Lecture 39

Prof Duncan Shaw

Meiosis and Recombination

Chromosomes pair up DNA replicationChiasmata form

Recombination

1st cell division

2nd cell divisionGametes

Result: meiosis generates new combinations of alleles

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

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

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

Autosomal dominant inheritance

Autosomal recessive inheritance

X-linked recessive inheritance