Transcript Genetics
Introduction Heredity: tendency of an individual to resemble his parents Variation: the differences between individuals of the same species Genetics: the study of how characters are transferred or inherited from one generation to the next Mendel’s Breeding Experiment Monohybrid Inheritance: The inheritance of just one pair of contrasting characters Cross 2 pure breeding parents with contrasting characters pure-breeding parents x purple first filial (F1) generation second filial (F2) generation white all purple 705 purple 224 white monohybrid ratio 3:1 Terms Genotype - the genetic composition Phenotype - the external appearance Homozygous (pure-breeding) – with the same alleles in the same character Heterozygous (hybrid) – with different alleles Terms • Dominant gene (or character) – which expresses its effect even in heterozygous condition • Recessive gene (or character) – which expresses its effect only in homozygous condition • Pedigree – it shows the inheritance of one or more characters in different generations Terms • Test cross – cross the organism with dominant character to an organism with homozygous recessive character – to test whether the organism with the dominant character is homozygous or heterozygous • Back cross – cross an organism with one of its parent Sex Determination • in human : by sex chromosomes • male : XY • female : XX XY XX • chromosomes other than sex chromosomes somatic chromosomes which control normal body characters • 22 pairs are identical in both sexes (human) – autosomes • The 23rd pair is different in male and female – sex chromosomes Twins • identical twins – one embryo separates into two and each develops into a new individual – they should have same characters and same sex • non-identical twins – formed from two different zygotes from two separate fertilizations – they usually have different characters – they may have different sexes Types of Variations No. of individuals • continuous variation – with a continuous range of intermediate values – eg. height, weight, intelligence quotient, heart beat continuous variation human height Types of Variations • discontinuous variation – the character is clear-cut & not showing intermediates – eg. tongue rolling, ear lobes and colour of corn albinism Causes of Variations • Meiosis – homologous chromosomes separate independently from each other and pass into a different gamete ( INDEPENDENT ASSORTMENT). As a result, a great variety of gametes are produced • Random Fertilization – since fertilization is a random process, there are many possible different combinations of genes in a zygote Causes of Variations • Mutation – genetic make-up may suddenly change – sometimes occur naturally – rate may greatly increased if the organism is exposed to radiation, certain chemicals or neutron bombardment – most mutation are harmful • Environmental Factors – cause variation in characters with continuous variation ~END~