Presentation on theme: "Genetic Inheritance & Variation"— Presentation transcript:
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 occurThere are 2 main processes that generate variation:MutationRecombination
2 Mutation and Recombination Mutation is a change in the genetic informationRecombination is a different arrangement of the same genetic materialThe cat sat on the matThe cat sat on the hat - mutationThe mat the cat sat on - recombinationFirst of all, we need to look at genetic inheritance…...
3 Mendel’s experimentsGregor Mendel (a 19th century Czech monk) worked out the basic laws of genetic inheritance by breeding pea plantsHe 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 seedsF1 generation: all smooth seeds. Crossedwith itself………...F2 generation: smooth and wrinkledin ratio 3:1
5 Mendel’s genetic hypothesis Genes come in pairs. Each of the parents has2 copies of this gene. The “A” form gives smoothseeds, the “a” form gives wrinkled.AAaaParents produce gametes (eggs, pollen)which have 1 copy of the gene.AaAaFertilisation produces the F1 generation, all smoothbecause the “A” form is dominant over “a”;“a” is recessiveAaEach F1 plant produces equal numbers of A and agametes 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).
6 Cross with two genes AABB aabb ab AaBb AB Ab aB ab AB Ab aB ab AB AB 4 types of gametesin equal numbers9/16 yellow/smooth3/16 green/smooth3/16 yellow/wrinkled1/16 green/wrinkled
7 Summary of Mendel’s experiments Genes in an organism come in pairsSome forms (“alleles”) of a gene are dominant over other alleles which are recessiveOne (at random) of each pair of genes goes into a gamete (segregation)Gametes meet randomly and fertiliseThe numbers and types of offspring in a cross are determined by the above lawsSeparate genes behave independently of each other (later, exceptions to this rule were found)
8 Genes and chromosomesGenes can have several different forms due to mutations in DNA sequence. These forms are called alleles. Property of having different forms is called polymorphismNormal 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 malesGametes (eggs, sperm, pollen) are haploid, i.e. they have a single copy of each chromosome
9 Autosomal dominant inheritance Person with trait in each generationMales and females equally likelyto show traitWhere 1 parent is heterozygous,about 50% of offspring show traitExample: Huntington’s disease
10 Autosomal recessive inheritance Trait may “skip” generationsMales and females equally likely to show traitHeterozygotes (“carriers”) do not show traitAbout 25% of offspring of 2 carriers will show traitExample: cystic fibrosis
11 X-linked recessive inheritance Carrier (heterozygous,unaffected) mothers pass the traitto about 50% of sonsTrait is never transmittedfrom father to sonIn the population, trait will be much more common in malesthan females. Example: muscular dystrophy
12 Jumping genesGenomes 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 bacteriaThey can also affect the expression of a gene near to where they jumpIf a transposon jumps in some cells but not others, can get a variegated phenotypeMaize (corn) cob