chapter 15 the chromosomal basis of inheritance...

28
CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Section A: Relating Mendelism to Chromosomes 1. Mendelian inheritance has its physical basis in the behavior of chromosomes during sexual life cycles 2. Morgan traced a gene to a specific chromosome 3. Linked genes tend to be inherited together because they are located on the same chromosome 4. Independent assortment of chromosomes and crossing over produce genetic recombinants 5. Geneticists use recombination data to map a chromosome’s genetic loci

Upload: trinhdung

Post on 10-Aug-2018

242 views

Category:

Documents


0 download

TRANSCRIPT

Page 1: CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/15A-RelatngMendelToChromo.pdf · THE CHROMOSOMAL BASIS OF INHERITANCE ... banding, geneticists

CHAPTER 15THE CHROMOSOMAL BASIS OF

INHERITANCE

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Section A: Relating Mendelism to Chromosomes1. Mendelian inheritance has its physical basis in the behavior of

chromosomes during sexual life cycles2. Morgan traced a gene to a specific chromosome3. Linked genes tend to be inherited together because they are located on the

same chromosome4. Independent assortment of chromosomes and crossing over produce genetic

recombinants5. Geneticists use recombination data to map a chromosome’s genetic loci

Page 2: CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/15A-RelatngMendelToChromo.pdf · THE CHROMOSOMAL BASIS OF INHERITANCE ... banding, geneticists

• It was not until 1900 that biology finally caught upwith Gregor Mendel.

• Independently, Karl Correns, Erich von Tschermak,and Hugo de Vries all found that Mendel hadexplained the same results 35 years before.

• Still, resistance remained about Mendel’s laws ofsegregation and independent assortment untilevidence had mounted that they had a physical basisin the behavior of chromosomes.

• Mendel’s hereditary factors are the genes located onchromosomes.

Introduction

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Page 3: CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/15A-RelatngMendelToChromo.pdf · THE CHROMOSOMAL BASIS OF INHERITANCE ... banding, geneticists

• Around 1900, cytologists and geneticists began tosee parallels between the behavior of chromosomesand the behavior of Mendel’s factors.• Chromosomes and genes are both present in pairs in

diploid cells.• Homologous chromosomes separate and alleles segregate

during meiosis.• Fertilization restores the paired condition for both

chromosomes and genes.

1. Mendelian inheritance has its physicalbasis in the behavior of chromosomesduring sexual life cycles

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Page 4: CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/15A-RelatngMendelToChromo.pdf · THE CHROMOSOMAL BASIS OF INHERITANCE ... banding, geneticists

• Around 1902, Walter Sutton, Theodor Boveri, andothers noted these parallels and a chromosometheory of inheritance began to take form.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 15.1

Page 5: CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/15A-RelatngMendelToChromo.pdf · THE CHROMOSOMAL BASIS OF INHERITANCE ... banding, geneticists

• Thomas Hunt Morgan was the first to associate aspecific gene with a specific chromosome in the early20th century.

• Like Mendel, Morgan made an insightful choice asan experimental animal, Drosophila melanogaster, afruit fly species that eats fungi on fruit.• Fruit flies are prolific breeders and have a generation time

of two weeks.• Fruit flies have three pairs of autosomes and a pair of sex

chromosomes (XX in females, XY in males).

2. Morgan traced a gene to a specificchromosome

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Page 6: CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/15A-RelatngMendelToChromo.pdf · THE CHROMOSOMAL BASIS OF INHERITANCE ... banding, geneticists

• Morgan spent a year looking for variantindividuals among the flies he was breeding.• He discovered a single male fly with white eyes instead

of the usual red.

• The normal character phenotype is the wild type.

• Alternativetraits aremutantphenotypes.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 15.2

Page 7: CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/15A-RelatngMendelToChromo.pdf · THE CHROMOSOMAL BASIS OF INHERITANCE ... banding, geneticists

• When Morgan crossed his white-eyed male with ared-eyed female, all the F1 offspring had red eyes,• The red allele appeared dominant to the white allele.

• Crosses between the F1 offspring produced theclassic 3:1 phenotypic ratio in the F2 offspring.

• Surprisingly, the white-eyed trait appeared only inmales.• All the females and half the males had red eyes.

• Morgan concluded that a fly’s eye color was linkedto its sex.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Page 8: CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/15A-RelatngMendelToChromo.pdf · THE CHROMOSOMAL BASIS OF INHERITANCE ... banding, geneticists

• Morgan deduced that thegene with the white-eyedmutation is on the Xchromosome alone, a sex-linked gene.• Females (XX) may have two

red-eyed alleles and have redeyes or may be heterozygousand have red eyes.

• Males (XY) have only asingle allele and will be redeyed if they have a red-eyedallele or white-eyed if theyhave a white-eyed allele.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 15.3

Page 9: CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/15A-RelatngMendelToChromo.pdf · THE CHROMOSOMAL BASIS OF INHERITANCE ... banding, geneticists

• Each chromosome has hundreds or thousands ofgenes.

