large scale chromosomal changes

Chromosomal changes

Variations from the normal condition in chromosome number or chromosome structure

Two major categories of chromosomal variation

Changes in chromosome number and changes in chromosome structure

Aneuploidy

Variation in the number of individual chromosomes

Euploidy

Variation in the number of complete chromosome sets

Monosomic

An individual missing one chromosome (2n-1)

Trisomic

An individual with one extra chromosome (2n+1)

Haploid

An organism with one set of chromosomes (n)

Diploid

An organism with two sets of chromosomes (2n)

Polyploid

An organism with more than two complete sets of chromosomes

Triploid

An organism with three chromosome sets (3n)

Tetraploid

An organism with four chromosome sets (4n)

Pentaploid

An organism with five chromosome sets (5n)

Hexaploid

An organism with six chromosome sets (6n)

Monoploidy

Presence of only one chromosome set in a species that is normally diploid

Examples of monoploid animals

Male bees, wasps, and ants

Parthenogenesis

Development of an embryo from an unfertilized egg

Why are monoploids vulnerable to deleterious alleles?

There is no masking of recessive harmful genes

Polyploidy

Condition of having multiple complete chromosome sets

Where is polyploidy most common?

Plants, especially grasses

Which polyploids are most common in plants?

Even-numbered polyploids such as 4n and 6n

Why are even polyploids more common than odd polyploids?

They generally have more regular meiosis and higher fertility

Two major classes of polyploids

Autopolyploids and allopolyploids

Autopolyploid

Polyploid produced by chromosome duplication within a species

How do autotetraploids arise?

By doubling chromosome number from 2n to 4n

Chemical commonly used to induce polyploidy

Colchicine

Triploid species are typically what type of polyploid?

Autopolyploids

Why are triploids often sterile?

Chromosomes cannot pair evenly during meiosis

Two chromosome pairing patterns in triploids

Trivalent formation or bivalent plus univalent formation

How does irregular pairing affect triploids?

It produces unbalanced gametes and sterility

Allopolyploid

Polyploid derived from hybridization between different species

Why are many allopolyploids fertile?

The chromosome sets segregate independently

Homeologous chromosomes

Chromosomes that are only partially homologous

Difference between homologous and homeologous chromosomes

Homeologous chromosomes are only partly homologous

Scientist who produced Raphanobrassica

G.D. Karpechenko

Raphanobrassica

Hybrid of radish and cabbage

Parent species of Raphanobrassica

Raphanus sativus and Brassica oleracea

Importance of allopolyploidy

Can produce new species

Brassica species demonstrate what evolutionary process?

Allopolyploid speciation

Common bread wheat scientific name

Triticum aestivum

Bread wheat chromosome composition

ABD genomes

Bread wheat is what kind of polyploid?

Allohexaploid

Bread wheat ploidy level

6x

Chromosome number contributed by each ancestral wheat genome

7 chromosomes

Center of origin of wheat

Fertile Crescent of the Middle East

Why is wheat globally important?

It is one of the world's major food crops

Benefits of polyploidy in agriculture

Larger seeds, fruits, and vegetative structures

Triticale

Hybrid crop derived from wheat and rye

Triticale chromosome number

56

Parent species of triticale

Wheat and rye

Advantage of triticale

Combines wheat yield with rye hardiness

Examples of common polyploid crops

Wheat, banana, strawberry, blueberry, cotton, rose, chrysanthemum, and tulip

Major disadvantage of many polyploids

Sterility

Commercial bananas are what ploidy level?

Triploid

Commercial banana chromosome number

3n = 33

Why are bananas seedless?

They are sterile triploids

Why are triploid bananas sterile?

Irregular meiosis produces chromosome imbalance

What meiotic structures form in triploid bananas?

Trivalents and univalents

Which polyploids are especially prone to sterility?

Odd-numbered polyploids such as 3n, 5n, and 7n

Aneuploidy

Change in chromosome number involving individual chromosomes

Cause of aneuploidy

Nondisjunction during meiosis

Nondisjunction

Failure of chromosomes to separate properly during meiosis

Hypoploid

Underrepresented chromosome condition

Hyperploid

Overrepresented chromosome condition

Nullisomy

Loss of both members of a homologous chromosome pair (2n-2)

Monosomy

Loss of a single chromosome (2n-1)

Trisomy

Gain of one chromosome (2n+1)

Turner syndrome chromosome composition

X

Turner syndrome sex

Female

Triple X syndrome chromosome composition

XXX

Triple X syndrome sex

Female

Klinefelter syndrome chromosome composition

XXY

Klinefelter syndrome sex

Male

Cause of Turner, Triple X, and Klinefelter syndromes

Nondisjunction of sex chromosomes

Down syndrome

Trisomy 21

Chromosome affected in Down syndrome

Chromosome 21

Gene dosage effect

Phenotypic change caused by an abnormal number of gene copies

Gene balance theory

Normal phenotype requires proper proportions of genes throughout the genome

Why is aneuploidy usually harmful?

It disrupts gene balance

Why does aneuploidy affect body shape more than euploidy?

It alters the relative dosage of specific genes

Deletion

Chromosomal mutation involving loss of a chromosome segment

Interstitial deletion

Loss of an internal chromosome segment

Terminal deletion

Loss of a chromosome tip

Pseudodominance

Expression of a recessive allele due to deletion of the dominant allele

Why are homozygous deletions often lethal?

Essential genes may be completely absent

Cri-du-chat syndrome

Disorder caused by deletion of the short arm of chromosome 5

Meaning of cri-du-chat

"Cry of the cat"

Prader-Willi syndrome

Associated with deletion in chromosome 15

Duplication

Chromosomal mutation causing a chromosome segment to be repeated

Barr mutation

Example of duplication in fruit flies

Inversion

Mutation where a chromosome segment is reversed 180 degrees

Pericentric inversion

Inversion that includes the centromere

Paracentric inversion

Inversion that excludes the centromere

Translocation

Movement of a chromosome segment to another chromosome

Effect of translocations on genes

They change linkage relationships

Reciprocal translocation

Exchange of segments between nonhomologous chromosomes

Nonhomologous chromosomes

Chromosomes that are not members of the same homologous pair

How do translocations affect inheritance?

They alter normal linkage and chromosome pairing

Colchicine

A chemical used to induce chromosome doubling

Original source of colchicine

Autumn crocus (Colchicum autumnale)

Medical use of colchicine

Treatment of gout