Presentation on theme: "Genetic Inheritance & Variation No 2 organisms in a species are the same (except clones or monozygotic twins) Genetic variation is essential for evolution."— Presentation transcript:
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…...
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)
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 3:1
Mendels genetic hypothesis Aa AAaa A a 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. 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 F2 plants. 1/4 of them are AA (smooth), 1/2 are Aa (smooth) and 1/4 are aa (wrinkled).
Cross with two genes AB ab AABBaabb AaBb ABabaBAb 4 types of gametes in equal numbers AB Ab aB ab AB Ab aB ab 9/16 yellow/smooth 3/16 green/smooth 3/16 yellow/wrinkled 1/16 green/wrinkled
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)
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: Huntingtons 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