Title: The next generation
1The next generation
- Chapters 9, 10, 17 in the course textbook,
especially pages 175-181, 201-204, 343-344
2Genetic 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
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4Terms 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
5Modes 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
(examples in my first lecture)
6Oogenesis 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
7Fertilisation
- 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
8Meiosis
- 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
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10Meiosis and Recombination
Result meiosis generates new combinations of
alleles
11The overall process
12Recombination 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
13Unlinked genes
14Linked genes
15Linkage to an autosomal dominant gene
A and a are alleles of a marker gene
AA
Aa
Yellow shading indicates affected with a genetic
disease (NOT caused by gene A/a)
Aa
aa
Allele a of the marker gene always segregates
with the disease, so the 2 genes must be linked
aa
Aa
aa
Aa
16An application of linkage
- Can do prenatal diagnosis for genetic disease
using a linked gene - Useful when you dont know exactly what gene is
causing the disease
?
Bb or bb
17How 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)
18Significance 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.
19Multiple 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
number
height