15 Lecture BIOL 1030-30 Gillette College

CAMPBELL

BIOLOGYReece • Urry • Cain • Wasserman • Minorsky • Jackson

EDITION

CAMPBELL

BIOLOGYReece • Urry • Cain • Wasserman • Minorsky • Jackson

EDITION

Chromosomal

Basis of

Inheritance

Lecture Presentation by

Nicole Tunbridge and

Kathleen Fitzpatrick

Locating Genes Along Chromosomes

Mendel’s “hereditary factors” were purely abstract

when first proposed

Today we can show that the factors—genes—are

located on chromosomes

The location of a particular gene can be seen by

tagging isolated chromosomes with a fluorescent

dye that highlights the gene

Figure 15.1

Figure 15.1a

Cytologists worked out the process of mitosis in

1875, using improved techniques of microscopy

Biologists began to see parallels between the

behavior of Mendel’s proposed hereditary factors

and chromosomes

Around 1902, Sutton and Boveri and others

independently noted the parallels and the

chromosome theory of inheritance began

to form

Figure 15.2P Generation

F1 Generation

Yellow-round

(YYRR)

Gametes

Meiosis

Fertilization

Green-wrinkled

seeds (yyrr)

Meiosis

Metaphase

Anaphase I

Metaphase

All F1 plants produce

yellow-round seeds (YyRr).

LAW OF

SEGREGATION

The two alleles for each

gene separate.

LAW OF INDEPENDENT

ASSORTMENT Alleles of

genes on nonhomologous

chromosomes assort

independently.

Y Y Y Y

R R R R

YR yr Yr yR14

F2 Generation

Fertilization

recombines the

R and r alleles at random.

An F1× F1 cross-fertilization Fertilization results

in the 9:3:3:1

phenotypic ratio in

the F2 generation.9 : 3 : 3 : 1

yy y y

Figure 15.2a

P Generation

Yellow-round

seeds (YYRR)

Meiosis

Fertilization

Green-wrinkled

seeds (yyrr)

rGametes

Figure 15.2b

F1 Generation

Meiosis

Metaphase

Anaphase I

Metaphase

All F1 plants produce

yellow-round seeds (YyRr).

LAW OFSEGREGATIONThe two alleles for each gene separate.

LAW OF INDEPENDENTASSORTMENT Alleles of genes on nonhomologouschromosomes assortindependently.

Y Y Y Y

R R R R

Figure 15.2c

LAW OF

SEGREGATION

LAW OF INDEPENDENTASSORTMENT

F2 Generation

Fertilization

recombines the

R and r alleles

at random.

An F1× F1 cross-fertilization Fertilization results

in the 9:3:3:1

phenotypic ratio in

the F2 generation.9 : 3 : 3 : 1

Concept 15.1: Morgan showed that Mendelianinheritance has its physical basis in the behavior of chromosomes: Scientific inquiry

The first solid evidence associating a specific gene

with a specific chromosome came in the early 20th

century from the work of Thomas Hunt Morgan

These early experiments provided convincing

evidence that the chromosomes are the location

of Mendel’s heritable factors

Morgan’s Choice of Experimental Organism

For his work, Morgan chose to study Drosophila

melanogaster, a common species of fruit fly

Several characteristics make fruit flies a

convenient organism for genetic studies

They produce many offspring

A generation can be bred every two weeks

They have only four pairs of chromosomes

Morgan 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

Figure 15.3a

Figure 15.3b

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 a 3:1 red to white eye

ratio, but only males had white eyes

Morgan determined that the white-eyed mutant

allele must be located on the X chromosome

Morgan’s finding supported the chromosome

theory of inheritance

Figure 15.4

Experiment Conclusion

Results

PGeneration

Generation

GenerationF2

Generation

All offspringhad red eyes.

Generation

Figure 15.4a

Experiment

Results

Generation

All offspring

had red eyes.

