The next generation
Chapters 9, 10, 17 in the course
textbook, especially pages 175-181,
201-204, 343-344
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 (mammals and some other organisms) 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 gives rise to 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
Recombination and linkage
• 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
Unlinked genes
A a B b
Parents
a a b b
Gametes: 100% ab
Gametes:
25% AB
25% Ab
25% aB
25% ab
25% AaBb
25% Aabb
25% aaBb
25% aabb
Linked genes
A a
B b
Parents
Gametes: 100% ab
Gametes:
50% AB
0% Ab
0% aB
50% ab
50% AaBb
50% aabb
a
b
a
b
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 or
bb
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)
Significance of genetic variation
• Some alleles directly cause specific traits, such as
(in humans) rare genetic diseases e.g. Cystic
fibrosis, sickle-cell anaemia; (in bacteria) ability to
grow on certain sugars
• Many alleles contribute to many traits of an
organism such as size, shape, intelligence,
behaviour, and risk of getting diseases e.g. (in
humans) cancer, heart disease, asthma
• Genetic variation is what evolution acts on.
Without it there would be no different species.
Multiple genes and quantitative traits
• Many traits like height, IQ show
a bell-shaped (normal)
distribution in population
• These are influenced by several
genes, so the overall effect
depends on the random selection
of alleles in an individual
• e.g. for height genes, you are
more likely to have a mixture of
tall and short alleles than all tall
or all short
height