Introduction to Genetics (BIOL0003) Notes - Changes in Chromosomes

Changes in Chromosome Number
Changes in Chromosome Number
 Aneuploidy – due to nondisjunction
o Formation of somatic cells
 Non-disjunction
 In meiotic division – failure of particular individual
chromosomes to go to the correct chromosome
end
o Results in sperm or egg with too many or
too few chromosomes
 Aneuploidy = presence of abnormal number of chromosomes in a cell
o Down’s syndrome
 Symptoms
 Short
 Breathing difficulty
 Narrow slanted eyes
 Trisomy 21
 Stain chromosomes in down syndrome child = extra
chromosome 21
o One of the smallest chromosomes
 Diagnostic test










 Egg production in females
o Mother makes all her eggs before birth – released during fertile period
 Released at later age – been in incomplete meiosis for longer =
increase in chromosomal abnormalities
 Translocation down’s syndrome
 Part of chromosome 21 has become attached to fathers sperm  attached to
another chromosomes  passed to another generation = leading to down’s
syndrome
 Mosaic down’s syndrome
 Patches of trisomy-21
o Non-disjunction took place early in development
 Individuals with mixtures of down’s chromosomes + normal chromosomes
 Prenatal diagnosis and termination

, Changes in Chromosome Number





 Sex chromosomes – x chromosome inactivation
o Klinefelter syndrome XXY
 Diploid sperm x haploid egg = XXY individual
 Diploid sperm – XY
 Haploid egg – X





o Turner syndrome XO
 O x X = XO individual
 Egg developed without a Y chromosome – O
 Haploid sperm – X





o Edwards syndrome
 Trisomy 18 – small chromosome
o Drosophila gynandromorph
 Non-disjunction in body cells
 Left side of animal is male – right side is female




o
o Sex determination in bees – haplodiploidy