As mentioned, chromosomes select characteristics such as sex (Men have different copies of the sex chromosome, X and Y wheras females have two X chromosomes) but also cause diseases through chromosomal abnormalities:
- Downs Syndrome – Caused by 3 copies of chromosome 21. This is referred to as trisomy.
- Turner Syndrome (women) – only 1 X chromosome.
- Klinefelter Syndrome (men) – YXX (trisomy) rather than YX.
- Cystic Fibrosis – 3 nucleotides removed in DELTAF508 gene – stopping production of phenylalanine.
- Sickle Cell Anaemia – A changed to T in gene for haemaglobin.
Materials can be traslocated from one chromosome to another, nucleotides added or removed or bases substituted. These changes can cause diseases and other genetic problems. Usually these are seen during protein synthesis.
– Down’s Syndrome
Downs Syndrome is a genetic disease caused by an extra copy (which may be complete or partial) or chromosome 21 (trisomy 21). The disease is often associated with lessened cognitive ability & physical development and features a common set of facial characteristics. Further implications of Down’s Syndrome vary greatly from one individual to another. Fertility is another affected function, with very few males able to successfully reproduce and only some females when mating with unaffected males. Incidence rates of the disease in their children are much greater at approximately half.
While treatment can be provided to improve a sufferers quality of life there is no cure.
Fig 2 - Trisomy 21 Causing Down's Syndrome (Female Karyotype)
It is estimated 1 in 800-1000 people are born with the disease, with several factors contributing to the likelyhood of a child having it. The most notable of these seems to be the age of the mother, with the chance of the disease increasing as a mother gets older.
The Mutation in Down’s Syndrome
There are several ways Down’s Syndrome has been discovered to occur. About 95% of all cases occur via the first route, Trisomy-21.
- Trisomy 21 – 95% of cases – Where the extra chromosome 21 is added to a gamete in nondisjunction (where either homologous chromosomes fail to come apart in meiosis 1 or sister chromatids fail to come apart during meiosis 2 or mitosis) event during production in the parent; then joining with a gamete from the other parent to produce an embryo with 47 chromosomes. The vast majority (~88%) of this mutation occurs in the mother.
- Mosiac Down’s Syndrome – 1-2% of cases – Where some of the cells in the embryo (and later body) have Trisomy-21 and some are normal. This can occur as Trisomy-21 above followed by a reversion to normal cells during cell division in the embryo; or the other way around where cell division in a normal embryo somehow change to Trisomy-21.
- Robertson Translocation – 2-3% of cases – In the karyotype of one of the parents, the long arm of chromosome 21 is attached to another chromosome (often 14) and following normal disjunctions during cell replication there is a high possibility of a child receiving the extra chromosome. This is also known as familial Down’s syndrome, as it is passed directly down and the parents show a normal phenotype – with this type there is no age effect and males are as likely as females to cause the disease in their offspring.
A final, very rare occurance is the duplication of a portion of chromosome 21, meaning that there are copies of some of the genes. If these are the genes responsible for the effects seen in Down’s syndrome then these effects will be expressed but otherwise the phenotype will be normal.
– Sickle Cell Anaemia
Sickle cell anaemia affects the red blood cells in the body, by producing cells which hold a rigid sickle shape rather than the usual doughnut. As this is a genetic disease based on a recessive allele there is a possibility for offspring to be carriers, suffer the disease or not carry it at all, depending on their parents. Sickle cell disease is caused by having both recessive alleles (SS) while people can also have sickle cell trait which means they are a carrier but do not show the effects of the disease (HbS).
As the cells are more rigid than normal, and combined with their unusual shape there are many complications which can occur within the body. These include blockages of blood vessels, increased destruction of blood cells (and so reduced oxygen capacity), problems with the spleen and a host of other blood & circulation related problems.
A sickled red blood cell sits among normal cells
It is interesting to note that the disease is found in higher levels in areas where Malaria is more common, as being a carrier (so the sickle cell trait rather than sickle cell disease) is a benefit as sickling of blood cells as they are attacked by malaria halts its spread.
Sickle cell disease is caused by a mutation on the haemoglobin gene – where A is changed to T at position 17 in a base substitution (mis-sense). This changes a glutamic acid on the protein (GAG) to a valine (GTG).
– Types of Mutation in DNA
Fig 1 - Showing different types of chromosomal mutation
Wild Type = Normal Sequence of DNA
- Point Mutations – Single nucleotide changes in the DNA strand which result in different codons.
- Miss-sense = Resulting in a different amino acid.
- Non-sense = Resulting in a STOP codon and possible termination of protein chain.
- Silent = Codon codes for the same amino acid as wild type so the protein is the same.
- Frameshift Base Insertions or Deletions = One nucleotide added or removed, resulting in the change of most of the following amino acids.