Chapter 15 Largescale chromosomal changes Fig 15 2

Chapter 15: Large-scale chromosomal changes Fig. 15 -2

Aberrant euploidy (usually polyploidy) and aneuploidy

Cell size typically reflects ploidy 2 N and 4 N grapes Fig. 15 -4 Fig. 15 -12

Types of polyploidy Autopolyploidy: multiple copies of identical chromosome sets; usually develop normally; cells are proportionately larger than diploid Alloploidy: multiple copies of non-identical chromosome sets; includes genomes of two different species; usually display “hybrid” characteristics

Autotriploids routinely generate aneuploid gametes (usually sterile) Fig. 15 -5

Autotetraploids are readily generated by suppressing mitotic spindle Fig. 15 -6

Autotetraploids routinely generate aneuploid gametes (usually sterile) Fig. 15 -7

Allopolyploids arise from interspecific hybridization + genome duplication Fig. 15 -8

Likely origins of modern hexaploid wheat Fig. 15 -10

Aneuploidy: extra or missing chromosomes (less than an entire haploid set) Examples: monosomy: 2 n – 1 (one chromosome has no homolog) trisomy: 2 n + 1 (three homologs for one chromosome)

Aneuploidy arises from meiotic nondisjunction, forming aneuploid gametes/spores Fig. 15 -13

Aneuploids produce aneuploid gametes/spores Fig. 15 -15

Viable human aneuploids are mostly limited to the smallest chromosomes and to the sex chromosomes Examples: trisomy-21: Down syndrome XO (no Y): Turner syndrome; primarily female; only viable human monosomic XXY: Klinefelter syndrome; primarily male

Down syndrome: the clinical manifestations of trisomy-21 Fig. 15 -17

The frequency of non-disjunction leading to trisomy-21 (and other aneuploidy) is correlated with maternal age Fig. 15 -18

Dosage compensation: mechanism for making X-linked gene expression equal in females (with two X chromosomes) and in males (with one X chromosome) In mammals: only one X chromosome is active in each cell In Drosophila: the activity of each X-linked gene copy is reduced in multi-X cells Thus, “gene balance” problems are alleviated in commonly occurring sex chromosome aneuploids

Chromosomal rearrangements • • Arise from double-strand DNA breaks Such artificial ends are very transient and rapidly join together Rejoining may restore the chromosome or may result in any imaginable combination of joined fragments Recovery of those products follows certain rules: 1. Each product must have no more nor less than one centromere (a mitotic and meiotic “must”) 2. Viability of the gametes/spore/zygote following meiosis is subject to gene balance effects (segmental aneuploids are usually poorly viable)

Types and origins of chromosomal rearrangements Unbalanced rearrangements Balanced rearrangements Fig. 15 -19

Consequences of inversions on neighboring genes Fig. 15 -20

Meiotic consequences of inversion heterozygosity Fig. 15 -21

Crossingover within inversion loops result in chromosome duplications/deletions Paracentric/Pericentric Crossover products yield inviable gametes/progeny • non-crossovers predominate • outside markers appear closer than they really are • crossingover is suppressed Fig. 15 -22

Meiosis in translocation heterozygotes can result in duplication/deletion gametes/spores Fig. 15 -24

Loops are also seen in synapsed homologs in deletion heterozygotes Fig. 15 -28 Deletions behave genetically as multi-gene loss-of-function mutations

Deletions are useful in physically mapping small chromosome regions Fig. 15 -29

Incidence of chromosome mutations in humans Fig. 15 -33

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Fig. 15 -