Abnormal Chromosome Numbers

• euploidy: changes in the number of sets of chromosomes per cell
• Each genome has one complete set of chromosomes
• Humans have 23 chromosomes per set
• Most higher organisms have 2 sets of chromosomes in each somatic cell and are thus diploid
• Gametes (eggs or sperm) have one set of chromosomes so are haploid
• Some cells in liver and bronchial tissues are tetraploid
• polyploid: any time more than 2 sets of chromosomes are found
• Polyploidy is common in plants
• A chemical called colchicine prevents spindle fibers from forming so chromosomes do not separate in mitosis or meiosis. It is used to trap chromosomes at metaphase for preparing human karyotypes and for doubling the chromosome composition of plants
• monosomy: missing one chromosome
• Monosomy in humans is 45 XX or XY - 21
• The only monosomy in humans that can survive are 21 and 22
• trisomy: one extra chromosome
• The most common trisomy is trisomy-21 (47 XX or XY + 21) aka Down's syndrome
• Trisomy 13 (Patau syndrome) people are extremely retarded and have numerous defects such as cleft palate and heart defects
• Trisomy 18 (Edwards syndrome) people have a life expectancy of about 2 months and are extremely retarded
• Normal cells contain two sets of chromosomes so they're 2N or diploid
• aneuploidy: having extra or missing individual chromosomes
• Changes that increase the number of whole sets of chromosomes creates polyploidy
• Gametes should have one set of chromosomes so are haploid or monoploid
• genome: one set of all the genes
• An extra or missing chromosome is generally extremely detrimental
• A missing autosome (2N-1) or monosomy in all somatic cells is generally lethal
• Trisomy (2N+1) is generally detrimental but some can survive
• Trisomy 21 is the most common in humans
• Trisomy 21 is known as Down's syndrome
• 1 in 700-800 live births have Down's syndrome
• Average IQ of Trisomy 21 is 50
• Chance of Down's syndrome increases as the mother gets older
• Edward's Syndrome (Trisomy 18) rarely survive 1 year
• Patau Syndrome (Trisomy 13) rarely survive one year
• Trisomy 22 can occur but fetus is usually aborted spontaneously
• Trisomy 16 accounts for 6% of all spontaneous abortions
• Mosaic individuals where not all cells have the extra chromosome are known and typically show less severe symptoms
• Very few interspecific crosses are successful in animals and if it does occur, the interspecific progeny are typically infertile
• Having more than 2 sets of chromosomes is very rare in animals
• Autopolyploids have extra copies of the same genome
• Most seedless fruits come from them being triploids
• When similar species can cross-hybridize, subsequent spontaneous or induced chromosome doubling creates a fertile plant that is "amphidiploid"
• People with Down's syndrome have 47 chromosomes, low IQ, and are short