GENE3200 - Chromosomal Variation (class 3)

chromosome number can vary a lot across organisms

no association of chromosome number with genome size or organism “complexity”

ploidy

-the number of chromosome sets

-haploid (1n): one set of chromosomes

-diploid (2n): two sets of chromosomes

-polyploid (3x, 4x, 5x, etc): more than two sets of chromosomes

three basic categories of chromosome mutations

chromosome rearrangement, aneuploidy, polyploidy

chromosome rearrangement

-duplications (2n = 6)

-chromosome structure is altered

aneuploidy

-trisomy (2n + 1 = 7)

-chromosome number is altered

polyploidy

-autotriploidy (3n = 9)

-one or more complete sets of chromosomes are added

changes in chromosome number

euploid, aneuploid, polyploidy

euploid

a cell/individual that contains the expected number of chromosomes (normal diploid: 2n)

aneuploid

-differ from expected by one or a few chromosomes

-nullisomy: 2n-2

-monosomy: 2n-1

-trisomy: 2n+1

____ causes aneuploidy

nondisjunction

aneuploidy in humans

-humans are sensitive to gene dosage changes, aneuploids usually do not survive

-multiple forms of sex chromosomes trisomies (XXY, XYY, XXX), and one type of sex chromosome monosomy occur (XO)

-only autosomal trisomies of chromosomes 13, 18, and 21 are seen in newborns, and no autosomal monosomies are observed (most other trisomies and monosomies can be detected in spontaneously aborted embryos

nondisjunction in mitosis

-mitotic nondisjunction is common

-nondisjunction in mitosis in soma → mosaic organism

-aneuploid cells found in almost all normal tissues (highest in developing brain, 30% of neurons

-increasingly associated with disease, common in cancer

-severity of phenotype varies

more about polyploidy

-complete extra sets of chromosomes

-can have additional sets of chromosome from the same species (autopolyploidy), so ploidy = 3n, 4n, 5n, etc

-can have additional sets of chromosomes from another species (allopolyploidy), so ploidy = n1+n2, 2n1+2n2, 3n1+n2, etc

-dosage issues less likely, everything is increased the same amount

-rare in animals, but very common in plants (very important in agriculture)

polyploid advantages in agriculture

rapid growth, more and bigger fruit, seedless varieties

polyploid disadvantages in agriculture

increased genome size, chromosome segregation

mechanisms that cause polyploidy

-double/triple fertilization

-cytokinesis failure in gamete precursor cells (nondisjunction of all chromosomes) (this can be induced with chemicals (colchicine)

-mating between parents of different ploidy

autopolyploid

all chromosomes from the same species

autopolyploids and meiosis

-even-numbered chromosome sets (4n, 6n) can be fertile (often give rise to new species, 4n: get all diploid (2n) gametes

-odd-numbered chromosome sets are usually sterile (unbalanced gametes)

• -some can be propagated through asexual reproduction

-more chromosomes → less likely gametes will be balanced (ex: not aneuploid at any chromosome)

meiosis in a triploid

results in sterility

allopolyploid

-hybrid of two different species

-two full sets of chromosomes but each set comes from a different species