Genetic Inheritance

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Genetic Inheritance Variation

• No 2 organisms in a species are the same (except
  clones or monozygotic twins)
• Genetic variation is essential for evolution and
  change to occur
• There are 2 main processes that generate
  variation
• Mutation
• Recombination

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Mutation and Recombination

• Mutation is a change in the genetic information
• Recombination is a different arrangement of the
  same genetic material
• The cat sat on the mat
• The cat sat on the hat - mutation
• The mat the cat sat on - recombination
• First of all, we need to look at genetic
  inheritance...

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Mendels experiments

• Gregor Mendel (a 19th century Czech monk) worked
  out the basic laws of genetic inheritance by
  breeding pea plants
• He chose simple characteristics that are
  determined by single genes (monogenic)
• Many characters such as height, IQ, disease
  susceptibility are determined by several genes
  (polygenic)

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Mendels first cross
P1 (parental) generation wrinkled seeds crossed
with smooth seeds
F1 generation all smooth seeds. Crossed with
itself...
F2 generation smooth and wrinkled in ratio 31
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Mendels genetic hypothesis
Genes come in pairs. Each of the parents has 2
copies of this gene. The A form gives
smooth seeds, the a form gives wrinkled.
AA
aa
Parents produce gametes (eggs, pollen) which have
1 copy of the gene.
A
a
Fertilisation produces the F1 generation, all
smooth because the A form is dominant over
a a is recessive
Each F1 plant produces equal numbers of A and a
gametes which fertilise at random to produce the
F2 plants. 1/4 of them are AA (smooth), 1/2 are
Aa (smooth) and 1/4 are aa (wrinkled).
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Cross with two genes
AABB
aabb
AB
ab
AaBb
AB
ab
aB
Ab
4 types of gametes in equal numbers
9/16 yellow/smooth 3/16 green/smooth 3/16
yellow/wrinkled 1/16 green/wrinkled
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Summary of Mendels experiments

• Genes in an organism come in pairs
• Some forms (alleles) of a gene are dominant
  over other alleles which are recessive
• One (at random) of each pair of genes goes into a
  gamete (segregation)
• Gametes meet randomly and fertilise
• The numbers and types of offspring in a cross are
  determined by the above laws
• Separate genes behave independently of each other
  (later, exceptions to this rule were found)

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Genes and chromosomes

• Genes can have several different forms due to
  mutations in DNA sequence. These forms are called
  alleles. Property of having different forms is
  called polymorphism
• Normal human body cells (somatic cells) are
  diploid 23 pairs of chromosomes
• Numbers 1-22 (autosomes)
• X and Y (sex chromosomes)
• XX in females, XY in males
• Gametes (eggs, sperm, pollen) are haploid, i.e.
  they have a single copy of each chromosome

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Autosomal dominant inheritance
Person with trait in each generation
Males and females equally likely to show trait
Where 1 parent is heterozygous, about 50 of
offspring show trait
Example Huntingtons disease
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Autosomal recessive inheritance

• Trait may skip generations
• Males and females equally likely to show trait
• Heterozygotes (carriers) do not show trait
• About 25 of offspring of 2 carriers will show
  trait
• Example cystic fibrosis

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X-linked recessive inheritance
Carrier (heterozygous, unaffected) mothers pass
the trait to about 50 of sons
Trait is never transmitted from father to son
In the population, trait will be much more common
in males than females. Example muscular dystrophy
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Jumping genes

• Genomes are not always stable. Some DNA sequences
  can jump from one place to another (transposons)
• Transposons can be responsible for things like
  antibiotic resistance in bacteria
• They can also affect the expression of a gene
  near to where they jump
• If a transposon jumps in some cells but not
  others, can get a variegated phenotype

Maize (corn) cob
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Transposon mechanism