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PowerPoint Lectures for Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Chapter 13Chapter 13
Meiosis and Sexual Life Cycles
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• Overview: Hereditary Similarity and Variation
• Living organisms
– Are distinguished by their ability to reproduce their own kind
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• Heredity
– Is the transmission of traits from one generation to the next
• Variation
– Shows that offspring differ somewhat in appearance from parents and siblings
Figure 13.1
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• Genetics
– Is the scientific study of heredity and hereditary variation
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• Concept 13.1: Offspring acquire genes from parents by inheriting chromosomes
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Inheritance of Genes
• Genes
– Are the units of heredity
– Are segments of DNA
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• Each gene in an organism’s DNA
– Has a specific locus on a certain chromosome
• We inherit
– One set of chromosomes from our mother and one set from our father
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Comparison of Asexual and Sexual Reproduction
• In asexual reproduction
– One parent produces genetically identical offspring by mitosis
Figure 13.2
Parent
Bud
0.5 mm
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• In sexual reproduction
– Two parents give rise to offspring that have unique combinations of genes inherited from the two parents
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• Concept 13.2: Fertilization and meiosis alternate in sexual life cycles
• A life cycle
– Is the generation-to-generation sequence of stages in the reproductive history of an organism
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Sets of Chromosomes in Human Cells
• In humans
– Each somatic cell has 46 chromosomes, made up of two sets
– One set of chromosomes comes from each parent
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5 µmPair of homologouschromosomes
Centromere
Sisterchromatids
Figure 13.3
• A karyotype
– Is an ordered, visual representation of the chromosomes in a cell
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• Homologous chromosomes
– Are the two chromosomes composing a pair
– Have the same characteristics
– May also be called autosomes
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• Sex chromosomes
– Are distinct from each other in their characteristics
– Are represented as X and Y
– Determine the sex of the individual, XX being female, XY being male
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• A diploid cell
– Has two sets of each of its chromosomes
– In a human has 46 chromosomes (2n = 46)
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• In a cell in which DNA synthesis has occurred
– All the chromosomes are duplicated and thus each consists of two identical sister chromatids
Figure 13.4
Key
Maternal set ofchromosomes (n = 3)
Paternal set ofchromosomes (n = 3)
2n = 6
Two sister chromatidsof one replicatedchromosome
Two nonsisterchromatids ina homologous pair
Pair of homologouschromosomes(one from each set)
Centromere
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• Unlike somatic cells
– Gametes, sperm and egg cells are haploid cells, containing only one set of chromosomes
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Behavior of Chromosome Sets in the Human Life Cycle
• At sexual maturity
– The ovaries and testes produce haploid gametes by meiosis
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• During fertilization
– These gametes, sperm and ovum, fuse, forming a diploid zygote
• The zygote
– Develops into an adult organism
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Figure 13.5
Key
Haploid (n)
Diploid (2n)
Haploid gametes (n = 23)
Ovum (n)
SpermCell (n)
MEIOSIS FERTILIZATION
Ovary Testis Diploidzygote(2n = 46)
Mitosis anddevelopment
Multicellular diploidadults (2n = 46)
• The human life cycle
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The Variety of Sexual Life Cycles
• The three main types of sexual life cycles
– Differ in the timing of meiosis and fertilization
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• In animals
– Meiosis occurs during gamete formation
– Gametes are the only haploid cells
Gametes
Figure 13.6 A
Diploidmulticellular
organism
Key
MEIOSIS FERTILIZATION
n
n
n
2n2nZygote
Haploid
Diploid
Mitosis
(a) Animals
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MEIOSIS FERTILIZATION
nn
n
nn
2n2n
Haploid multicellularorganism (gametophyte)
Mitosis Mitosis
SporesGametes
Mitosis
Zygote
Diploidmulticellularorganism(sporophyte)
(b) Plants and some algaeFigure 13.6 B
• Plants and some algae
– Exhibit an alternation of generations
– The life cycle includes both diploid and haploid multicellular stages
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MEIOSIS FERTILIZATION
nn
n
n
n
2n
Haploid multicellularorganism
Mitosis Mitosis
Gametes
Zygote(c) Most fungi and some protistsFigure 13.6 C
• In most fungi and some protists
– Meiosis produces haploid cells that give rise to a haploid multicellular adult organism
– The haploid adult carries out mitosis, producing cells that will become gametes
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• Concept 13.3: Meiosis reduces the number of chromosome sets from diploid to haploid
