Download notes

Lecture 39
Prof Duncan Shaw
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
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