Chromosome+Mutations


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Learning Outcomes
 * describe and explain chromosome mutations
 * describe the nature and consequences of translocations, inversions, duplications and deletions
 * describe and explain aneuploidy arising as a result of nin-disjunction during meiosis
 * define Barr Bodies and explain their significance to the expression of aneuploides
 * define a syndrome
 * describe example os aneuploidy in autosomes e.g. Downs Syndrome, Patau Syndrome and sex chromosomes e.g. Turner Syndrome, Klinfelters Syndrome

__Collaborative Notes__

//__Chromosome Mutations__//
A mutation is a change in the genetic sequence of a genome. A chromosome mutation involves the rearrangement of whole blocks of DNA rather than individual bases within a gene. There are many types of chromosome mutations such as deletion, insertion, translocation and duplication. Each types of mutation results in a change in the number and /or sequence of a set of genes on the chromosome.

__Deletion__ In deletion, genes of a chromosome are permanently lost. A break occurs at two points on the chromosome and the middle piece falls out, then the two ends of the chromosome rejoin. This way the middle part of the chromosome is lost permanently.

__Inversion__ In inversion, the order of a particular order of gene is rotated. One piece of the chromosome falls out and rotates through 180 degrees and rejoins. In this type of mutation no genetin code is lost.

__Translocation__ In translocation several genes are moved between different chromosomes. In this process a peice of one chromosome breaks off and joins on to another chromosome. This will cause major problems when the chromosomes are passed on to gametes. Some will recieve extra genes, while some will lack some genes.

__Duplication__ When duplication occurs, a segment is lost from one chromosome and is added to its homologue. The chromosome with the segment removed, becomes incomplete. As a result some gametes will recieve double the genes while the other will have no genes for the affected segment. So NOT FAIR!!!!!!!!

__Anueploidy:__ Anueuploidy is an abnormal number of chromosomes. It is a condition where the chromosome number is not an exact multiple of the normal haploid set for the species. Anueploidy results fromt he non-disjunction of homologous chromosomes during meiosis. The two common forms are:
 * 1) //Monosomy//

Monosomy is a form of anueploidy where the individual lack half the number of chromosomes as compared to a normal individual. Monosomy leads to the following syndromes
 * Turner Syndrome (XO)
 * Klinefelter Syndrome (XXY)

Trisomy involves the presence of 3 copies of each chromosome instead of 2 copies of each chromosome. Trosomy leads to syndromes such as:
 * 1) //Trisomy//
 * Down Syndrome
 * Edward Syndrome
 * Patau Syndrome

= __Polyploidy__ = =Where every chromosome in the set is represented more than one time= **Autopolyploidy**
 * all sets of chromosomes are from the same species.
 * Failure of chromosomes to separate during cell division because the spindle doesn't function properly


 * Diploid gamete + Haploid gamete = triploid zygote (2n+n=3n)
 * Diploid gamete + Diploid gamete = Tetraploid (2n+ 2n = 4n

Sometimes Meiosis 1 is normal, but the spindles fail to pull the chromatids apart in Meiosis 2.

**Reduced Fertility in Autoployploids:** Triploids (3n) are highly infertile becasue of the very low probability that meiosis will produce cells with balanced chromosome sets. E.g. Bananas whose 3 sets of chromosomes prevent meiosis from taking place and thus do not produce fertile gametes. Therefore, bananas don't have seeds.

**Allopolyploidy:**
 * All sets of chromosomes originate from two (or more) ancestral species.
 * The resulting hybrid is sterile.
 * Example: crossing a radish (2n= 18) and a cabbage (2n= 18). The hybrid is sterile, since each chromosome had no homologue to pair with in meiosis. Normal chromosome segregation couldn't occur therefore, no viable gametes were formed.
 * However, if self fertilisation is possible, then a viable fertile hybrid is produced. This is because when chromosome doubling occurs, non disjunction means that each chromosome would have a partner which was IDENTICAL. The fully fertile hybrid has 2 sets of cabbage and 2 sets of radish chromosomes (amphidiploid)

**Polyploidy in Animals:**

Polyploidy results in miscarriage in humans, triploids being the most common human chromosomal mutation. It is rare and mostly occurs in animals like leeches and crustaceans that reproduce without fertilisation (parthogenically).

//__**Summary:**__// __Changes in chromosome number:__ 1. Anueploidy (only involves certain chromosomes) a) Monosomy (2n-1) b) Trisomy (2n+1) c) Tetrasomy (2n+2)

2. Polyploidy (involves entire chromosomal set) a) Autopolyploidy (chromosomes derived from one species) b) Allopolyploidy (chromosomes derives from more than one species)