• Meiosis
– Takes place in two sets of divisions, meiosis I and meiosis II
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The Stages of Meiosis
• An overview of meiosis
Figure 13.7
Interphase
Homologous pairof chromosomesin diploid parent cell
Chromosomesreplicate
Homologous pair of replicated chromosomes
Sisterchromatids Diploid cell with
replicatedchromosomes
1
2
Homologous chromosomes separate
Haploid cells withreplicated chromosomes
Sister chromatids separate
Haploid cells with unreplicated chromosomes
Meiosis I
Meiosis II
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• Meiosis I
– Reduces the number of chromosomes from diploid to haploid
• Meiosis II
– Produces four haploid daughter cells
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Centrosomes(with centriole pairs)
Sisterchromatids
Chiasmata
Spindle
Tetrad
Nuclearenvelope
Chromatin
Centromere(with kinetochore)
Microtubuleattached tokinetochore
Tertads line up
Metaphaseplate
Homologouschromosomesseparate
Sister chromatidsremain attached
Pairs of homologouschromosomes split up
Chromosomes duplicateHomologous chromosomes
(red and blue) pair and exchangesegments; 2n = 6 in this example
INTERPHASE MEIOSIS I: Separates homologous chromosomes
PROPHASE I METAPHASE I ANAPHASE I
• Interphase and meiosis I
Figure 13.8
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TELOPHASE I ANDCYTOKINESIS
PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II ANDCYTOKINESIS
MEIOSIS II: Separates sister chromatids
Cleavagefurrow Sister chromatids
separate
Haploid daughter cellsforming
During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing single chromosomes
Two haploid cellsform; chromosomesare still doubleFigure 13.8
• Telophase I, cytokinesis, and meiosis II
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A Comparison of Mitosis and Meiosis
• Meiosis and mitosis can be distinguished from mitosis
– By three events in Meiosis l
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• Synapsis and crossing over
– Homologous chromosomes physically connect and exchange genetic information
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• Tetrads on the metaphase plate
– At metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the metaphase plates
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• Separation of homologues
– At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell
– In anaphase II of meiosis, the sister chromatids separate
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Figure 13.9
MITOSIS MEIOSIS
Prophase
Duplicated chromosome(two sister chromatids)
Chromosomereplication
Chromosomereplication
Parent cell(before chromosome replication)
Chiasma (site ofcrossing over)
MEIOSIS I
Prophase I
Tetrad formed bysynapsis of homologouschromosomes
Metaphase
Chromosomespositioned at themetaphase plate
Tetradspositioned at themetaphase plate
Metaphase I
Anaphase ITelophase I
Haploidn = 3
MEIOSIS II
Daughtercells of
meiosis I
Homologuesseparateduringanaphase I;sisterchromatidsremain together
Daughter cells of meiosis II
n n n n
Sister chromatids separate during anaphase II
AnaphaseTelophase
Sister chromatidsseparate duringanaphase
2n 2nDaughter cells
of mitosis
2n = 6
• A comparison of mitosis and meiosis
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• Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution
• Reshuffling of genetic material in meiosis
– Produces genetic variation
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Origins of Genetic Variation Among Offspring
• In species that produce sexually
– The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises each generation
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Independent Assortment of Chromosomes
• Homologous pairs of chromosomes
– Orient randomly at metaphase I of meiosis
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• In independent assortment
– Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs
Figure 13.10
Key
Maternal set ofchromosomesPaternal set ofchromosomes
Possibility 1
Two equally probable arrangements ofchromosomes at
metaphase I
Possibility 2
Metaphase II
Daughtercells
Combination 1 Combination 2 Combination 3 Combination 4
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Crossing Over
• Crossing over
– Produces recombinant chromosomes that carry genes derived from two different parents
Figure 13.11
Prophase Iof meiosis
Nonsisterchromatids
Tetrad
Chiasma,site ofcrossingover
Metaphase I
Metaphase II
Daughtercells
Recombinantchromosomes
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Random Fertilization
• The fusion of gametes
– Will produce a zygote with any of about 64 trillion diploid combinations
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Evolutionary Significance of Genetic Variation Within Populations
• Genetic variation
– Is the raw material for evolution by natural selection
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• Mutations
– Are the original source of genetic variation
• Sexual reproduction
– Produces new combinations of variant genes, adding more genetic diversity
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PowerPoint Lectures for Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Chapter 14Chapter 14
Mendel and the Gene Idea
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• Overview: Drawing from the Deck of Genes
• What genetic principles account for the transmission of traits from parents to offspring?
