Chromosomal basis of inheritance
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Chromosomal Basis of Inheritance. Biology 112. Locating Genes Along Chromosomes. Mendel’s “hereditary factors” were genes, though this wasn’t known during his time Today know that genes are located on chromosomes

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Chromosomal basis of inheritance

Chromosomal Basis of Inheritance

Biology 112


Locating genes along chromosomes

Locating Genes Along Chromosomes

  • Mendel’s “hereditary factors” were genes, though this wasn’t known during his time

  • Today know that 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


Mendelian inheritance has its physical basis in the behavior of chromosomes

Mendelian inheritance has its physical basis in the behavior of chromosomes

  • The chromosome theory of inheritance states:

    • Mendelian genes have specific loci (positions) on chromosomes

    • Chromosomes undergo segregation and independent assortment

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


Chromosomal basis of inheritance

Fig. 15-2

P Generation

Yellow-round

seeds (YYRR)

Green-wrinkled

seeds (yyrr)

y

Y

r

R

r

R

Y

y

Meiosis

Fertilization

r

y

Y

R

Gametes

All F1 plants produce

yellow-round seeds (YyRr)

F1 Generation

R

R

y

y

r

r

Y

Y

LAW OF SEGREGATION

The two alleles for each gene

separate during gamete

formation.

LAW OF INDEPENDENT

ASSORTMENT Alleles of genes

on nonhomologous

chromosomes assort

independently during gamete

formation.

Meiosis

r

r

R

R

Metaphase I

y

Y

y

Y

1

1

r

r

R

R

Anaphase I

Y

y

Y

y

Metaphase II

r

R

R

r

2

2

y

Y

y

Y

y

Y

Y

y

Y

y

y

Y

Gametes

r

R

r

r

R

r

R

R

1/4

1/4

1/4

1/4

yr

yR

YR

Yr

F2 Generation

An F1  F1 cross-fertilization

3

3

: 1

: 3

9

: 3


Thomas hunt morgan

Thomas Hunt Morgan

  • Drosophila melanogaster


Morgan s experimental evidence

Morgan’s Experimental Evidence

  • The first solid evidence associating a specific gene with a specific chromosome came from Thomas Hunt Morgan

  • Morgan’s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel’s heritable factors

  • Several characteristics make fruit flies a convenient organism for genetic studies:

    • They breed at a high rate

    • A generation can be bred every two weeks

    • They have only four pairs of chromosomes


Chromosomal basis of inheritance

  • Morgan noted wild type, or normal, phenotypes that were common in the fly populations

  • Traits alternative to the wild type are called mutant phenotypes


Sex linked genes

Sex-Linked Genes

  • Sex-Linked genes: Genes located on sex chromosomes.

    • This term is commonly applied to genes on the X chromosome

  • 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

  • Morgan determined that the white-eyed mutant allele must be located on the X chromosome


Chromosomal basis of inheritance

Fig. 15-4

EXPERIMENT

P

Generation

F1

All offspring had red eyes

Generation

RESULTS

F2

Generation

CONCLUSION

+

P

w

w

X

X

Generation

X

Y

+

w

w

Sperm

Eggs

+

+

F1

w

w

+

Generation

w

w

+

w

Sperm

Eggs

+

+

w

w

+

F2

w

Generation

+

w

w

w

w

+

w


Linked genes

Linked genes

  • Each chromosome has hundreds or thousands of genes

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

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


Linkage affects inheritance

Linkage Affects Inheritance

  • Morgan did some 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


Chromosomal basis of inheritance

  • 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

  • However, nonparental phenotypes were also produced

  • Genetic recombinationoccured: the production of offspring with combinations of traits differing from either parent


Chromosomal basis of inheritance

Fig. 15-UN1

b vg

b+vg+

Parents

in testcross

b vg

b vg

b vg

b+vg+

Most

offspring

or

b vg

b vg


Chromosomal basis of inheritance

Fig. 15-9-4

EXPERIMENT

P Generation (homozygous)

Wild type

(gray body,

normal wings)

