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

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…...

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)

Mendel’s first cross

P1 (parental) generation: wrinkled seedscrossed with smooth seeds

F1 generation: all smooth seeds. Crossedwith itself………...

F2 generation: smooth and wrinkled in ratio 3:1

Mendel’s genetic hypothesis

Aa

AA aa

A a

A a

A

a

Genes come in pairs. Each of the parents has2 copies of this gene. The “A” form gives smoothseeds, the “a” form gives wrinkled.

Parents produce gametes (eggs, pollen)which have 1 copy of the gene.

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 F2plants. 1/4 of them are AA (smooth), 1/2 are Aa(smooth) and 1/4 are aa (wrinkled).

Cross with two genes

AB ab

AABB aabb

AaBb

AB abaBAb

4 types of gametesin equal numbers

AB Ab aB ab

AB

Ab

aB

ab

9/16 yellow/smooth3/16 green/smooth3/16 yellow/wrinkled1/16 green/wrinkled

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)

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

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

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

X-linked recessive inheritance

Carrier (heterozygous, unaffected) mothers pass the traitto about 50% of sons

Trait is never transmittedfrom father to son

In the population, trait will be much more common in malesthan females. Example: muscular dystrophy

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

Transposon mechanism