14 mendel text

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Mendel and the Gene Idea

What genetic principles account for the transmission of traits from parents to offspring?

• 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


• An alternative to the blending model is the “particulate” hypothesis of inheritance: the gene idea

– Parents pass on discrete heritable units, genes


• Gregor Mendel

– Documented a particulate mechanism of inheritance through his experiments with garden peas

Figure 14.1


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


• 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


• Some genetic vocabulary

– Character: a heritable feature, such as flower color

– Trait: a variant of a character, such as purple or white flowers


• 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”


• 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


• The hybrid offspring of the P generation

– Are called the F1 generation

• When F1 individuals self-pollinate

– The F2 generation is produced


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


• 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.


• 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


• Mendel observed the same pattern

– In many other pea plant characters

Table 14.1


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


• 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


• Second, for each character

– An organism inherits two alleles, one from each parent

– A genetic locus is actually represented twice


• 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


• Fourth, the law of segregation

– The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes


• 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


• 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.


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


• An organism’s phenotype

– Is its physical appearance

• An organism’s genotype

– Is its genetic makeup


• 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


The Testcross

• In pea plants with purple flowers

– The genotype is not immediately obvious

• 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


• 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


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


• 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


• How are two characters transmitted from parents to offspring?

– As a package?

– Independently?


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


• Using the information from a dihybrid cross, Mendel developed the law of independent assortment

– Each pair of alleles segregates independently during gamete formation


• The laws of probability govern Mendelian inheritance

• Mendel’s laws of segregation and independent assortment

– Reflect the rules of probability


• 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


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


• 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


The Spectrum of Dominance

• Complete dominance

– Occurs when the phenotypes of the heterozygote and dominant homozygote are identical

• In codominance

– Two dominant alleles affect the phenotype in separate, distinguishable ways

• The human blood group

- Is an example of codominance


• 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


• Frequency of Dominant Alleles

• Dominant alleles

– Are not necessarily more common in populations than recessive alleles


Multiple Alleles

• The ABO blood group in humans

– Is determined by multiple alleles

Table 14.2


Pleiotropy

• In pleiotropy

– A gene has multiple phenotypic effects

• Some traits (polygenic)

– May be determined by two or more genes

• In epistasis

– A gene at one locus alters the phenotypic expression of a gene at a second locus


• 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


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


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


• The norm of reaction

– Is the phenotypic range of a particular genotype that is influenced by the environment

Figure 14.13


• Multifactorial characters

– Are those that are influenced by both genetic and environmental factors

• An organism’s phenotype

– Includes its physical appearance, internal anatomy, physiology, and behavior

– Reflects its overall genotype and unique environmental history


• Even in more complex inheritance patterns

– Mendel’s fundamental laws of segregation and independent assortment still apply


• 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


Different Colors?


Shades skin color


Shades skin color


A&E skin color


A&E skin color


A&E skin color


One Generation


Skin tones – Nat. Geo.

‘Yet with the effects of human migrations and cultural habits, people in one place can show tremendous variation in skin tone – like students from the Washington International Primary

School.” ‘Unmasking Skin,’ Joel L. Swerdlow, National Geographic, Nov. 2002 p46-47.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 55


Tower of Babel


Punnit Square - grays






Eye Shapes


Q 664 abcnews

www.abcnews.com, Science page, "We're all the same," 9/10/98

What the facts show is that there are

differences among us, but they stem from culture, not race.

Q 664


Biological Fact


Acts 17:26

Acts 17:26And hath made of one blood

all nations of men for to dwell on all the face of the earth, and hath determined the

times before appointed, and the bounds of their

habitation;


Races?


Biblical View Acts 17:26






Pedigree Analysis

• A pedigree

– Is a family tree that describes the interrelationships of parents and children across generations


• 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)

PedigreesCan also be used to make predictions about future offspring


Recessively Inherited Disorders

• Many genetic disorders

– Are inherited in a recessive manner

• 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


Cystic Fibrosis

• Symptoms of cystic fibrosis include

– Mucus buildup in the some internal organs

– Abnormal absorption of nutrients in the small intestine


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


Mating of Close Relatives

• Matings between relatives

– Can increase the probability of the appearance of a genetic disease

– Are called consanguineous matings


Dominantly Inherited Disorders

• Some human disorders

– Are due to dominant alleles


• One example is achondroplasia

– A form of dwarfism that is lethal when homozygous for the dominant allele

Figure 14.15


• 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


Multifactorial Disorders

• Many human diseases

– Have both genetic and environment components

• Examples include

– Heart disease and cancer


Genetic Testing and Counseling

• Genetic counselors

– Can provide information to prospective parents concerned about a family history for a specific disease


