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Genetic Linkage and Recombination • Mendel was lucky - the genes he chose all segregated independently • This is not true of all genes - many genes are linked • In humans, there are 23 pairs of chromosomes and about 35000 pairs of genes - each chromosome has a few hundred to a few thousand genes • Genes close together on the same chromosome are linked and do not segregate independently Terms & Definitions • Genes can have several different forms due to mutations in the DNA. These forms are called alleles. Property of having different forms is called polymorphism • Organism with 2 copies of the same allele of a gene in diploid cells is homozygous for the gene • Organism with different alleles of a gene in diploid cells is heterozygous for the gene • Males are hemizygous for genes on X and Y chromosomes Modes of inheritance • Dominant alleles affect the phenotype when present in 1 copy (heterozygous), e.g. Huntington’s disease • Recessive alleles affect the phenotype only when present in 2 copies (homozygous), e.g. cystic fibrosis • Can tell whether dominant or recessive by studying Mode of Inheritance in families (examples in my first lecture) Oogenesis & spermatogenesis (animals) • Oogenesis is the process of egg formation • Spermatogenesis is the process of sperm formation • Both go through several stages, with (in mammals) different timing in males than females • Sperms go through more cell divisions than eggs do - more chance of mutation Fertilisation • 2 haploid cells (egg, sperm) form 1 diploid cell (the zygote) which develops into the embryo • Whether sperm contained an X or Y chromosome determines if embryo is female or male • Embryo contains an assortment of genes from each original parent - more genetic diversity • Mitochondria (and their DNA) come only from mother via the egg - maternal inheritance Meiosis • Process of cell division in germ cells, to produce eggs or sperm (gametes) • 1 diploid cell 2 haploid cells • Goes through several defined stages • Chromosomes are passed on as re-arranged copies due to recombination - creates genetic diversity Meiosis and Recombination Chromosomes pair up DNA replication Chiasmata form Recombination 1st cell division Gametes 2nd cell division Result: meiosis generates new combinations of alleles The overall process Mum Egg Dad Sperm Development to adult Fertilisation Meiosis Recombination How much genetic variation? • About 35,000 genes in humans • If each gene has only 2 alleles (probably an underestimate), then: – Number of possible genotypes = 335,000 = 1016,700 • Far more than all the atoms in the Universe! • Essentially, we are all genetically unique (except identical twins) Recombination • The closer together 2 genes are on the same chromosome, the less likely there is to be a recombination between them - such genes are linked and do not segregate independently • Genes that are far apart are likely to have a recombination between them and will segregate independently - such genes are unlinked • Genes on separate chromosomes are unlinked Linkage to an autosomal dominant gene AA aa Aa A and a are alleles of a “marker” gene Aa Yellow shading indicates affected with a genetic disease (NOT caused by gene A/a) Aa aa Aa aa Allele a of the marker gene always segregates with the disease, so the 2 genes must be linked An application of linkage • Can do prenatal diagnosis for genetic disease using a linked gene • Useful when you don’t know exactly what gene is causing the disease BB bb bb Bb ? Bb bb