chapter 15 the chromosomal basis of inheritance. chromosomal genetics part i- sex linkage part ii:...

24
Chapter 15 The Chromosomal Basis of Inheritance

Upload: beryl-watson

Post on 01-Jan-2016

231 views

Category:

Documents


3 download

TRANSCRIPT

Page 1: Chapter 15 The Chromosomal Basis of Inheritance. Chromosomal genetics Part I- Sex linkage Part II: Linkage Part III: Chromosomal aberrations

Chapter 15The Chromosomal Basis of Inheritance

Page 2: Chapter 15 The Chromosomal Basis of Inheritance. Chromosomal genetics Part I- Sex linkage Part II: Linkage Part III: Chromosomal aberrations

Chromosomal genetics

• Part I- Sex linkage

• Part II: Linkage

• Part III: Chromosomal aberrations

Page 3: Chapter 15 The Chromosomal Basis of Inheritance. Chromosomal genetics Part I- Sex linkage Part II: Linkage Part III: Chromosomal aberrations

Dihybrid Pea Cross

seed color:yellow (dominant)green (recessive)

seed shape:round (dominant)wrinkled (recessive)

Page 4: Chapter 15 The Chromosomal Basis of Inheritance. Chromosomal genetics Part I- Sex linkage Part II: Linkage Part III: Chromosomal aberrations

P Generation Yellow-roundseeds (YYRR)

Y

F1 Generation

Y

R R

R Y

r

r

r

y

y

y

Meiosis

Fertilization

Gametes

Green-wrinkledseeds ( yyrr)

All F1 plants produceyellow-round seeds (YyRr)

R R

YY

r ry y

Meiosis

R R

Y Y

r r

y y

Metaphase I

Y Y

R Rrr

y y

Anaphase I

r r

y Y

Metaphase IIR

Y

R

y

yyy

RR

YY

rrrr

yYY

R R

yRYryrYR1/41/4

1/41/4

F2 Generation

Gametes

An F1 F1 cross-fertilization

9 : 3 : 3 : 1

LAW OF INDEPENDENTASSORTMENT Alleles of genes

on nonhomologouschromosomes assort

independently during gameteformation.

LAW OF SEGREGATIONThe two alleles for each geneseparate during gameteformation.

1

2

33

2

1

The chromosome theory of inheritance

states that:

Mendelian genes have specific loci (positions) on chromosomes

It is the chromosomes that undergo segregation and independent assortment

Page 5: Chapter 15 The Chromosomal Basis of Inheritance. Chromosomal genetics Part I- Sex linkage Part II: Linkage Part III: Chromosomal aberrations

Sutton Boveri

Page 6: Chapter 15 The Chromosomal Basis of Inheritance. Chromosomal genetics Part I- Sex linkage Part II: Linkage Part III: Chromosomal aberrations

Linked genes

Each chromosome has 100-1000’s of genes (except Y chromosome)

Genes located on the same chromosome that tend to be inherited together are called

linked genes

Morgan did 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(genes are not on sex chromosome)

Page 7: Chapter 15 The Chromosomal Basis of Inheritance. Chromosomal genetics Part I- Sex linkage Part II: Linkage Part III: Chromosomal aberrations
Page 8: Chapter 15 The Chromosomal Basis of Inheritance. Chromosomal genetics Part I- Sex linkage Part II: Linkage Part III: Chromosomal aberrations

EXPERIMENTP Generation (homozygous)

Wild type(gray body,normal wings)

Double mutant(black body,vestigial wings)

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

EXPERIMENTP Generation (homozygous)

Wild type(gray body,normal wings)

Double mutant(black body,vestigial wings)

b b vg vg

b b vg vg

Double mutantTESTCROSS

b+ b+ vg+ vg+

F1 dihybrid(wild type)

b+ b vg+ vg

EXPERIMENTP Generation (homozygous)

Wild type(gray body,normal wings)