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


Tests for Identifying Carriers

• For a growing number of diseases

– Tests are available that identify carriers and help define the odds more accurately

• 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


• 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


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


• Overview: Locating Genes on Chromosomes

• Genes

– Are located on chromosomes

– Can be visualized using certain techniques

Figure 15.1


• Mendelian inheritance has its physical basis in the behavior of chromosomes

• Several researchers proposed in the early 1900s that genes are located on chromosomes

• The behavior of chromosomes during meiosis was said to account for Mendel’s laws of segregation and independent assortment


• The chromosome theory of inheritance states that

– Mendelian genes have specific loci on chromosomes

– Chromosomes undergo segregation and independent assortment


• The chromosomal basis of Mendel’s laws

Figure 15.2

Yellow-roundseeds (YYRR)

Green-wrinkledseeds (yyrr)

Meiosis

Fertilization

Gametes

All F1 plants produceyellow-round seeds (YyRr)

P Generation

F1 Generation

Meiosis

Two equallyprobable

arrangementsof chromosomesat metaphase I

LAW OF SEGREGATION LAW OF INDEPENDENT ASSORTMENT

Anaphase I

Metaphase II

Fertilization among the F1 plants

9 : 3 : 3 : 1

14

14

14

14

YR yr yr yR

Gametes

Y

RRY

y

r

r

y

R Y y r

Ry

Y

r

Ry

Y

r

R

Y

r

y

r R

Y y

R

Y

r

y

R

Y

Y

R R

Y

r

y

r

y

R

y

r

Y

r

Y

r

Y

r

Y

R

y

R

y

R

y

r

Y

F2 Generation

Starting with two true-breeding pea plants,we follow two genes through the F1 and F2 generations. The two genes specify seed color (allele Y for yellow and allele y forgreen) and seed shape (allele R for round and allele r for wrinkled). These two genes are on different chromosomes. (Peas have seven chromosome pairs, but only two pairs are illustrated here.)

The R and r alleles segregate at anaphase I, yielding two types of daughter cells for this locus.

1

Each gamete gets one long chromosome with either the R or r allele.

2

Fertilizationrecombines the R and r alleles at random.

3

Alleles at both loci segregatein anaphase I, yielding four types of daughter cells depending on the chromosomearrangement at metaphase I. Compare the arrangement of the R and r alleles in the cellson the left and right

1

Each gamete gets a long and a short chromosome in one of four allele combinations.

2

Fertilization results in the 9:3:3:1 phenotypic ratio in the F2 generation.

3


Morgan’s Experimental Evidence: Scientific Inquiry

• Thomas Hunt Morgan

– Provided convincing evidence that chromosomes are the location of Mendel’s heritable factors


Morgan’s Choice of Experimental Organism

• Morgan worked with fruit flies

– Because they breed at a high rate

– A new generation can be bred every two weeks

– They have only four pairs of chromosomes


• Morgan first observed and noted

– Wild type, or normal, phenotypes that were common in the fly populations

• Traits alternative to the wild type

– Are called mutant phenotypes

Figure 15.3


Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair

• In one experiment Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type)

– The F1 generation all had red eyes

– The F2 generation showed the 3:1 red:white eye ratio, but only males had white eyes


Figure 15.4

The F2 generation showed a typical Mendelian 3:1 ratio of red eyes to white eyes. However, no females displayed the white-eye trait; they all had red eyes. Half the males had white eyes,and half had red eyes.

Morgan then bred an F1 red-eyed female to an F1 red-eyed male toproduce the F2 generation.

RESULTS

PGeneration

F1

Generation

X

F2

Generation

Morgan mated a wild-type (red-eyed) female with a mutant white-eyed male. The F1 offspring all had red eyes.EXPERIMENT

• Morgan determined

– That the white-eye mutant allele must be located on the X chromosome


CONCLUSION Since all F1 offspring had red eyes, the mutant white-eye trait (w) must be recessive to the wild-type red-eye trait (w+). Since the recessive trait—white eyes—was expressed only in males in the F2 generation, Morgan hypothesized that the eye-color gene is located on the X chromosome and that there is no corresponding locus on the Y chromosome, as diagrammed here.

