ANEUPLOIDY IN HUMANS

==Common Autosomal Aneuploids:== [image:http://i.imgur.com/MvTg4Je.jpg?2] • Down Syndrome (Trisomy 21) • Edward Syndrome (Trisomy 18) • Patau Syndrome (Trisomy 13) ~''(left: an 8 year old boy with characteristic facial features of downs syndrome)''~ ==Common Sex Chromosome Aneuploids:== [image:http://i.imgur.com/o96skJJ.png?1] • Turner Syndrome (Monosomy = XO) • Klinefelter Syndrome (Trisomy = XXY) ~''(left: a young girls with webbing of the neck, characteristic of turner syndrome)''~ You may notice that any change to the number of chromosomes results in a ‘syndrome’. A syndrome is a condition that affects many different recognisable features. As each chromosome contains many genes, aneuploidy affects several characteristics and produces many symptoms. Secondly, you will see that most cases of aneuploidy are trisomy (have an extra chromosome) rather than monosomy (missing a chromosome). This is because a loss of genetic information, especially on this scale (a whole chromosome!), is most likely lethal. '''Maternal Age Effect''' The incidence of various aneuploidy based syndromes shows a marked increase with the mother’s age. This is known as the maternal age effect and interestingly, the father’s age has very little effect on the incidence of aneuploidies. The reason for this is that females contain all their egg cells (ovum cells) at birth, whereas males produce new sperm continuously throughput their life. In actual fact, the ovum cells are not quite complete but are suspended at late prophase of meiosis I (see phases of meiosis, Mendelian genetics). Every month as a part of the menstrual cycle the process of meiosis is allow to continue for a few cells, resulting in the release of a mature egg cell (ovum). Scientists now believe that the longer the chromosomes have been suspended, the more ‘sticky’ they become. As a result the chromosomes are less likely to separate, resulting in a non-disjunction and thus potentially an aneuploid individual. '''Chromosomal Abnormalities & Miscarriage''' Only approximately 83% of conceptions (fertilisations) result in a live birth. Approximately 15% of conceptions undergo a spontaneous miscarriage. Of these over half have severe chromosomal abnormalities (missing or additional chromosomes, even additional chromosome sets). While many of these would naturally be lethal, it is believed that the mother’s body actually has some mechanism or recognising and rejecting those conceptions with chromosomal abnormalities.