Fig. 15-2

Chapter 15: Large-scale chromosomal changes

Aberrant euploidy (usually polyploidy) and aneuploidy

Fig. 15-12

Cell size typicallyreflects ploidy

Fig. 15-4

2N and 4N grapes

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

Fig. 15-5

Autotriploids routinely generate aneuploid gametes(usually sterile)

Fig. 15-6

Autotetraploids are readily generatedby suppressing mitotic spindle

Fig. 15-7

Autotetraploids routinely generate aneuploid gametes(usually sterile)

Fig. 15-8

Allopolyploids arise from interspecific hybridization + genome duplication

Likely origins of modern hexaploid wheat

Fig. 15-10

Aneuploidy: extra or missing chromosomes (less than an entire haploid set)

Examples:

monosomy: 2n – 1(one chromosome has no homolog)

trisomy: 2n + 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)

Fig. 15-19

Types and origins of chromosomal rearrangements

Unbalancedrearrangements

Balancedrearrangements

Fig. 15-20

Consequences of inversions on neighboring genes

Fig. 15-21

Meiotic consequences of inversion heterozygosity

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

Deletions behave genetically as multi-gene loss-of-function mutations

Fig. 15-28

Deletions are useful in physically mapping small chromosome regions

Fig. 15-29

Fig. 15-33

Incidence of chromosome mutations in humans

Fig. 15-

Fig. 15-