1. Lecture 39 Prof Duncan Shaw

2. Meiosis and Recombination Chromosomes pair upDNA replication Chiasmata form Recombination 1st cell division 2nd cell divisionGametes Result: meiosis generates new combinations of alleles

3. Terms & Definitions Genes can have several different forms due to small changes in DNA sequence. 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

4. Phenotype & Genotype The phenotype of an organism is its observable properties The genotype is the set of alleles it has for all of its genes (35,000 in humans) The relationship between genotype and phenotype is what genetics is all about The effect that alleles have on the phenotype depends on whether they are dominant or recessive

5. Modes of inheritance Dominant alleles affect the phenotype when present in 1 copy (heterozygous), e.g. Huntingtons 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

6. Autosomal dominant inheritance

8. Autosomal recessive inheritance

10. X-linked recessive inheritance