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The next generation

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Mendel was lucky - the genes he chose all segregated independently ... Essentially, we are all genetically unique (except identical twins) ... – PowerPoint PPT presentation

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Title: The next generation


1
The next generation
  • Chapters 9, 10, 17 in the course textbook,
    especially pages 175-181, 201-204, 343-344

2
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

3
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4
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

5
Modes 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)

6
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

7
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

8
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

9
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10
Meiosis and Recombination
Result meiosis generates new combinations of
alleles
11
The overall process
12
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

13
Unlinked genes
14
Linked genes
15
Linkage 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
16
An 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
17
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)

18
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.

19
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

number
height
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