• Genes located on the same chromosome, linkedgenes, tend to be inherited together because thechromosome is passed along as a unit.

• Results of crosses with linked genes deviate fromthose expected according to independent assortment.

3. Linked genes tend to be inheritedtogether because they are located on thesame chromosome

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Page 10: CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/15A-RelatngMendelToChromo.pdf · THE CHROMOSOMAL BASIS OF INHERITANCE ... banding, geneticists

• Morgan observed this linkage and its deviationswhen he followed the inheritance of characters forbody color and wing size.• The wild-type body color is gray (b+) and the mutant

black (b).

• The wild-type wing size is normal (vg+) and the mutanthas vestigial wings (vg).

• Morgan crossed F1 heterozygous females(b+bvg+vg) with homozygous recessive males(bbvgvg).

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Page 11: CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/15A-RelatngMendelToChromo.pdf · THE CHROMOSOMAL BASIS OF INHERITANCE ... banding, geneticists

• According to independent assortment, this shouldproduce 4 phenotypes in a 1:1:1:1 ratio.

• Surprisingly, Morgan observed a large number ofwild-type (gray-normal) and double-mutant (black-vestigial) flies among the offspring.• These phenotypes correspond to those of the parents.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Page 12: CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/15A-RelatngMendelToChromo.pdf · THE CHROMOSOMAL BASIS OF INHERITANCE ... banding, geneticists

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 15.4

Page 13: CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/15A-RelatngMendelToChromo.pdf · THE CHROMOSOMAL BASIS OF INHERITANCE ... banding, geneticists

• Morgan reasoned that body color and wing shapeare usually inherited together because their genesare on the same chromosome.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Page 14: CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/15A-RelatngMendelToChromo.pdf · THE CHROMOSOMAL BASIS OF INHERITANCE ... banding, geneticists

• The other two phenotypes (gray-vestigial andblack-normal) were fewer than expected fromindependent assortment (and totally unexpectedfrom dependent assortment).

• These new phenotypic variations must be the resultof crossing over.

Page 15: CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/15A-RelatngMendelToChromo.pdf · THE CHROMOSOMAL BASIS OF INHERITANCE ... banding, geneticists

• The production of offspring with new combinationsof traits inherited from two parents is geneticrecombination.

• Genetic recombination can result from independentassortment of genes located on nonhomologouschromosomes or from crossing over of genes locatedon homologous chromosomes.

4. Independent assortment of chromosomesand crossing over produce geneticrecombinants

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Page 16: CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/15A-RelatngMendelToChromo.pdf · THE CHROMOSOMAL BASIS OF INHERITANCE ... banding, geneticists

• Mendel’s dihybrid cross experiments produced someoffspring that had a combination of traits that did notmatch either parent in the P generation.• If the P generation consists of a yellow-round parent

(YYRR) crossed with a green-wrinkled seed parent(yyrr), all F1 plants have yellow-round seeds (YyRr).

• A cross between an F1 plant and a homozygous recessiveplant (a test-cross) produces four phenotypes.

• Half are be parental types, with phenotypes that matchthe original P parents, either with yellow-round seeds orgreen-wrinkled seeds.

• Half are recombinants, new combination of parentaltraits, with yellow-wrinkled or green-round seeds.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Page 17: CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/15A-RelatngMendelToChromo.pdf · THE CHROMOSOMAL BASIS OF INHERITANCE ... banding, geneticists

• A 50% frequency of recombination is observed forany two genes located on different(nonhomologous) chromosomes.

• The physical basis of recombination betweenunlinked genes is the random orientation ofhomologous chromosomes at metaphase 1.• The F1 parent (YyRr) can produce gametes with four

different combinations of alleles.

• These include YR, Yr, yR, and yr.

• The orientation of the tetrad containing the seed colorgene has no bearing on the orientation on the tetradwith the seed shape gene.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Page 18: CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/15A-RelatngMendelToChromo.pdf · THE CHROMOSOMAL BASIS OF INHERITANCE ... banding, geneticists

• In contrast, linked genes, genes located on thesame chromosome, tend to move together throughmeiosis and fertilization.

• Under normal Mendelian genetic rules, we wouldnot expect linked genes to recombine intoassortments of alleles not found in the parents.• If the seed color and seed coat genes were linked, we

would expect the F1 offspring to produce only two typesof gametes, YR and yr when the tetrads separate.

• One homologous chromosome from a P generationparent carries the Y and R alleles on the samechromosome and the other homologous chromosomefrom the other P parent carries the y and r alleles.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Page 19: CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/15A-RelatngMendelToChromo.pdf · THE CHROMOSOMAL BASIS OF INHERITANCE ... banding, geneticists

• The results of Morgan’s testcross for body colorand wing shape did not conform to eitherindependent assortment or complete linkage.• Under independent assortment the testcross should

produce a 1:1:1:1 phenotypic ratio.

