1. 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

2. 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…...

3. Mendel’s 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)

4. Mendel’s 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 3:1

5. Mendel’s 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 Aa
Fertilisation produces the F1 generation, all smooth
because the “A” form is dominant over “a”;
“a” is recessive A a
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).

6. Cross with two genes AABB aabb ab AaBb AB Ab aB ab AB Ab 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

7. Summary of Mendel’s 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)

8. 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

9. 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: Huntington’s disease

10. 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

11. 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

12. 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

13. Transposon mechanism