Figure 15.4b

Generation

Conclusion

Concept 15.2: Sex-linked genes exhibit unique patterns of inheritance

Morgan’s discovery of a trait that correlated with

the sex of flies was key to the development of the

chromosome theory of inheritance

In humans and some other animals, there is a

chromosomal basis of sex determination

The Chromosomal Basis of Sex

In humans and other mammals, there are two

varieties of sex chromosomes: a larger X

chromosome and a smaller Y chromosome

A person with two X chromosomes develops as a

female, while a male develops from a zygote with

one X and one Y

Only the ends of the Y chromosome have regions

that are homologous with corresponding regions

of the X chromosome

Figure 15.5

Figure 15.6

Parents

(a) The X-Y system

(b) The X-0 system

(c) The Z-W system

(d) The haplo-diploid system

Zygotes (offspring)

Sperm Egg

22 +XX

76 +ZW

76 +ZZ

32(Diploid)

16(Haploid)

Figure 15.6a

Parents

(a) The X-Y system

(b) The X-0 system

Zygotes (offspring)

Sperm Egg

Figure 15.6b

(c) The Z-W system

(d) The haplo-diploid system

76 +ZW

76 +ZZ

32(Diploid)

16(Haploid)

Short segments at the ends of the Y

chromosomes are homologous with the X,

allowing the two to behave like homologues during

meiosis in males

A gene on the Y chromosome called SRY (sex-

determining region on the Y) is responsible for

development of the testes in an embryo

A gene that is located on either sex chromosome

is called a sex-linked gene

Genes on the Y chromosome are called Y-linked

genes; there are few of these

Genes on the X chromosome are called X-linked

Inheritance of X-Linked Genes

X chromosomes have genes for many characters

unrelated to sex, whereas most Y-linked genes

are related to sex determination

X-linked genes follow specific patterns of

inheritance

For a recessive X-linked trait to be expressed

A female needs two copies of the allele

(homozygous)

A male needs only one copy of the allele

(hemizygous)

X-linked recessive disorders are much more

common in males than in females

Figure 15.7

(b) (c)

Sperm Sperm

Eggs Eggs

XNXN XnY

XNY XnY

Some disorders caused by recessive alleles on

the X chromosome in humans

Color blindness (mostly X-linked)

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

The inactive X condenses into a Barr body

If a female is heterozygous for a particular gene

located on the X chromosome, she will be a

mosaic for that character

Figure 15.8X chromosomes

Allele for

orange fur

Allele for

black fur

Cell division and

X chromosome

inactivation

Early embryo:

Two cell

populations

in adult cat:

Active XInactive

Black fur Orange fur

Active X

Figure 15.8a

Concept 15.3: 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 (except the Y chromosome)

Genes located on the same chromosome that tend

to be inherited together are called linked genes

How Linkage Affects Inheritance

Morgan did other experiments with fruit flies to see

how linkage affects inheritance of two characters

Morgan crossed flies that differed in traits of body

color and wing size

Figure 15.9Experiment

Results

P Generation(homozygous)

Wild type (graybody, normal wings)

Double mutant(black body, vestigial wings)

F1 dihybrid testcross

Wild-type F1 dihybrid

(gray body, normal wings)

Homozygous

recessive (black

body, vestigial wings)

Testcrossoffspring Eggs

Wild type(gray-normal)

Black-vestigial

Gray-vestigial

Black-normal

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

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

b+ b+ vg+ vg+

b+ b vg+ vg

b b vg vg

PREDICTED RATIOS

Genes on differentchromosomes:

Genes on the samechromosome:

1 : 1 : 1 : 1

1 : 1 : 0 : 0

965 : 944 : 206 : 185

Figure 15.9a

Experiment

P Generation(homozygous)Wild type (graybody, normal wings)

Double mutant(black body, vestigial wings)

Wild-type F1 dihybrid

(gray body, normal

wings)

