Chapter 6 – Chromosomal Mutations
• An organism or cell is euploid when it has one complete set of chromosomes, or exact multiples
of complete sets.
– Eukaryotes that are haploid or diploid are euploid, as are other organisms with variable
numbers of chromosome sets.
• Aneuploidy results from variations in the number of individual chromosomes (not sets), so that
the chromosome number is not an exact multiple of the haploid set of chromosomes.
• Aneuploidy can occur due to nondisjunction during meiosis.
– Nondisjunction in meiosis I will produce four gametes, two with a chromosome
duplicated, and two that are missing a chromosome.
• Fusion of a normal gamete with one containing a chromosomal duplication will
produce a zygote with three copies of that chromosome, and two of all the
• Fusion of a normal gamete with one missing a chromosome will result in a
zygote with only one copy of that chromosome and two of all others.
• Nondisjunction during meiosis II produces two normal gametes and two that are abnormal (one
with two sibling chromosomes and one with that chromosome missing).
– Fusion of abnormal gametes with normal ones will produce the genotypes discussed
– Normal gametes are also produced and when fertilized will produce normal zygotes.
• More complex gametic chromosome composition can result when:
– More than one chromosome is involved.
– Nondisjunction occurs in both meiotic divisions.
– Nondisjunction occurs in mitosis (result is somatic cells with unusual chromosome
• Autosomal aneuploidy is not well tolerated in animals, and in mammals is detected mainly after
spontaneous abortion. Aneuploidy is much better tolerated in plants.
• There are four main types of aneuploidy:
– Monosomy involves loss of a single chromosome (2N-1)
– Nullisomy involves loss of one homologous chromosome pair (cell is 2N-2)
– Trisomy involves one extra chromosome, so the cell has three copies of one, and two of
all the others (2N + 1)
– Tetrasomy involves an extra chromosome pair, so the cell has four copies of one and
two of all the others
• Some types of aneuploidy have serious meioitic consequences.
– A monosomic cell (2N-1)
• May produce gametes that are N (normal) and N-1 (monosomic).
• Or, the unpaired chromosome may be lost completely, producing gametes that
are all N-1.
Trisomy-21 (Down Syndrome)
• Occurs in an estimated 3,510 per 1 million conceptions and 1,430 per 1 million live births.
• Down syndrome individuals are characterized by:
– Low IQ
– Epicanthal folds over eyes
– Short and broad hands
– Below-average height
– Simian Crease
Trisomy-13 (Patau sydrome)
• Occurs in 2/10,000 live births, most die within the first three months.
– Cleft lip and palate.
– Small eyes
– Polydactyly (extra fingers and toes).
– Mental and developmental retardation.
– Cardiac and other abnormalities.
Trisomy-18 (Edwards syndrome)
• Occurs in 2.5/10,000 live births.
• 90% die within 6 months.
• 80% are female.
– Small size with multiple congenital malformations throughout the body.
– Clenched fists.
– Elongated skull.
– Low-set ears.
– Mental and developmental retardation. Monoploidy and Polyploidy
• Can result when either round of meiotic division lacks cytokinesis, or when meiotic
nondisjunction occurs for all chromosomes.
– Complete nondisjunction in meiosis I will produce 1/2 gametes with normal
chromosomes, 1/4 with two sets and 1/4 with no sets of chromosomes.
– Gametes with two sets of chromosomes fused with a normal gamete produces a triploid
– Fusion of two gametes that each have two sets of chromosomes produces a tetraploid
• Polyploidy of somatic cells can result from mitotic nondisjunction of complete chromosome
• Two clases based on number of chromosome sets:
– Even-number polyploids are more likely to be at least partially fertile, because the
potential exists for equal segregation of homologs during meiosis.
– Odd-number polyploids will always have unpaired chromosomes. Balanced gametes
are rare and these organisms are usually sterile or have increased zygote death.
• Unstable in meiosis because random segregation means that balanced gametes (either N or 2N)
are rare. n
– The probability of a triploid organism producing a haploid gamete is (1/2) where n is
the number of chromosomes.
– Triploidy is always lethal in humans, accounting for 15-20% of spontaneous abortions
and 1/10,000 live births, with most dying in the first month.
– Tetraploidy in humans is also lethal, usually before birth, accounting for 5% of
Polyploidy in Plants
• Polyploidy is more common in plants, probably due to self fertilization.
– Allows even-number polyploids to produce fertile gametes and reproduce.
• Two types:
– Autopolyploidy and Allopolyploidy.
• Results when all sets of the chromosomes are from the same species.
– Usually due to meiotic error.
– Fusion of a diploid gamete with a haploid one produces a triploid organism.
• Eg. Grasses, garden flowers, crop plants and forest trees.
• Eg. “Seedless” fruits such as bananas, grapes and watermelons.
• Results when the chromosomes are from two different organisms.
– Fusion of haploid gametes followed by chromosome doubling.
• Eg. Fusion of haploid gametes from plant 1 and plant 2 produces and N + 1 2
hybrid plant. No chromosomal pairing occurs in meiosis, viable gametes are not
produced and the plants are sterile. • Rarely, division error doubles the chromosome sets (2N + 2N ). The diploid sets
function normally in meiosis and fertile allotetraploid plants result.