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• One possible explanation of heredity is a “blending” hypothesis
– The idea that genetic material contributed by two parents mixes in a manner analogous to the way blue and yellow paints blend to make green
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• An alternative to the blending model is the “particulate” hypothesis of inheritance: the gene idea
– Parents pass on discrete heritable units, genes
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• Gregor Mendel
– Documented a particulate mechanism of inheritance through his experiments with garden peas
Figure 14.1
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• Concept 14.1: Mendel used the scientific approach to identify two laws of inheritance
• Mendel discovered the basic principles of heredity
– By breeding garden peas in carefully planned experiments
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Mendel’s Experimental, Quantitative Approach
• Mendel chose to work with peas
– Because they are available in many varieties
– Because he could strictly control which plants mated with which
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• Crossing pea plants
Figure 14.2
1
5
4
3
2
Removed stamensfrom purple flower
Transferred sperm-bearing pollen fromstamens of white flower to egg-bearing carpel of purple flower
Parentalgeneration(P)
Pollinated carpelmatured into pod
Carpel(female)
Stamens(male)
Planted seedsfrom pod
Examinedoffspring:all purpleflowers
Firstgenerationoffspring(F1)
APPLICATION By crossing (mating) two true-breedingvarieties of an organism, scientists can study patterns ofinheritance. In this example, Mendel crossed pea plantsthat varied in flower color.
TECHNIQUETECHNIQUE
When pollen from a white flower fertilizeseggs of a purple flower, the first-generation hybrids all have purpleflowers. The result is the same for the reciprocal cross, the transferof pollen from purple flowers to white flowers.
TECHNIQUERESULTS
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• Some genetic vocabulary
– Character: a heritable feature, such as flower color
– Trait: a variant of a character, such as purple or white flowers
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• Mendel chose to track
– Only those characters that varied in an “either-or” manner
• Mendel also made sure that
– He started his experiments with varieties that were “true-breeding”
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• In a typical breeding experiment
– Mendel mated two contrasting, true-breeding varieties, a process called hybridization
• The true-breeding parents
– Are called the P generation
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• The hybrid offspring of the P generation
– Are called the F1 generation
• When F1 individuals self-pollinate
– The F2 generation is produced
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The Law of Segregation
• When Mendel crossed contrasting, true-breeding white and purple flowered pea plants
– All of the offspring were purple
• When Mendel crossed the F1 plants
– Many of the plants had purple flowers, but some had white flowers
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• Mendel discovered
– A ratio of about three to one, purple to white flowers, in the F2 generation
Figure 14.3
P Generation
(true-breeding parents) Purple
flowersWhiteflowers
F1 Generation (hybrids)
All plants hadpurple flowers
F2 Generation
EXPERIMENT True-breeding purple-flowered pea plants andwhite-flowered pea plants were crossed (symbolized by ). Theresulting F1 hybrids were allowed to self-pollinate or were cross-pollinated with other F1 hybrids. Flower color was then observedin the F2 generation.
RESULTS Both purple-flowered plants and white-flowered plants appeared in the F2 generation. In Mendel’sexperiment, 705 plants had purple flowers, and 224 had whiteflowers, a ratio of about 3 purple : 1 white.
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• Mendel reasoned that
– In the F1 plants, only the purple flower factor was affecting flower color in these hybrids
– Purple flower color was dominant, and white flower color was recessive
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• Mendel observed the same pattern
– In many other pea plant characters
Table 14.1
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Mendel’s Model
• Mendel developed a hypothesis
– To explain the 3:1 inheritance pattern that he observed among the F2 offspring
• Four related concepts make up this model
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• First, alternative versions of genes
– Account for variations in inherited characters, which are now called alleles
Figure 14.4
Allele for purple flowers
Locus for flower-color gene
Homologouspair ofchromosomes
Allele for white flowers
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• Second, for each character
– An organism inherits two alleles, one from each parent
– A genetic locus is actually represented twice
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• Third, if the two alleles at a locus differ
– Then one, the dominant allele, determines the organism’s appearance
– The other allele, the recessive allele, has no noticeable effect on the organism’s appearance
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• Fourth, the law of segregation
– The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes
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• Does Mendel’s segregation model account for the 3:1 ratio he observed in the F2 generation of his numerous crosses?