• If completely linked, we should expect to see a 1:1:0:0ratio with only parental phenotypes among offspring.

• Most of the offspring had parental phenotypes,suggesting linkage between the genes.

• However, 17% of the flies were recombinants,suggesting incomplete linkage.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Page 20: CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/15A-RelatngMendelToChromo.pdf · THE CHROMOSOMAL BASIS OF INHERITANCE ... banding, geneticists

• Morgan proposed that some mechanismoccasionally exchanged segments betweenhomologous chromosomes.• This switched alleles between homologous chromosomes.

• The actual mechanism, crossing over duringprophase I, results in the production of more types ofgametes than one would predict by Mendelian rulesalone.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 15.5a

Page 21: CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/15A-RelatngMendelToChromo.pdf · THE CHROMOSOMAL BASIS OF INHERITANCE ... banding, geneticists

• The occasional production of recombinant gametesduring prophase I accounts for the occurrence ofrecombinant phenotypes in Morgan’s testcross.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 15.5b

Page 22: CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/15A-RelatngMendelToChromo.pdf · THE CHROMOSOMAL BASIS OF INHERITANCE ... banding, geneticists

• One of Morgan’s students, Alfred Sturtevant, usedcrossing over of linked genes to develop a methodfor constructing a chromosome map.

• This map is an ordered list of the genetic loci along aparticular chromosome.

5. Geneticists can use recombination data tomap a chromosome’s genetic loci

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Page 23: CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/15A-RelatngMendelToChromo.pdf · THE CHROMOSOMAL BASIS OF INHERITANCE ... banding, geneticists

• Sturtevant hypothesized that the frequency ofrecombinant offspring reflected the distancesbetween genes on a chromosome.

• The farther apart two genes are, the higher theprobability that a crossover will occur betweenthem and therefore a higher recombinationfrequency.• The greater the distance between two genes, the more

points between them where crossing over can occur.

• Sturtevant used recombination frequencies fromfruit fly crosses to map the relative position ofgenes along chromosomes, a linkage map.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Page 24: CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/15A-RelatngMendelToChromo.pdf · THE CHROMOSOMAL BASIS OF INHERITANCE ... banding, geneticists

• Sturtevant used the test cross design to map therelative position of three fruit fly genes, body color(b), wing size (vg), and eye color (cn).• The recombination frequency between cn and b is 9%.• The recombination frequency between cn and vg is

9.5%.• The recombination

frequency betweenb and vg is 17%.

• The only possiblearrangement of thesethree genes placesthe eye color genebetween the other two.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 15.6

Page 25: CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/15A-RelatngMendelToChromo.pdf · THE CHROMOSOMAL BASIS OF INHERITANCE ... banding, geneticists

• Sturtevant expressed the distance between genes,the recombination frequency, as map units.• One map unit (sometimes called a centimorgan) is

equivalent to a 1% recombination frequency.

• You may notice that the three recombinationfrequencies in our mapping example are not quiteadditive: 9% (b-cn) + 9.5% (cn-vg) > 17% (b-vg).

• This results from multiple crossing over events.• A second crossing over “cancels out” the first and

reduced the observed number of recombinant offspring.

• Genes father apart (for example, b-vg) are more likely toexperience multiple crossing over events.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Page 26: CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/15A-RelatngMendelToChromo.pdf · THE CHROMOSOMAL BASIS OF INHERITANCE ... banding, geneticists

• Some genes on a chromosome are so far apart thata crossover between them is virtually certain.

• In this case, the frequency of recombinationreaches is its maximum value of 50% and thegenes act as if found on separate chromosomes andare inherited independently.• In fact, several genes studies by Mendel are located on

the same chromosome.

• For example, seed color and flower color are farenough apart that linkage is not observed.

• Plant height and pod shape should show linkage, butMendel never reported results of this cross.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Page 27: CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/15A-RelatngMendelToChromo.pdf · THE CHROMOSOMAL BASIS OF INHERITANCE ... banding, geneticists

• Genes located far apart on a chromosome aremapped by adding the recombination frequenciesbetween the distant genes and intervening genes.

• Sturtevant and hiscolleagues were ableto map the linearpositions of genes inDrosophila into fourgroups, one for eachchromosome.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 15.7

Page 28: CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE …lhsteacher.lexingtonma.org/Pohlman/15A-RelatngMendelToChromo.pdf · THE CHROMOSOMAL BASIS OF INHERITANCE ... banding, geneticists

• A linkage map provides an imperfect picture of achromosome.

• Map units indicate relative distance and order, notprecise locations of genes.• The frequency of crossing over is not actually uniform

over the length of a chromosome.

• Combined with other methods like chromosomalbanding, geneticists can develop cytological maps.• These indicated the positions of genes with respect to

chromosomal features.

• More recent techniques show the absolute distancesbetween gene loci in DNA nucleotides.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings