Genetics 10 - Changes in Chromosome number

By the end of these lectures, you should be able to:

Understand the mechanisms which produce aneuploidy
Describe the origin and effects of aneuploidy in humans such as Down’s, Turner’s, Edward’s syndrome and Klinefelter’s syndromes
Know examples of the role of polyploidisation in crop and invertebrate/vertebrate evolution

Aneuploidy: Individuals with chromosome number different from normal - excess or deficiency - can be associated with abnormalities.
Nondisjunction: An error in cell division in which chromosomes fail to separate and migrate to opposite poles; responsible for trisomy and monosomy.

Down’s syndrome (trisomy 21) - very common (about 1 out of every 800 births) – many physical (including single palmer crease and simpler fingerprints), behavioural (more affectionate) and physiological changes (including rapid ageing) – incidence of DS increases with mother’s age - can also be associated with a translocation.
Klinefelter’s syndrome; XXY = male – often mild conditions have a number of female characteristics such as breast development and female type pubic hair; XYY = male.
Turner’s syndrome; XO = female – associated with many developmental changes; XXX = female have 2 barr bodies.
Trisomy is generally lethal and results in the abortion of the zygote early in development, but some trisomy that survive:
Edward’s syndrome (trisomy 18) occurs in 1/6000 births and results in heart abnormalities, kidney malformations and other organ abnormalities.
Patau syndrome (trisomy 13) occurs in approximately 1/10000 births and is characterized by severe intellectual disability, cleft lip and palate, and a range of other physical anomalies.

Somatic aneuploidy; Drosophila (and finch) gynandromorphs = organisms with both female and male characteristics due to some body cells having XX and others XO, arising from XX nondisjunction in early development; red-eye side female and white-eye side male.
Haplodiploidy sex determination in ants, bees and wasps (males are haploid n, females are diploid 2n).
Polyploidy: Changes in number of chromosome sets; Usually lethal in humans; odd numbers of chromosomes are associated with infertility as cannot pair during meiosis, but can reproduce asexually.
- Example: banana is triploid (thus cannot reproduce sexually); cabbage is tetraploid
Autopolyploid: polypoid condition resulting from the duplication of one diploid set of chromosomes.
Allopolyploidy: Polypoid condition resulting from the union of 2 or more distinct chromosome sets (i.e. different species) with a subsequent doubling of chromosome number.

Significant origin of crops around 10,000 years ago in the Fertile Crescent (Middle East).
Allopolyploidy in Crops
- Associated with the origin of wheats.
- Einkorn hybridized with another species to produce Emmer wheat.
- Further hybridization resulted in today's hexaploid wheat.
- Triticale is a recent allopolyploid of Emmer wheat and rye.
- Other polyploids:
- Banana (triploid and sterile)
- Maize (tetraploid)
- Strawberry (octaploid)
Relatively few non-plant polyploids including oysters (tetraploids mate with diploids and produce sterile triploids).
Polyploidy associated with some human cancers (e.g., adenocarcinoma cells have double the expected DNA).

Hawaii has a chain of volcanoes; newest island is about 5 million years old.
High density of Drosophila species.
Drosophila from different islands exhibit different chromosome inversions, useful for reconstructing their evolutionary history.

Little change in DNA amount among different primate genomes, but structural changes (inversions and translocations) are present.
Human chromosome 2 is a fusion of chimpanzee chromosomes 2a and 2b.
Similarities of human chromosomes with other organisms (e.g., mice, pufferfish, sea anemones).

Provides raw genetic information for the process of evolution.
Numerous polyploidisation events observed in fish (e.g., salmon is hexaploid), contributing to their diversity.