– We can answer this question using a Punnett square
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• Mendel’s law of segregation, probability and the Punnett square
Figure 14.5
P Generation
F1 Generation
F2 Generation
P p
P p
P p
P
p
PpPP
ppPp
Appearance:Genetic makeup:
Purple flowersPP
White flowerspp
Purple flowersPp
Appearance:Genetic makeup:
Gametes:
Gametes:
F1 sperm
F1 eggs
1/21/2
Each true-breeding plant of the parental generation has identicalalleles, PP or pp.
Gametes (circles) each contain only one allele for the flower-color gene. In this case, every gamete produced by one parent has the same allele.
Union of the parental gametes produces F1 hybrids having a Pp combination. Because the purple-flower allele is dominant, allthese hybrids have purple flowers.
When the hybrid plants producegametes, the two alleles segregate, half the gametes receiving the P allele and the other half the p allele.
3 : 1
Random combination of the gametesresults in the 3:1 ratio that Mendelobserved in the F2 generation.
This box, a Punnett square, shows all possible combinations of alleles in offspring that result from an F1 F1 (Pp Pp) cross. Each square represents an equally probable product of fertilization. For example, the bottomleft box shows the genetic combinationresulting from a p egg fertilized bya P sperm.
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Useful Genetic Vocabulary
• An organism that is homozygous for a particular gene
– Has a pair of identical alleles for that gene
– Exhibits true-breeding
• An organism that is heterozygous for a particular gene
– Has a pair of alleles that are different for that gene
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• An organism’s phenotype
– Is its physical appearance
• An organism’s genotype
– Is its genetic makeup
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• Phenotype versus genotype
Figure 14.6
3
1 1
2
1
Phenotype
Purple
Purple
Purple
White
Genotype
PP(homozygous)
Pp(heterozygous)
Pp(heterozygous)
pp(homozygous)
Ratio 3:1 Ratio 1:2:1
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The Testcross
• In pea plants with purple flowers
– The genotype is not immediately obvious
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• A testcross
– Allows us to determine the genotype of an organism with the dominant phenotype, but unknown genotype
– Crosses an individual with the dominant phenotype with an individual that is homozygous recessive for a trait
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• The testcross
Figure 14.7
Dominant phenotype,unknown genotype:
PP or Pp?
Recessive phenotype,known genotype:
pp
If PP,then all offspring
purple:
If Pp,then 1⁄2 offspring purpleand 1⁄2 offspring white:
p p
P
P
Pp Pp
PpPp
pp pp
PpPpP
p
p p
APPLICATION An organism that exhibits a dominant trait,such as purple flowers in pea plants, can be either homozygous forthe dominant allele or heterozygous. To determine the organism’sgenotype, geneticists can perform a testcross.
TECHNIQUE In a testcross, the individual with theunknown genotype is crossed with a homozygous individualexpressing the recessive trait (white flowers in this example). By observing the phenotypes of the offspring resulting from this cross, we can deduce the genotype of the purple-flowered parent.
RESULTS
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The Law of Independent Assortment
• Mendel derived the law of segregation
– By following a single trait
• The F1 offspring produced in this cross
– Were monohybrids, heterozygous for one character
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• Mendel identified his second law of inheritance
– By following two characters at the same time
• Crossing two, true-breeding parents differing in two characters
– Produces dihybrids in the F1 generation, heterozygous for both characters
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• How are two characters transmitted from parents to offspring?
– As a package?
– Independently?
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YYRRP Generation
Gametes YR yr
yyrr
YyRrHypothesis ofdependentassortment
Hypothesis ofindependentassortment
F2 Generation(predictedoffspring)
1⁄2 YR
YR
yr
1 ⁄2
1 ⁄2
1⁄2 yr
YYRR YyRr
yyrrYyRr
3 ⁄4 1 ⁄4
Sperm
Eggs
Phenotypic ratio 3:1
YR1 ⁄4
Yr1 ⁄4
yR1 ⁄4
yr1 ⁄4
9 ⁄163 ⁄16
3 ⁄161 ⁄16
YYRR YYRr YyRR YyRr
YyrrYyRrYYrrYYrr
YyRR YyRr yyRR yyRr
yyrryyRrYyrrYyRr
Phenotypic ratio 9:3:3:1
315 108 101 32 Phenotypic ratio approximately 9:3:3:1
F1 Generation
EggsYR Yr yR yr1 ⁄4 1 ⁄4 1 ⁄4 1 ⁄4
Sperm
RESULTS
CONCLUSION The results support the hypothesis of independent assortment. The alleles for seed color and seed shape sort into gametes independently of each other.
