fig. 15-2
DESCRIPTION
Chapter 15: Large-scale chromosomal changes. Fig. 15-2. Aberrant euploidy (usually polyploidy) and aneuploidy. Cell size typically reflects ploidy. 2N and 4N grapes. Fig. 15-4. Fig. 15-12. Types of polyploidy Autopolyploidy : multiple copies of identical - PowerPoint PPT PresentationTRANSCRIPT
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
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)
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
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
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
Loops are also seen in synapsed homologs in deletion heterozygotes
Deletions behave genetically as multi-gene loss-of-function mutations
Fig. 15-28