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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 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 seeds crossed with smooth seeds F1 generation: all smooth seeds. Crossed with itself………... F2 generation: smooth and wrinkled in ratio 3:1 Mendel’s genetic hypothesis AA aa A A A a 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. a Parents produce gametes (eggs, pollen) which have 1 copy of the gene. Aa Fertilisation produces the F1 generation, all smooth because the “A” form is dominant over “a”; “a” is recessive 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). Cross with two genes AABB AB aabb Ab aB ab AB AB ab Ab AaBb AB Ab aB ab 4 types of gametes in equal numbers aB ab 9/16 yellow/smooth 3/16 green/smooth 3/16 yellow/wrinkled 1/16 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 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 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