Double mutant

(black body,

vestigial wings)

b b vg vg

b+ b+ vg+ vg+

F1 dihybrid

(wild type)

Double mutant

TESTCROSS

b+ b vg+ vg

b b vg vg

Testcross

offspring

b+ vg+

b vg

b+ vg

b vg+

Eggs

Black-

normal

Wild type

(gray-normal)

Black-

vestigial

Gray-

vestigial

b vg

Sperm

b b vg+ vg

b+ b vg+ vg

b b vg vg

b+ b vg vg

PREDICTED RATIOS

1

If genes are located on different chromosomes:

1

:

:

:

1

1

If genes are located on the same chromosome and

parental alleles are always inherited together:

1

0

:

1

:

0

:

965

:

:

185

:

944

206

RESULTS


Recombination of unlinked genes independent assortment of chromosomes

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


Recombination of linked genes crossing over

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


Chromosomal basis of inheritance

Fig. 15-10

Testcross

parents

Gray body, normal wings

(F1 dihybrid)

Black body, vestigial wings

(double mutant)

bvg

b+vg+

bvg

b vg

Replication

of chromo-

somes

Replication

of chromo-

somes

b+vg+

bvg

b+vg+

bvg

bvg

bvg

bvg

bvg

Meiosis I

b+vg+

Meiosis I and II

b+vg

bvg+

bvg

Meiosis II

Recombinant

chromosomes

bvg

bvg+

b+vg

b+vg+

Eggs

Testcross

offspring

965

Wild type

(gray-normal)

944

Black-

vestigial

206

Gray-

vestigial

185

Black-

normal

bvg

b+vg+

bvg

b+vg

bvg+

bvg

bvg

bvg

bvg

Sperm

Parental-type offspring

Recombinant offspring

391 recombinants

Recombination

frequency

 100 = 17%

=

2,300 total offspring


Chromosomal basis of inheritance

Fig. 15-10b

Recombinant

chromosomes

b+vg+

b+vg

bvg+

bvg

Eggs

944

Black-

vestigial

965

Wild type

(gray-normal)

185

Black-

normal

206

Gray-

vestigial

Testcross

offspring

bvg

b+vg+

bvg

bvg+

b+vg

bvg

bvg

bvg

bvg

Sperm

Recombinant offspring

Parental-type offspring

391 recombinants

Recombination

frequency

 100 = 17%

=

2,300 total offspring


Mapping the distance between genes using recombination data

Mapping the Distance Between Genes Using Recombination Data

  • Genetic map, an ordered list of the genetic loci along a particular chromosome

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


Chromosomal basis of inheritance

  • 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


Chromosomal basis of inheritance

Fig. 15-11

RESULTS

Recombination

frequencies

9%

9.5%

Chromosome

17%

b

cn

vg


Chromosomal basis of inheritance

Fig. 15-12

Mutant phenotypes

Short

aristae

Cinnabar

eyes

Vestigial

wings

Brown

eyes

Black

body

0

48.5

57.5

67.0

104.5

Gray

body

Red

eyes

Normal

wings

Red

eyes

Long aristae

(appendages

on head)

Wild-type phenotypes


Chromosomal basis of inheritance

  • 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


Chromosomal basis of inheritance

  • Using methods like chromosomal banding, geneticists can develop cytogenetic maps of chromosomes

  • Cytogenetic maps indicate the positions of genes with respect to chromosomal features


Sex systems in animals

Sex systems in animals


Chromosomal basis of inheritance

Fig. 15-6

44 +

XX

44 +

XY

Parents

22 +

X

22 +

X

22 +

Y

or

+

Sperm

Egg

44 +

XX

44 +

XY

or

Zygotes (offspring)

(a) The X-Y system

22 +

XX

22 +

X

(b) The X-0 system

76 +

ZW

76 +

ZZ

(c) The Z-W system

32

(Diploid)

16

(Haploid)

