Describe the normal karyotype, chromosome banding and nomenclature.
In a normal karyotype, there will be 22 pairs of autosomal chromosomes and one pair of sex chromosomes (XX or XY) which makes up 46 chromosomes altogether (DIPLOID). In sex cells there are 23 chromosomes (HAPLOID).
I n order to identify chromosomes and genes on chromosomes, G (Giemsa) banding is used. Bands are labelled according to the chromosome number, then the arm (the p arm is the shorter one and the q arm is the longer one), and then the distance from the centromere:
Aberrations cause: - 60% of all early spontaneous miscarriages.
- 4.5% of all still births (dead when born)
- 7.5% of all conceptions, 0.6% of live births.
Draw a diagram of a balanced translocation and explain why these generally not deleterious?
A balanced translocation occurs when two pieces of DNA from the chromosomes become exchanged.
These do not normally cause any negative effect if any, because all the DNA of both chromosomes is present.
They can however be associated with cancer if they occur in haploid cells.
Draw a diagram showing possible meiotic products from someone with a balanced translocation.
Offspring from a balanced translocation may have an unbalanced set of chromosomes (50%).
Affected conceptions may miscarry, or be mentally retarded/multiple malformations if live-born.
Describe how 3 different chromosome aberrations lead to Down syndrome
Monosomy - lossof a single chromosome is almost always lethal
Describe why sex determination is not solely based on sex chromosome karyotype
The SRY Gene The gene that codes for phenotypically being male is the SRY gene which is situated at the end of the Y chromosome (hence anyone with a Y chromosome is male). It is activated at 6 weeks post conception and codes for the production of male testis.