Homozygous

recessive (black

body, vestigial

wings)

b+ b+ vg+ vg+

b+ b vg+ vg

b b vg vg

Figure 15.9b

Experiment

Results

Testcrossoffspring Eggs

Wild type(gray-normal)

Black-vestigial

Gray-vestigial

Black-normal

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

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

PREDICTED RATIOS

Genes on differentchromosomes:

Genes on the samechromosome:

1 : 1 : 1 : 1

1 : 1 : 0 : 0

965 : 944 : 206 : 185

Morgan found that body color and wing size are

usually inherited together in specific combinations

(parental phenotypes)

He noted that these genes do not assort

independently, and reasoned that they were on

the same chromosome

Figure 15.UN01

Most offspring

F1 dihybrid female

and homozygous

recessive male

in testcross

b+ vg+

However, nonparental phenotypes were also

produced

Understanding this result involves exploring

genetic recombination, the production of

offspring with combinations of traits differing from

either parent

Genetic Recombination and Linkage

The genetic findings of Mendel and Morgan relate

to the chromosomal basis of recombination

Recombination of Unlinked Genes: Independent Assortment of Chromosomes

Offspring with a phenotype matching one of the

parental phenotypes are called parental types

Offspring with nonparental phenotypes (new

combinations of traits) are called recombinant

types, or recombinants

A 50% frequency of recombination is observed for

any two genes on different chromosomes

Figure 15.UN02

Gametes from yellow-round

dihybrid parent (YyRr)

Gametes from testcrosshomozygousrecessiveparent (yyrr)

Parental-

offspring

Recombinant

offspring

yyRrYyRr Yyrryyrr

YR yr Yr yR

Recombination of Linked Genes: Crossing Over

Morgan discovered that genes can be linked, but

the linkage was incomplete, because some

recombinant phenotypes were observed

He proposed that some process must occasionally

break the physical connection between genes on

the same chromosome

That mechanism was the crossing over of

homologous chromosomes

Figure 15.10P generation

(homozygous)

Wild type (gray body,

normal wings)

b+ vg+

Double mutant (black body,

vestigial wings)

Wild-type F1

dihybrid (gray body,

normal wings)

F1 dihybrid

testcross

Homozygous recessive

(black body,

vestigial wings)

Replication of

chromosomes

Meiosis I

Meiosis I and II

Meiosis II

Replication of

chromosomes

Recombinant

chromosomes

Testcross

offspring

Parental-type

offspring

Recombinant

offspringRecombination

frequency

391 recombinants

2,300 total offspring× 100 = 17%=

Wild type

(gray-normal)

Black-

vestigial

185Black-normal

b+ vg+

b vgb vgb vgb vg

b vgb+ vg+ b+ vg b vg+

Figure 15.10a

P generation (homozygous)

Wild type (gray body,

normal wings)

b+ vg+

Double mutant (black body,

vestigial wings)

Wild-type F1

dihybrid (gray body,

normal wings)

b+ vg+

Figure 15.10b

Wild-type F1

dihybrid

(gray body,

normal wings)

Homozygous recessive(black body,vestigial wings)

Meiosis I

Meiosis I and II

Meiosis II

Recombinant

chromosomes

b+ vg+

Figure 15.10c

Meiosis II Recombinant

chromosomes

Eggsb+vg+ b vg b+ vg b vg+

Testcross

offspring

Parental-typeoffspring

Recombinantoffspring

Recombination

frequency× 100 = 17%=

965Wild type

(gray-normal)