Double mutant(black body,vestigial wings)

b b vg vg

b b vg vg

Double mutantTESTCROSS

b+ b+ vg+ vg+

F1 dihybrid(wild type)

b+ b vg+ vg

Testcrossoffspring Eggs b+ vg+ b vg b+ vg b vg+

Black-normal

Gray-vestigial

Black-vestigial

Wild type(gray-normal)

b vg

Sperm

b+ b vg+ vg b b vgvg b+ b vgvg b b vg+vg

EXPERIMENTP Generation (homozygous)

RESULTS

Wild type(gray body,normal wings)

Double mutant(black body,vestigial wings)

b b vg vg

b b vg vg

Double mutantTESTCROSS

b+ b+ vg+ vg+

F1 dihybrid(wild type)

b+ b vg+ vg

Testcrossoffspring Eggs b+ vg+ b vg b+ vg b vg+

Black-normal

Gray-vestigial

Black-vestigial

Wild type(gray-normal)

b vg

Sperm

PREDICTED RATIOS

If genes are located on different chromosomes:

If genes are located on the same chromosome andparental alleles are always inherited together:

1

1

1

1

1 1

0 0

965 944 206 185

:

:

:

:

:

:

:

:

:

b+ b vg+ vg b b vgvg b+ b vgvg b b vg+vg

Page 9: Chapter 15 The Chromosomal Basis of Inheritance. Chromosomal genetics Part I- Sex linkage Part II: Linkage Part III: Chromosomal aberrations

• From the results, Morgan reasoned that body color and wing size are usually inherited together in specific combinations (parental phenotypes) because the genes are on the same chromosome

However, nonparental phenotypes were also produced

Understanding this result involves exploring

genetic recombination,

production of offspring with combinations of traits differing from either parent

Page 10: Chapter 15 The Chromosomal Basis of Inheritance. Chromosomal genetics Part I- Sex linkage Part II: Linkage Part III: Chromosomal aberrations

Recombination of Unlinked Genes: Independent Assortment of Chromosomes

• Mendel observed that combinations of traits in some offspring differ from either parent

• 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

Page 11: Chapter 15 The Chromosomal Basis of Inheritance. Chromosomal genetics Part I- Sex linkage Part II: Linkage Part III: Chromosomal aberrations

Recombination of Linked Genes: Crossing Over

• Morgan discovered that genes can be linked, but the linkage was incomplete, as evident from recombinant phenotypes

• Morgan proposed that some process must sometimes break the physical connection between genes on the same chromosome

• That mechanism was the crossing over of homologous chromosomes

Page 12: Chapter 15 The Chromosomal Basis of Inheritance. Chromosomal genetics Part I- Sex linkage Part II: Linkage Part III: Chromosomal aberrations

• linkage map-

is a genetic map of a chromosome (an ordered list of genetic loci) based on recombination frequencies

• Distances between genes can be expressed as map units; one map unit, or centimorgan, represents a 1% recombination frequency

• the farther apart 2 genes are, the higher the probability that a crossover will occur between them, and the higher the recombination frequency

cinnabarbody-color wing-size

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

Page 13: Chapter 15 The Chromosomal Basis of Inheritance. Chromosomal genetics Part I- Sex linkage Part II: Linkage Part III: Chromosomal aberrations

Using methods like chromosomal banding, geneticists can develop

cytogenetic maps,

indicate the positions of genes with respect to chromosomal features

• Recombination frequencies can be used to make linkage maps of fruit fly genes

• portray the order of genes along a chromosome, but does not accurately portray the precise location of those genes

chromosome II

Page 14: Chapter 15 The Chromosomal Basis of Inheritance. Chromosomal genetics Part I- Sex linkage Part II: Linkage Part III: Chromosomal aberrations

Alterations of chromosome #

• 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

• Large-scale chromosomal alterations often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders

all gametes will be abnormal ½ gametes will be abnormal/ normal

Page 15: Chapter 15 The Chromosomal Basis of Inheritance. Chromosomal genetics Part I- Sex linkage Part II: Linkage Part III: Chromosomal aberrations

• aneuploidy-

result of fertilization of gametes in which nondisjunction occurred, offspring have an abnormal # of a particular chromosome

– trisomic zygote-

three copies of a particular chromosome

– monosomic zygote-

has only one copy of a particular chromosome

• polyploidy-

an organism has more than 2 complete sets of chromosomes

Page 16: Chapter 15 The Chromosomal Basis of Inheritance. Chromosomal genetics Part I- Sex linkage Part II: Linkage Part III: Chromosomal aberrations

(a) Deletion

(b) Duplication

(c) Inversion

(d) Translocation

A deletion removes a chromosomal segment.

