BMS2042 Lecture 10 & 11: Cytogenetics, Sex Determination, Sex Chromosomes and Aneuploidy Lecture Outline 1. Human cytogenetics 2. Sex chromosomes & sex determination mechanisms 3. Dosage compensation and X inactivation 4. Linking genetic maps to chromosomes using somatic cell hybrids 5. Aneuploidy (change in chromosome number) 1. Human Cytogenetics Cytogenetics Cytogenetics à examination/visualisation of chromosomes. Number of chromosomes varies in different organisms. • Humans have 46, 23 pairs of homologues o 22 pairs of autosomes -­‐ same in both sexes.
o 1 pair of sex chromosomes -­‐ differ between sexes: § XX in females, XY in males. Chromosomes vary in size and centromere position Metacentric – in middle Acrocentric – towards end Telocentric – at end G Banding Centromere position and by size isn’t sufficient to distinguish all chromosomes. • Individual chromosomes in mammals (including humans) can be identified by Giemsa stain. • Need to pre-­‐treat chromosomes on a slide with agent that differentially loosens DNA-­‐protein interaction (separate DNA from behind wound around histones) • Then stain with Giemsa à G-­‐bands (dark and pale). Cell at metaphase (left),
and karyotype (right) of a normal human male (2n=46) Note: The banding pattern on the left and right chromatids are slightly different because the left represents the metaphase stage and the right represents the prometaphase stage. BMS2042 Nomenclature of Human Karyotypes Nomenclature of Human Chromosomes Nomenclature of Human Karyotypes Lists in order:
1. Number of chromosomes per cell 2. Complement of sex chromosomes 3. Any abnormal chromosomes Example: • 46, XY is a normal male
• 46, XX is a normal female
• 47, XY, +21 is a male with extra copy of chromosome 21 Lists in order:
1. p -­‐ short arm; q -­‐ long arm 2. Each arm sub-­‐divided into regions by landmark G-­‐ bands (arbitrary) 3. Each band (dark and pale) numbered within regions, starting from the centromere Chromosome Painting More recent method of distinguishing between chromosomes. Extension of fluorescent in situ hybridisation (FISH) procedure: 1. make a probe that contains many sequences from an individual chromosome (isolate the chromosome cytologically, or obtain DNA from library of clones from that chromosome) 2. label probe chemically with fluorescent dye 3. hybridise probe to chromosome spreads in situ. Observe that only one chromosome type fluoresces (at many different sites along its length). Chromosome has been "painted". Example: In Humans • Five different dyes developed e.g. A, B, C, D and E. • Various combinations of these dyes used to label probes for each chromosome -­‐ 24 required for 1-­‐
22, X and Y. • Each combination gives different colour: o A, B, C, D, E
AB, AC, AD, AE, BC, BD, BE, CD, CE
ABC, ABD, ABE, ACD, ACE, ADE, BCD, BCE, BDE, CDE 2. Sex Chromosomes & Sex Determination Wide range of reproductive modes found: • Some organisms entirely asexual • Some alternate between short periods of sexual reproduction and long periods of asexual reproduction. • Most diploid organisms have only sexual reproduction Requires sexual differentiation or phenotypic dimorphism of the two sexes. Basis of Sex Heteromorphic chromosomes (sex chromosomes) distinguish the two sexes. But, genes (not just those on Determination: sex chromosomes) rather than chromosomes are the basis of sex determination. Genes BMS2042 Sex Chromosome Systems in some animal species A wide variety of mechanisms have evolved in animal species -­‐ evolved separately on many occasions. • Genetic sex determining systems normally have populations of ½ females and ½ males (~equal). o one sex is heterogametic, the other is homogametic. • Non-­‐genetic systems can have unequal numbers of the two sexes. Sex chromosome systems in some animal species Temperature There are 3 General Patterns of Temperature-­‐Dependent Sex Determination: Dependent Sex Determination BMS2042