944Black-

vestigial

206Gray-

vestigial

185Black-normal

b vgb vgb vgb vg

b vgb+ vg+ b+ vg b vg+

391 recombinants

2,300 total offspring

Animation: Crossing Over

New Combinations of Alleles: Variation for Natural Selection

Recombinant chromosomes bring alleles together

in new combinations in gametes

Random fertilization increases even further the

number of variant combinations that can be

produced

This abundance of genetic variation is the raw

material upon which natural selection works

Mapping the Distance Between Genes Using Recombination Data: Scientific Inquiry

Alfred Sturtevant, one of Morgan’s students,

constructed a genetic map, an ordered list of

the genetic loci along a particular chromosome

Sturtevant predicted that the farther apart two

genes are, the higher the probability that a

crossover will occur between them and therefore

the higher the recombination frequency

A linkage map is a genetic map of a chromosome

based on recombination frequencies

Distances between genes can be expressed as

map units; one map unit, or centimorgan,

represents a 1% recombination frequency

Map units indicate relative distance and order,

not precise locations of genes

Figure 15.11

Chromosome

Results

Recombination

frequencies

9% 9.5%

b cn vg

Genes that are far apart on the same chromosome

can have a recombination frequency near 50%

Such genes are physically linked, but genetically

unlinked, and behave as if found on different

chromosomes

Sturtevant used recombination frequencies to

make linkage maps of fruit fly genes

He and his colleagues found that the genes

clustered into four groups of linked genes (linkage

groups)

The linkage maps, combined with the fact that

there are four chromosomes in Drosophila,

provided additional evidence that genes are

located on chromosomes

Figure 15.12

Mutant phenotypes

Wild-type phenotypes

aristae

Maroon

Cinnabar

Vestigial

curved

aristae

(appendages

on head)

bodyRed

Normal

0 16.5 48.5 57.5 67.0 75.5 104.5

Concept 15.4: Alterations of chromosome number or structure cause some genetic disorders

Large-scale chromosomal alterations in humans

and other mammals often lead to spontaneous

abortions (miscarriages) or cause a variety of

developmental disorders

Plants tolerate such genetic changes better than

animals do

Abnormal Chromosome Number

In nondisjunction, pairs of homologous

chromosomes do not separate normally during

meiosis

As a result, one gamete receives two of the same

type of chromosome, and another gamete

receives no copy

Figure 15.13-1Meiosis I

Nondisjunction

Meiosis II

Nondisjunction

disjunction

Meiosis II

Nondisjunction

disjunction

Gametes

Number of chromosomes

(a) Nondisjunction of homo-

logous chromosomes in

meiosis I

(b) Nondisjunction of sister

chromatids in meiosis II

n + 1 n + 1 n + 1 n nn − 1 n − 1 n − 1

Video: Nondisjunction in Mitosis

Aneuploidy results from the fertilization of

gametes in which nondisjunction occurred

Offspring with this condition have an abnormal

number of a particular chromosome

A monosomic zygote has only one copy of a

particular chromosome

A trisomic zygote has three copies of a particular

chromosome

Polyploidy is a condition in which an organism

has more than two complete sets of chromosomes

Triploidy (3n) is three sets of chromosomes

Tetraploidy (4n) is four sets of chromosomes

Polyploidy is common in plants, but not animals

Polyploids are more normal in appearance than

aneuploids

Alterations of Chromosome Structure

Breakage of a chromosome can lead to four types

of changes in chromosome structure

Deletion removes a chromosomal segment

Duplication repeats a segment

Inversion reverses orientation of a segment within

a chromosome

Translocation moves a segment from one

chromosome to another

Figure 15.14

(a) Deletion (c) Inversion

(b) Duplication (d) Translocation

A deletion removes a

chromosomal segment.An inversion reverses a

segment within a chromosome.

A duplication repeats

a segment. A translocation moves a

segment from one chromosome

to a nonhomologous

chromosome.

A B C D E F G H

A B C E F G H

A B C D E F G H

A B C B C D E F G H

A B C D E F G H

A D C B E F G H

A B C D E F G H M N O P Q R

M N O C D E F G H A B P Q R

Figure 15.14a

(a) Deletion

(b) Duplication

A deletion removes a

chromosomal segment.

A duplication repeats

a segment.

