Chromosomes Abnormalities in Chromosomal Inheritance

Chromosomes p and q arms are both called telomeres On either side of the centromeres are proteins called kinetochore – where the mitotic/meiotic spindle attaches

Location of Genes Genes are located on chromosomes in particular positions called loci (locus in singular) These genes are given names that consist of letters and numbers that signify their function An average human chromosome can contain a large number of genes arranged in a particular order (between 70 – 2600) Genes that are located on the same chromosome are said to belong to the same linkage group

Linkage Groups In humans, there are 22 autosomal linkage groups plus X and Y linkage groups X and Y chromosomes typically carry many genes relating to sex characteristics

Sex Determination In mammals, sex chromosomes are X, Y: –XX female, XY male In birds and some reptiles, sex chromosomes are W, Z –WZ female, ZZ male Some reptiles do not have sex chromosomes. Instead, sex is determined by the temperature at which they are incubated: –Higher temp = male; lower temp = female

Faulty Gamete Production More likely to happen in later years, especially women over 35 years of age This can result in chromosomal defects, leading to genetic disorders Most defects relate to unsuccessful separation of homologous pairs (non- disjunction) –Both copies end up in one gamete, leaving the other deficient of a copy of that particular pair –Chromosomes don’t break apart properly

Fertilisation with faulty Gametes When one or more faulty gametes fuse, the result is an abnormal zygote: –Wrong number of chromosomes –Changes to chromosomes –Rearrangements of chromosomes

Wrong Number of Chromosomes Additional Chromosomes (Additions): –When an additional copy of a particular chromosome is inherited, it is called a trisomy Missing Chromosomes (Deletions): –When a member of a typical pair of chromosomes is missing, it is called a monosomy

Examples - Additions Down’s Syndrome – trisomy 21 –(47, +21) Edward’s Syndrome - trisomy 18 –(47, +18) Patau Syndrome – trisomy 13 –(47, +13) Klinefelter’s Syndrome – XXY –(47, XXY) Extra Y Chromosome – XYY –(47, XYY)

Examples - Deletions Turner Syndrome –(45, XO) Deleted Chromosome 17 –(45, -17)

Changes to Chromosomes Sometimes chromosomal abnormalities involve only part of a chromosome: –Duplication (added parts – part of the chromosome is repeated) –Deletion (missing parts of chromosomes)

Examples - Deletions Wolf-Hirschhorn Syndrome –Missing part of number 4 Cri-du-chat Syndrome –Missing part of number 5

Rearrangements of Chromosomes Occasionally, parts of chromosomes can break off and attach to other chromosomes. This is called translocation One form of Down’s Syndrome can be inherited in this way, where part of chromosome 21 is translocated to Chromosome 14. (45, 14/21)

Other abnormalities If both copies of a homologous pair of chromosomes is inherited from one parent, problems in development can occur Eg. Angelman syndrome – both chromosome 15 copies are inherited from one parent

Diagnosis Most of these abnormalities can be detected by a karyotype image Sometimes, specialised stains are used to pinpoint more subtle differences You need to be able to recognise an abnormal karyotype