A duplication repeats a segment.

An inversion reverses a segment within a chromosome.

A translocation moves a segment from onechromosome 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 D E F G H B C

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

GM N O C H FED A B P Q R

Breakage of a chromosome can lead to 4 types of changes in chromosome structure:

Page 17: Chapter 15 The Chromosomal Basis of Inheritance. Chromosomal genetics Part I- Sex linkage Part II: Linkage Part III: Chromosomal aberrations
Page 18: Chapter 15 The Chromosomal Basis of Inheritance. Chromosomal genetics Part I- Sex linkage Part II: Linkage Part III: Chromosomal aberrations

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

Page 19: Chapter 15 The Chromosomal Basis of Inheritance. Chromosomal genetics Part I- Sex linkage Part II: Linkage Part III: Chromosomal aberrations

Down Syndrome

• Down syndrome (trisomy 21) is a condition that results from three copies of chromosome 21

• It affects about one out of every

700 children born in the US

• The frequency of Down syndrome increases with the age of the mother, a correlation that has not been explained

Page 20: Chapter 15 The Chromosomal Basis of Inheritance. Chromosomal genetics Part I- Sex linkage Part II: Linkage Part III: Chromosomal aberrations

– 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

• Monosomy X-

called Turner syndrome, produces X0 females, who are sterile; it is the only known viable monosomy in humans

Page 21: Chapter 15 The Chromosomal Basis of Inheritance. Chromosomal genetics Part I- Sex linkage Part II: Linkage Part III: Chromosomal aberrations

Disorders Caused by Structurally Altered Chromosomes

• Certain cancers, including chronic myelogenous leukemia (CML), are caused by translocations of chromosomes

Page 22: Chapter 15 The Chromosomal Basis of Inheritance. Chromosomal genetics Part I- Sex linkage Part II: Linkage Part III: Chromosomal aberrations

Some inheritance patterns are exceptions to the standard chromosome theory

• There are two normal exceptions to Mendelian genetics that involve genes located:

1. in the nucleus

2. outside the nucleus mitochondria plastids (chloroplasts)

Page 23: Chapter 15 The Chromosomal Basis of Inheritance. Chromosomal genetics Part I- Sex linkage Part II: Linkage Part III: Chromosomal aberrations

Normal lgf2 allele(expressed)

Normal lgf2 allele(not expressed)

Wild-type mouse(normal size)

Paternalchromosome

Maternalchromosome

A wild-type mouse is homozygous for the normal lgf2 allele.

Normal lgf2 allele(expressed)

Mutant lgf2 allele(not expressed)

Normal size mouse

Paternal

Maternal

Mutant lgf2 allele(expressed)

Normal lgf2 allele(not expressed)

Dwarf mouse

Paternal

Maternal

When a normal lgf2 allele is inherited from the father, heterozygousmice grow to normal size. But when a mutant allele is inheritedfrom the father, heterozygous mice have the dwarf phenotype.

For a few mammalian traits, the phenotype depends on which parent passed along the alleles for those traits

Such variation in phenotype is genomic imprinting

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

insulin-like growth factor

It appears that imprinting is the result of the methylation (addition –CH3) of DNA

Page 24: Chapter 15 The Chromosomal Basis of Inheritance. Chromosomal genetics Part I- Sex linkage Part II: Linkage Part III: Chromosomal aberrations

Inheritance of Organelle Genes

• Extranuclear genes are genes found in organelles in the cytoplasm

• The inheritance of traits controlled by extranuclear genes depends on the maternal parent 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