EXPERIMENT Two true-breeding pea plants—one with yellow-round seeds and the other with green-wrinkled seeds—were crossed, producing dihybrid F1 plants. Self-pollination of the F1 dihybrids, which are heterozygous for both characters, produced the F2 generation. The two hypotheses predict different phenotypic ratios. Note that yellow color (Y) and round shape (R) are dominant.
• A dihybrid cross
– Illustrates the inheritance of two characters
• Produces four phenotypes in the F2 generation
Figure 14.8
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• Using the information from a dihybrid cross, Mendel developed the law of independent assortment
– Each pair of alleles segregates independently during gamete formation
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• Concept 14.2: The laws of probability govern Mendelian inheritance
• Mendel’s laws of segregation and independent assortment
– Reflect the rules of probability
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The Multiplication and Addition Rules Applied to Monohybrid Crosses• The multiplication rule
– States that the probability that two or more independent events will occur together is the product of their individual probabilities
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• Probability in a monohybrid cross
– Can be determined using this rule
Rr
Segregation ofalleles into eggs
Rr
Segregation ofalleles into sperm
R r
rR
RR
R1⁄2
1⁄2 1⁄2
1⁄41⁄4
1⁄4 1⁄4
1⁄2 rr
R rr
Sperm
Eggs
Figure 14.9
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• The rule of addition
– States that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities
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Solving Complex Genetics Problems with the Rules of Probability• We can apply the rules of probability
– To predict the outcome of crosses involving multiple characters
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• A dihybrid or other multicharacter cross
– Is equivalent to two or more independent monohybrid crosses occurring simultaneously
• In calculating the chances for various genotypes from such crosses
– Each character first is considered separately and then the individual probabilities are multiplied together
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• Concept 14.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics
• The relationship between genotype and phenotype is rarely simple
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Extending Mendelian Genetics for a Single Gene
• The inheritance of characters by a single gene
– May deviate from simple Mendelian patterns
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The Spectrum of Dominance
• Complete dominance
– Occurs when the phenotypes of the heterozygote and dominant homozygote are identical
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• In codominance
– Two dominant alleles affect the phenotype in separate, distinguishable ways
• The human blood group MN
– Is an example of codominance
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• In incomplete dominance
– The phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
Figure 14.10
P Generation
F1 Generation
F2 Generation
RedCRCR
Gametes CR CW
WhiteCWCW
PinkCRCW
Sperm
CR
CR
CR
Cw
CR
CRGametes1⁄2 1⁄2
1⁄2
1⁄2
1⁄2
Eggs1⁄2
CR CR CR CW
CW CWCR CW
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• The Relation Between Dominance and Phenotype
• Dominant and recessive alleles
– Do not really “interact”
– Lead to synthesis of different proteins that produce a phenotype
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• Frequency of Dominant Alleles
• Dominant alleles
– Are not necessarily more common in populations than recessive alleles
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Multiple Alleles
• Most genes exist in populations
– In more than two allelic forms
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• The ABO blood group in humans
– Is determined by multiple alleles
Table 14.2
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Pleiotropy
• In pleiotropy
– A gene has multiple phenotypic effects
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Extending Mendelian Genetics for Two or More Genes
• Some traits
– May be determined by two or more genes
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Epistasis
• In epistasis
– A gene at one locus alters the phenotypic expression of a gene at a second locus
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• An example of epistasis
Figure 14.11
BC bC Bc bc1⁄41⁄41⁄41⁄4
BC
bC
Bc
bc
1⁄4
1⁄4
1⁄4
1⁄4
BBCc BbCc BBcc Bbcc
Bbcc bbccbbCcBbCc
BbCC bbCC BbCc bbCc
BBCC BbCC BBCc BbCc
9⁄163⁄16
4⁄16
BbCc BbCc
Sperm
Eggs
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Polygenic Inheritance
• Many human characters
– Vary in the population along a continuum and are called quantitative characters
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AaBbCc AaBbCc
aabbcc Aabbcc AaBbcc AaBbCc AABbCcAABBCcAABBCC
20⁄64
15⁄64
6⁄64
1⁄64
Fra
cti o
n o
f p
rog
en
y
• Quantitative variation usually indicates polygenic inheritance
– An additive effect of two or more genes on a single phenotype
Figure 14.12
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Nature and Nurture: The Environmental Impact on Phenotype• Another departure from simple Mendelian
genetics arises
– When the phenotype for a character depends on environment as well as on genotype
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• The norm of reaction