A B C D E F G H

A B C E F G H

A B C D E F G H

A B C B C D E F G H

Figure 15.14b

(c) Inversion

(d) Translocation

An inversion reverses a

segment within a chromosome.

A translocation moves a

segment from one chromosome

to a nonhomologous

chromosome.

A B C D E F G H

A D C B E F G H

A B C D E F G H M N O P Q R

M N O C D E F G H A B P Q R

Human Disorders Due to Chromosomal Alterations

Alterations of chromosome number and structure

are associated with some serious disorders

Some types of aneuploidy appear to upset the

genetic balance less than others, resulting in

individuals surviving to birth and beyond

These surviving individuals have a set of

symptoms, or syndrome, characteristic of the type

of aneuploidy

Down Syndrome (Trisomy 21)

Down syndrome is an aneuploid condition that

results from three copies of chromosome 21

It affects about one out of every 700 children born

in the United States

The frequency of Down syndrome increases with

the age of the mother, a correlation that has not

been explained

Figure 15.15

Aneuploidy of Sex Chromosomes

Nondisjunction of sex chromosomes produces a

variety of aneuploid conditions

XXX females are healthy, with no unusual physical

features

Klinefelter syndrome is the result of an extra

chromosome in a male, producing XXY individuals

Monosomy X, called Turner syndrome, produces

X0 females, who are sterile; it is the only known

viable monosomy in humans

Disorders Caused by Structurally Altered Chromosomes

The syndrome cri du chat (“cry of the cat”), results

from a specific deletion in chromosome 5

A child born with this syndrome is severely

intellectually disabled and has a catlike cry;

individuals usually die in infancy or early childhood

Certain cancers, including chronic myelogenous

leukemia (CML), are caused by translocations

of chromosomes

Figure 15.16

Normal chromosome 9

Normal chromosome 22

Reciprocal translocation

Translocated chromosome 9

Translocated chromosome 22

(Philadelphia chromosome)

Concept 15.5: Some inheritance patterns are exceptions to standard Mendelian inheritance

There are two normal exceptions to Mendelian

genetics

One exception involves genes located in the

nucleus, and the other exception involves genes

located outside the nucleus

In both cases, the sex of the parent contributing an

allele is a factor in the pattern of inheritance

Genomic Imprinting

For a few mammalian traits, the phenotype

depends on which parent passed along the alleles

for those traits

Such variation in phenotype is called genomic

imprinting

Genomic imprinting involves the silencing of

certain genes depending on which parent passes

them on

Figure 15.17Normal Igf2 allele

is expressed.

Normal Igf2 allele

is expressed.

Normal Igf2 allele

is not expressed.

Normal Igf2 allele

is not expressed.

Mutant Igf2 allele

is not expressed.

Mutant Igf2 allele

is expressed.

Mutant Igf2 allele

inherited from motherMutant Igf2 allele

inherited from father

Normal-sized mouse

(wild type)

Normal-sized mouse (wild type) Dwarf mouse (mutant)

Paternalchromosome

Maternalchromosome

(a) Homozygote

(b) Heterozygotes

It appears that imprinting is the result of the

methylation (addition of —CH3) of cysteine

nucleotides

Genomic imprinting is thought to affect only a

small fraction of mammalian genes

Most imprinted genes are critical for embryonic

development

Inheritance of Organelle Genes

Extranuclear genes (or cytoplasmic genes) are

found in organelles in the cytoplasm

Mitochondria, chloroplasts, and other plant

plastids carry small circular DNA molecules

Extranuclear genes are inherited maternally

because the zygote’s cytoplasm comes from

the egg

The first evidence of extranuclear genes came

from studies on the inheritance of yellow or white

patches on leaves of an otherwise green plant

Some defects in mitochondrial genes prevent cells

from making enough ATP and result in diseases

that affect the muscular and nervous systems

For example, mitochondrial myopathy and Leber’s

hereditary optic neuropathy

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