Download meiosis generates new combinations of alleles

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