Chromosome Rearrangements And Change In Chromosome Number

Chromosome rearrangements and change in chromosome number

Chromosomal Mutations • Deletion – part of the chromosome is missing – Starts with breaks in the chromosome – Radiation, heat, viruses, chemicals, etc. – May cause an unpaired loop – May give rise to pseudodominance – Cri-du-chat syndrome (Chrom. #5)

Chromosomal Mutations • Duplication – doubling of a segment of a chromosome – Tandem – Reverse tandem – Terminal tandem – Position Effect of barring eyes in Drosophila

Chromosomal Mutations • Inversion – results when a segment of a chromosome is excised and then reintegrated in an orientation 180 degrees from the original orientation. – Pericentric inversion – Paracentric inversion – Resulting from a dicentric bridge

Chromosomal Mutations • Translocation – change in position of chromosome segments and the gene sequences they contain to a different location – Nonreciprocal intrachromosomal – Nonreciprocal interchromaosomal – Reciprocal interchromosomal

Variation in Chromosomal Number • Euploidy – the correct number of sets of chromosomes in an organism. • Monoploidy – only one set of chromosomes when there should be more: – Male wasps, ants, and bees are monoploids because they develop from unfertilized eggs – Used in plant-breeding experiments/colchicine (inhibits mitotic spindle)

Variation in Chromosomal Number Polyploidy – having three or more sets of chromosomes – Usually occurs due to a breakdown of the mitotic spindle – Usually occurs in plants (self-fertilization) – Even numbered polyploids have a better chance of being fertile than odd numbered polyploids – In humans, triploidy is usually lethal.

Variation in Chromosomal Number Polyploidy (continued) • Autopolyploidy – All sets of chromosomes originate in the same species/defect in meiosis – Example – a diploid gamete fuses with a haploid gamete to produce a triploid zygote (bananas and seedless fruit)

Variation in Chromosomal Number Polyploidy (continued) • Allopolyploidy – Sets of chromosomes originate from different species though usually related – Because of differences between chromosomes, the hybrid, no crossing over occurs and no viable gametes produced making hybrids sterile – Occasionally, two sets of different chromosomes will double, producing tissues of 2N + 2N2 / produces fully fertile allotetraploid, 2N + 2N2, plants

Variation in Chromosomal Number • Aneuploidy – variation in the number of chromosomes – Nullisomy - loss of one homologous chromosome pair; 2N – 2 – Monosomy – loss of a single chromosome;

2N – 1 – Trisomy – single extra chromosome; 2N + 1 – Tetrasomy – extra chromosome pair; 2N + 2

Changing to Chromosome number or ‘ploidy’ This system counts what is there before duplication

Euploids (complete sets of chromosomes) Monoploid (x) Diploid (2x) Triploid (3x) Tetraploid (4x)

“Polyploid”

Aneuploid Incomplete set or extra Monosomy(2n-1) Trisomy(2n+1)

Aneuploidy is harmful, polyploidy may be tolerated:

Balance of gene products (RNA, proteins) is important. Some polyploids you might know: 1 out of 3 flowering plants bananas (triploid) coffee, peanuts, McIntosh apples, Grapes (tetraploid) wheat (hexaploid) strawberries (diploid-octaploid) Unusual in most animals Found in lizards, amphibians and Fish