(d) The haplo-diploid system


Inheritance of sex linked genes

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

  • In humans, sex-linked usually refers to a gene on the larger X chromosome


Chromosomal basis of inheritance

  • Sex-linked genes follow specific patterns of inheritance

  • For a recessive sex-linked trait to be expressed

    • A female needs two copies of the allele

    • A male needs only one copy of the allele

  • Sex-linked recessive disorders are much more common in males than in females

  • Some disorders caused by recessive alleles on the X chromosome in humans:

    • Color blindness

    • Duchenne muscular dystrophy

    • Hemophilia


Chromosomal basis of inheritance

Fig. 15-7

XnY

XnY

XNXN

XNXn

XNY

XNXn

Sperm

Sperm

Sperm

Xn

Y

XN

Xn

Y

Y

Eggs

Eggs

XNXN

Eggs

XNXn

XNY

XN

XNY

XN

XNXn

XNY

XN

XnXn

XnY

XNXn

XNY

XnXN

XnY

Xn

Xn

XN

(a)

(b)

(c)


Chromosomal basis of inheritance

  • Some disorders caused by recessive alleles on the X chromosome in humans:

    • Color blindness

    • Duchenne muscular dystrophy

    • Hemophilia


X inactivation in female mammals

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

  • DNA Methylation is the mechanism for X-inactivation

    • (methyl groups added to cytosine nucleotide in the DNA molecule)


Chromosomal basis of inheritance

Fig. 15-8

X chromosomes

Allele for

orange fur

Early embryo:

Allele for

black fur

Cell division and

X chromosome

inactivation

Two cell

populations

in adult cat:

Active X

Inactive X

Active X

Black fur

Orange fur


Chromosomal alterations

Chromosomal Alterations

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


Abnormal chromosome number

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

  • Nondisjunction can also occur in mitosis


Chromosomal basis of inheritance

Fig. 15-13-3

Meiosis I

Nondisjunction

Meiosis II

Nondisjunction

Gametes

n – 1

n + 1

n – 1

n

n

n + 1

n + 1

n – 1

Number of chromosomes

(b) Nondisjunction of sister

chromatids in meiosis II

(a) Nondisjunction of homologous

chromosomes in meiosis I

Nondisjunction video


Aneuploidy

Aneuploidy

  • 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

Polyploidy

  • 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

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 a segment within a chromosome

    • Translocation moves a segment from one chromosome to another


Chromosomal basis of inheritance

Fig. 15-15

A B C D E F G H

A B C E F G H

Deletion

(a)

A B C D E F G H

A B C B C D E F G H

Duplication

(b)

A B C D E F G H

A D C B E F G H

Inversion

(c)

A B C D E F G H

M N O C D E F G H

(d)

Reciprocal

translocation

M N O P Q R

A B P Q R


Human disorders due to chromosomal alterations

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 (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.


Aneuploidy of sex chromosomes

Aneuploidy of Sex Chromosomes

  • Nondisjunction of sex chromosomes produces a variety of aneuploidconditions

  • Males

    • Klinefeltersyndrome is the result of an extra chromosome in a male, producing sterile XXY individuals (1 in 2000 births?)

    • Extra Y (XYY)…apparently normal males (1 in 1000 births?)

  • Females

    • MonosomyX, called Turner syndrome, produces X0 females, who are sterile; it is the only known viable monosomy in humans (1 in 5000 births?)

    • Triple X Syndrome (XXX)…apparently normal females (1 in 1000 births?)


Genomic imprinting

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 that are “stamped” with an imprint during gamete production

    • Causes certain genes to be differently expressed in the offspring depending upon whether the alleles were inherited from the ovum or from the sperm cell.


Triplet repeats

Triplet Repeats

  • Fragile-X syndrome (CGG)

  • Huntington’s disease (CAG)


Extranuclear genes

Extranuclear Genes

  • Extranuclear genes (or cytoplasmic genes)are genes found in organelles in the cytoplasm

  • Mitochondria, chloroplasts carry small circular DNA molecules

  • Extranuclear genes are inherited maternally because the zygote’s cytoplasm comes from the egg


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