Chapter 11Chromosome Mutations

• Changes in chromosome number• Chromosomal rearrangements• Evolution of genomes

7 AABBCCC2n+1Trisomic

5 ABBCC2n-1Monosomic

12 AAAABBBBCCCC4nTetraploid

9 AAABBBCCC3nTriploid

6 AABBCC2nDiploid

3 ABCnMonoploid

ChromosomenumberConstitutionDesignationName

Aberrant chromosome constitutions of a normally diploid organism

Changes in chromosome number

About 70% of the plant species can beconsidered as polyploids.

Polyploidization is often used in plantbreeding.

Euploidy

Organisms with multiples of the basicchromosome set are called euploid.

The size of epidermal leaf cells of tobaccoincreases with the ploidy level.

Generating monoploid plants by tissue culture

Pollen (haploid) are grown in tissue culture in the presence of certain planthormones. Haploid embryoids develop that can be grown into mature monoploidplants. Advantage for plant breeding?

The use of colchicine to generate a diploid from a monoploid.

Colchicine disrupts spindle fibre formation during mitosis, preventing the migrationof chromatids after the centromere is split. A single cell results that contains pair ofidentical chromosomes that are homozygous at all loci.

The use of colchicine to generate a diploid from a monoploid.

Why are monoploid plants produced? Is there any genetic advantage?

Monoploids

Male bees, wasps and ants are monoploid. These males developparthogenetically from unfertilized eggs.However, in most species, monoploids are abnormal individuals.

Polyploids are distiguished in

autopolyploids,that contain multiple chromosome sets from one species and

allopolyploidsthat are composed of chromosome sets from two or more differentspecies. Allopolyploids form only between closely related species.

Triploidsare usually autopolyploids. They can result from a cross of a tetraploidwith a diploid. Polyploids with odd numbers of chromosome sets aresterile, because their gametes are aneuploid. (Note that each chromosomeis really a pair of chromatids).

Meiotic pairing possibilities in tetraploids. (Note that each chromosome is really a pair of chromatids).

Functional gametes

Nonfunctionalgametes

Complex segregation ratios result from tetraploids

The amphidiploid (or allotetraploid) Raphanobrassica originates fromcabbage (Brassica) and radish (Raphanus)

The amphidiploid Triticale originates fromrye (Secale) and wheat (Triticum)

The evolution of modernhexaploid wheat, in whichamphidiploids are producedat two points. A, B, D aredifferent chromosome sets.

Aneuploidy

Chromosome number differs from wildtype by part of the chromosome set.

Aneuploid organisms are producedmainly by nondisjunction at meiosis.

Monosomics (2n-1)

Turner syndrome (X0)

Trisomics (2n+1)

Klinefelter syndrome (XXY)

Trisomics (2n+1)

Down syndrome (trisomy 21)

Maternal age and the incidence of trisomy 21

What is the reason that the sex chromosomes or chromosome 21are often observed in human aneuploids?

The concept of gene balance.

Dosage compensationX-chromosome inactivation

Chromosomal rearrangements

Origins of chromosomal rearrangements

Origins of chromosomal rearrangements

normal sequence

paracentric

pericentric

Inversions

A B C D E F

A B C E D F

A D C B E F

Inversion heterozygotes pair in a loop at meiosis

Synaptonemal complexes in mouse heterozygous for a paracentric inversion

Meiotic products resulting from asingle crossover within a paracentricinversion loop. Two nonsisterchromatids cross over within the loop.

Meiotic products resulting from a meiosis with a single crossoverwithin a pericentric inversion loop.

Message

Heterozygous inversions are characterized by reduced fertilityfrom unbalanced or deleted meiotic products.

Reciprocal translocations

The meiotic products resultingfrom two most commonlyencountered chromosomesegregation patterns in areciprocal translocationheterozygote.

A semisterile maize plant producing normal and aborted pollen. The clear pollen grainscontain chromosomally unballanced meiotic products of a reciprocal translocationheterozygote. The opaque grains are normal.

Message

Heterozygous reciprocaltranslocations arecharacterized by semi-sterility.

Message

Chromosomal rearrangements lead to reduced fertility andcontribute to speciation.

Position-effect variegation

Evolution of genomes and speciation

1. The genomes are made up of many duplicated segments

2. The genomes of two related species are characterized by many structural changes.

Chromosomal synteny

Chromosomal segments of two species that contain similar genesin an identical order are said to be synthenic.

Syntheny of human and mouse chromosomes