of maternal and paternal chromosomes is 2 23 , so each gamete you produce in your lifetime has roughly 8 million possible combinations of chromosomes inherited
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Crossing Over o Crossing over produces recombinant chromosomes

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 Recombinant chromosomes: individual chromosomes that carry genes derived from two different parents o Crossing over begins early in prophase I
 Each gene on one homologue is aligned precisely with the corresponding gene on the other homologue o In crossing over, the DNA molecules of two nonsister chromatids are broken at the same place and then rejoined to each other’s DNA
 The segment of each sister chromatid from the break point to the end is joint to the rest of the other chromatid
 Two homologous segments trade places, producing chromosomes with new combinations of maternal and paternal genes o In humans, an average of one to three crossover events occur per chromosome pair, depending on the size of the chromosomes and the position of their centromeres
 At metaphase II, chromosomes that contain one or more recombinant chromatids can be oriented in two alternative ways with respect to their chromosomes
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Independent assortment of these nonsister chromatids increases even more the number of genetic types of daughter cells that can result from meiosis
Random Fertilization o In humans, each male and female gamete represents one of approximately 8 million possible chromosome combinations due to independent assortment during meiosis
 Fusion of male and female gamete during fertilization will produce a zygote with an yof about 64 trillion diploid combinations
Evolutionary Significance of Genetic Variation within Populations
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Darwin recognized that a population evolves through the differential reproductive success of its variant members o Generally, those best suited to the local environment leave the most offspring, thus transmitting their genes
 Natural selection results in the accumulation of those genetic variations favored by the environment
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As the environment changes, population may survive if at least some of its members can cope effectively with the new conditions
Although Darwin realized that heritable variation is what makes evolution possible, he could not explain why offspring resemble their parents