1. Variation in individual chromosome number:
Aneuploidy not generally well-tolerated in animals; primarily detected after spontaneous abortion.
Sex chromosome aneuploidy occurs more often than autosome aneuploidy (inactivation of X compensates).
Four main types of aneuploidy:
Nullisomy = loss of one homologous chromosome pair.
Monosomy = loss of a single chromosome.
Trisomy = one extra chromosome.
Tetrasomy = one extra chromosome pair.

2. Fig. 16.11, Examples of aneuploidy.

3. Variation in individual chromosome number:
Down Syndrome (trisomy-21, OMIM ):
Occurs in 1/286 conceptions and 1/699 live births.
Probability of non-disjunction trisomy-21 occurring varies with age of ovaries and testes.
Trisomy-21 also occurs by Robertsonian translocation  joins long arm of chromosome 21 with long arm of chromosome 14 or 15.
Familial down syndrome arises when carrier parents (heterozygotes) mate with normal parents.
1/2 gametes are inviable.
1/3 of live offspring are trisomy-21; 1/3 are carrier heterozygotes, and 1/3 are normal.

4. Segregation patterns for familial trisomy-21
Fig 14 14 21 21
Trisomy
Inviable
Carrier
Normal
Fig ,
Segregation patterns for familial trisomy-21

5. Relationship between age of mother and risk of trisomy-21:
16-26
7.7/10,000
27-34
4/10,000
35-39
~3/1000
40-44
1/100
45-47
~3/100

6. Trisomy-13 - Patau Syndrome
2/10,000 live births
Trisomy-18 - Edwards Syndrome
2.5/10,000 live births

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10. Variation in complete sets of chromosomes:
Monoploidy = one of each chromosome (no homologous pair)
Polyploidy = more than one pair of each chromosome.
Fig

11. Variation in complete sets of chromosomes:
Monoploidy and polyploidy:
Result from either (1) meiotic division without cell division or (2) non-disjunction for all chromosomes.
Lethal in most animals. Monoploidy is rare in adult diploid species because recessive lethal mutations are expressed.
Polyploidy tolerated in plants because of self-fertilization; plays an important role in plant speciation and diversification.
Two lineages of plants become reproductively isolated following genome duplication, can lead to instantaneous speciation.
Examples include
15% of angiosperm speciation events
31% of ferns
crops like canola, wheat, cotton

12. Viable
Self-fertile

14. Odd-numbered polyploids have unpaired chromosomes and usually are sterile.
Most seedless fruits we eat are triploid.