– Is the phenotypic range of a particular genotype that is influenced by the environment
Figure 14.13
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• Multifactorial characters
– Are those that are influenced by both genetic and environmental factors
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Integrating a Mendelian View of Heredity and Variation
• An organism’s phenotype
– Includes its physical appearance, internal anatomy, physiology, and behavior
– Reflects its overall genotype and unique environmental history
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• Even in more complex inheritance patterns
– Mendel’s fundamental laws of segregation and independent assortment still apply
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• Concept 14.4: Many human traits follow Mendelian patterns of inheritance
• Humans are not convenient subjects for genetic research
– However, the study of human genetics continues to advance
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Pedigree Analysis
• A pedigree
– Is a family tree that describes the interrelationships of parents and children across generations
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• Inheritance patterns of particular traits
– Can be traced and described using pedigrees
Figure 14.14 A, B
Ww ww ww Ww
wwWwWwwwwwWw
WWor
Ww
ww
First generation(grandparents)
Second generation(parents plus aunts
and uncles)
Thirdgeneration
(two sisters)
Ff Ff ff Ff
ffFfFfffFfFF or Ff
ff FForFf
Widow’s peak No Widow’s peak Attached earlobe Free earlobe
(a) Dominant trait (widow’s peak) (b) Recessive trait (attached earlobe)
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• Pedigrees
– Can also be used to make predictions about future offspring
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Recessively Inherited Disorders
• Many genetic disorders
– Are inherited in a recessive manner
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• Recessively inherited disorders
– Show up only in individuals homozygous for the allele
• Carriers
– Are heterozygous individuals who carry the recessive allele but are phenotypically normal
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Cystic Fibrosis
• Symptoms of cystic fibrosis include
– Mucus buildup in the some internal organs
– Abnormal absorption of nutrients in the small intestine
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Sickle-Cell Disease
• Sickle-cell disease
– Affects one out of 400 African-Americans
– Is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells
• Symptoms include
– Physical weakness, pain, organ damage, and even paralysis
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Mating of Close Relatives
• Matings between relatives
– Can increase the probability of the appearance of a genetic disease
– Are called consanguineous matings
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Dominantly Inherited Disorders
• Some human disorders
– Are due to dominant alleles
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• One example is achondroplasia
– A form of dwarfism that is lethal when homozygous for the dominant allele
Figure 14.15
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• Huntington’s disease
– Is a degenerative disease of the nervous system
– Has no obvious phenotypic effects until about 35 to 40 years of age
Figure 14.16
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Multifactorial Disorders
• Many human diseases
– Have both genetic and environment components
• Examples include
– Heart disease and cancer
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Genetic Testing and Counseling
• Genetic counselors
– Can provide information to prospective parents concerned about a family history for a specific disease
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Counseling Based on Mendelian Genetics and Probability Rules• Using family histories
– Genetic counselors help couples determine the odds that their children will have genetic disorders
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Tests for Identifying Carriers
• For a growing number of diseases
– Tests are available that identify carriers and help define the odds more accurately
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Fetal Testing
• In amniocentesis
– The liquid that bathes the fetus is removed and tested
• In chorionic villus sampling (CVS)
– A sample of the placenta is removed and tested
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• Fetal testing
Figure 14.17 A, B
(a) Amniocentesis
Amnioticfluidwithdrawn
Fetus
Placenta Uterus Cervix
Centrifugation
A sample ofamniotic fluid canbe taken starting atthe 14th to 16thweek of pregnancy.
(b) Chorionic villus sampling (CVS)
FluidFetalcells
Biochemical tests can bePerformed immediately onthe amniotic fluid or lateron the cultured cells.
Fetal cells must be culturedfor several weeks to obtainsufficient numbers forkaryotyping.
Severalweeks
Biochemicaltests
Severalhours
Fetalcells
Placenta Chorionic viIIi
A sample of chorionic villustissue can be taken as earlyas the 8th to 10th week ofpregnancy.
Suction tubeInserted throughcervix
Fetus
Karyotyping and biochemicaltests can be performed onthe fetal cells immediately,providing results within a dayor so.
Karyotyping
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Newborn Screening
• Some genetic disorders can be detected at birth
– By simple tests that are now routinely performed in most hospitals in the United States