PGeneration

F1

Generation

F2

Generation

Ova(eggs)

Ova(eggs)

Sperm

Sperm

XX X

XY

WW+

W+

W

W+W+ W+

W+

W+

W+

W+

W+

W

W+

W W

W


• Morgan’s discovery that transmission of the X chromosome in fruit flies correlates with inheritance of the eye-color trait

– Was the first solid evidence indicating that a specific gene is associated with a specific chromosome


• Linked genes tend to be inherited together because they are located near each other on the same chromosome

• Each chromosome

– Has hundreds or thousands of genes


How Linkage Affects Inheritance: Scientific Inquiry

• Morgan did other experiments with fruit flies

– To see how linkage affects the inheritance of two different characters


• Morgan crossed flies

– That differed in traits of two different characters

Double mutant(black body,vestigial wings)


Wild type(gray body,

normal wings)

P Generation(homozygous)

b+ b+ vg+ vg+

x

b b vg vg

F1 dihybrid(wild type)(gray body, normal wings)

b+ b vg+ vgb b vg vg

TESTCROSSx

b+vg+ b vg b+ vg b vg+

b vg

b+ b vg+ vg b b vg vg b+ b vg vgb b vg+ vg

965Wild type

(gray-normal)

944Black-

vestigial

206Gray-

vestigial

185Black-normal

Sperm

Parental-typeoffspring

Recombinant (nonparental-type)offspring

RESULTS

EXPERIMENT Morgan first mated true-breedingwild-type flies with black, vestigial-winged flies to produce heterozygous F1 dihybrids, all of which are wild-type in appearance. He then mated wild-type F1 dihybrid females with black, vestigial-winged males, producing 2,300 F2 offspring, which he “scored” (classified according to phenotype).

CONCLUSION If these two genes were on different chromosomes, the alleles from the F1 dihybrid would sort into gametes independently, and we would expect to see equal numbers of the four types of offspring. If these two genes were on the same chromosome, we would expect each allele combination, B+ vg+ and b vg, to stay together as gametes formed. In this case, onlyoffspring with parental phenotypes would be produced. Since most offspring had a parental phenotype, Morganconcluded that the genes for body color and wing sizeare located on the same chromosome. However, the production of a small number of offspring with nonparental phenotypes indicated that some mechanism occasionally breaks the linkage between genes on the same chromosome.

Figure 15.5




• Morgan determined that

– Genes that are close together on the same chromosome are linked and do not assort independently

– Unlinked genes are either on separate chromosomes or are far apart on the same chromosome and assort independently

Parentsin testcross

b+ vg+

b vg

b+ vg+

b vg

b vg

b vg

b vg

b vg

Mostoffspring

X

or


Recombination of Unlinked Genes: Independent Assortment of Chromosomes

• When Mendel followed the inheritance of two characters

– He observed that some offspring have combinations of traits that do not match either parent in the P generation

Gametes from green-wrinkled homozygousrecessive parent (yyrr)

Gametes from yellow-roundheterozygous parent (YyRr)

Parental-type offspring

Recombinantoffspring

YyRr yyrr Yyrr yyRr

YR yr Yr yR

yr


• Recombinant offspring

– Are those that show new combinations of the parental traits

• When 50% of all offspring are recombinants

– Geneticists say that there is a 50% frequency of recombination


Recombination of Linked Genes: Crossing Over

• Morgan discovered that genes can be linked

– But due to the appearance of recombinant phenotypes, the linkage appeared incomplete


• Morgan proposed that

– Some process must occasionally break the physical connection between genes on the same chromosome

– Crossing over of homologous chromosomes was the mechanism


Figure 15.6

Testcrossparents

Gray body,normal wings(F1 dihybrid)

b+ vg+

b vgReplication ofchromosomes

b+ vg

b+vg+

b

vg

vgMeiosis I: Crossingover between b and vgloci produces new allelecombinations.

Meiosis II: Segregationof chromatids producesrecombinant gameteswith the new allelecombinations.

Recombinantchromosome

b+vg+ b vg b+ vg b vg+

b vg

Sperm

b vg

b vgReplication ofchromosomesvg

vg

b

b

bvg

b vg

Meiosis I and II:Even if crossing overoccurs, no new allelecombinations areproduced.

