Explore the world of genetics with these concise notes tailored for Year 1 students in the Introduction to Genetics (BIOL0003) module at University College London. This document focuses specifically on the dynamic topic of changes in chromosomes, encompassing alterations in chromosome number and st...
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
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