OvaGametes

Testcrossoffspring

Sperm

b+ vg+ b vg b+ vg b vg+

965Wild type

(gray-normal)b+ vg+

b vg b vg b vg b vg

b vg+b+ vg+b vg+

944Black-

vestigial

206Gray-

vestigial

185Black-normal Recombination

frequency =391 recombinants

2,300 total offspring 100 = 17%

Parental-type offspring Recombinant offspring

Ova

b vg

Black body,vestigial wings(double mutant)

b

• Linked genes

– Exhibit recombination frequencies less than 50%


Linkage Mapping: Using Recombination Data: Scientific Inquiry

• A genetic map

– Is an ordered list of the genetic loci along a particular chromosome

– Can be developed using recombination frequencies


• A linkage map

– Is the actual map of a chromosome based on recombination frequencies

Recombinationfrequencies

9% 9.5%

17%

b cn vgChromosome

The b–vg recombination frequency is slightly less than the sum of the b–cn and cn–vg frequencies because double crossovers are fairly likely to occur between b and vg in matings tracking these two genes. A second crossoverwould “cancel out” the first and thus reduce the observed b–vg recombination frequency.

In this example, the observed recombination frequencies between three Drosophila gene pairs (b–cn 9%, cn–vg 9.5%, and b–vg 17%) best fit a linear order in which cn is positioned about halfway between the other two genes:

RESULTS

A linkage map shows the relative locations of genes along a chromosome.APPLICATION

TECHNIQUE A linkage map is based on the assumption that the probability of a crossover between twogenetic loci is proportional to the distance separating the loci. The recombination frequencies used to constructa linkage map for a particular chromosome are obtained from experimental crosses, such as the cross depictedin Figure 15.6. The distances between genes are expressed as map units (centimorgans), with one map unitequivalent to a 1% recombination frequency. Genes are arranged on the chromosome in the order that best fits the data.

Figure 15.7


• The farther apart genes are on a chromosome

– The more likely they are to be separated during crossing over


• Many fruit fly genes

– Were mapped initially using recombination frequencies

Figure 15.8

Mutant phenotypes

Short aristae

Black body

Cinnabareyes

Vestigialwings

Brown eyes

Long aristae(appendageson head)

Gray body

Redeyes

Normalwings

Redeyes

Wild-type phenotypes

IIY

I

X IVIII

0 48.5 57.5 67.0 104.5


• Sex-linked genes exhibit unique patterns of inheritance

• An organism’s sex

– Is an inherited phenotypic character determined by the presence or absence of certain chromosomes


• In humans and other mammals

– There are two varieties of sex chromosomes, X and Y

Figure 15.9a

(a) The X-Y system

44 +XY

44 +XX

Parents

22 +X

22 +Y

22 +XY

Sperm Ova

44 +XX

44 +XY

Zygotes(offspring)


• Different systems of sex determination

– Are found in other organisms

Figure 15.9b–d

22 +XX

22 +X

76 +ZZ

76 +ZW

16(Haploid)

16(Diploid)

(b) The X–0 system

(c) The Z–W system

(d) The haplo-diploid system


Inheritance of Sex-Linked Genes

• The sex chromosomes

– Have genes for many characters unrelated to sex

• A gene located on either sex chromosome

– Is called a sex-linked gene


• Sex-linked genes

– Follow specific patterns of inheritance

Figure 15.10a–c

XAXA XaY

Xa Y

XAXa XAY

XAYXAYa

XA

XA

Ova

Sperm

XAXa XAY

Ova XA

Xa

XAXA XAY

XaYXaYA

XA YSperm

XAXa XaY

Ova

Xa Y

XAXa XAY

XaYXaYa

XA

Xa

A father with the disorder will transmit the mutant allele to all daughters but to no sons. When the mother is a dominant homozygote, the daughters will have the normal phenotype but will be carriers of the mutation.

If a carrier mates with a male of normal phenotype, there is a 50% chance that each daughter will be a carrier like her mother, and a 50% chance that each son will have the disorder.

If a carrier mates with a male who has the disorder, there is a 50% chance that each child born to them will have the disorder, regardless of sex. Daughters who do not have the disorder will be carriers, where as males without the disorder will be completely free of the recessive allele.

(a)

(b)

(c)

Sperm


• Some recessive alleles found on the X chromosome in humans cause certain types of disorders

– Color blindness

– Duchenne muscular dystrophy

– Hemophilia


X inactivation in Female Mammals

• In mammalian females

– One of the two X chromosomes in each cell is randomly inactivated during embryonic development


• If a female is heterozygous for a particular gene located on the X chromosome

– She will be a mosaic for that character

Two cell populationsin adult cat:

Active X

Orangefur

Inactive X

Early embryo:X chromosomes

Allele forblack fur

Cell divisionand X

chromosomeinactivation

Active X

Blackfur

Inactive X

Figure 15.11


• Alterations of chromosome number or structure cause some genetic disorders

• Large-scale chromosomal alterations

– Often lead to spontaneous abortions or cause a variety of developmental disorders


Abnormal Chromosome Number

• When nondisjunction occurs

– Pairs of homologous chromosomes do not separate normally during meiosis

– Gametes contain two copies or no copies of a particular chromosome

Figure 15.12a, b

Meiosis I

Nondisjunction

Meiosis II

Nondisjunction

Gametes

n + 1n + 1 n 1 n – 1 n + 1 n –1 n nNumber of chromosomes

Nondisjunction of homologouschromosomes in meiosis I

Nondisjunction of sisterchromatids in meiosis II

(a) (b)


• Aneuploidy

– Results from the fertilization of gametes in which nondisjunction occurred

– Is a condition in which offspring have an abnormal number of a particular chromosome


• If a zygote is trisomic

– It has three copies of a particular chromosome

• If a zygote is monosomic

– It has only one copy of a particular chromosome


• Polyploidy

– Is a condition in which there are more than two complete sets of chromosomes in an organism

Figure 15.13


Alterations of Chromosome Structure

• Breakage of a chromosome can lead to four types of changes in chromosome structure

– Deletion

– Duplication

– Inversion

– Translocation


• Alterations of chromosome structure

Figure 15.14a–d

A B C D E F G HDeletion

A B C E G HF

A B C D E F G HDuplication

A B C B D EC F G H

A

A

M N O P Q R

B C D E F G H

B C D E F G HInversion

Reciprocaltranslocation

A B P Q R

M N O C D E F G H

A D C B E F HG

(a) A deletion removes a chromosomal segment.

(b) A duplication repeats a segment.

(c) An inversion reverses a segment within a chromosome.

(d) A translocation moves a segment fromone chromosome to another,nonhomologous one. In a reciprocal

translocation, the most common type,nonhomologous chromosomes exchangefragments. Nonreciprocal translocationsalso occur, in which a chromosome transfers a fragment without receiving afragment in return.


Human Disorders Due to Chromosomal Alterations

• Alterations of chromosome number and structure

– Are associated with a number of serious human disorders


Down Syndrome

• Down syndrome

– Is usually the result of an extra chromosome 21, trisomy 21

Figure 15.15


Aneuploidy of Sex Chromosomes

• Nondisjunction of sex chromosomes

– Produces a variety of aneuploid conditions


• Klinefelter syndrome

– Is the result of an extra chromosome in a male, producing XXY individuals

• Turner syndrome

– Is the result of monosomy X, producing an X0 karyotype


Disorders Caused by Structurally Altered Chromosomes

• Cri du chat

– Is a disorder caused by a deletion in a chromosome


• Certain cancers

– Are caused by translocations of chromosomes

Figure 15.16

Normal chromosome 9Reciprocal

translocation

Translocated chromosome 9

Philadelphiachromosome

Normal chromosome 22 Translocated chromosome 22


• Some inheritance patterns are exceptions to the standard chromosome theory

• Two normal exceptions to Mendelian genetics include

– Genes located in the nucleus

– Genes located outside the nucleus


Genomic Imprinting

• In mammals

– The phenotypic effects of certain genes depend on which allele is inherited from the mother and which is inherited from the father


• Genomic imprinting

– Involves the silencing of certain genes that are “stamped” with an imprint during gamete production

Figure 15.17a, b

(a) A wild-type mouse is homozygous for the normal igf2 allele.

Normal Igf2 allele(expressed)

Normal Igf2 allelewith imprint(not expressed)

Paternalchromosome

Maternalchromosome

Wild-type mouse(normal size)

Normal Igf2 allele

Paternal

Maternal

Mutant lgf2 allele

Mutant lgf2 allele

Paternal

Maternal

Dwarf mouseNormal Igf2 allelewith imprint

Normal size mouse

(b) When a normal Igf2 allele is inherited from the father, heterozygous mice grow to normal size. But when a mutant allele is inherited from the father, heterozygous mice have the dwarf phenotype.


Inheritance of Organelle Genes

• Extranuclear genes

– Are genes found in organelles in the cytoplasm


• The inheritance of traits controlled by genes present in the chloroplasts or mitochondria

– Depends solely on the maternal parent because the zygote’s cytoplasm comes from the egg

Figure 15.18


• Some diseases affecting the muscular and nervous systems

– Are caused by defects in mitochondrial genes that prevent cells from making enough ATP