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Transcript
Patterns of inheritance: Mendel and beyond Contrasting characters in peas Experiment with garden peas I In this case a truebreeding tall plant was crossed with a truebreeding short plant. All of the plants in the next generation were tall. These are the results Mendel obtained when he crossed two heterozygotes. ‘Paint-pot’ theory of inheritance Mendel’s finding was contrary to the prediction of the ‘Paint-pot’ theory of inheritance Mendel’s 1st lawlaw of segregation Mendel’s law of segregation explanation of monohybrid ratio Mendel described patterns of inheritance in the 1860s, but it wasn’t until the early 1900s that inherited traits, genes, were linked to cellular structures called chromosomes. The number of chromosomes varies among species, but all chromosomes contain genes arranged linearly at specific locations, called loci. This is how geneticists today represent the cross from the previous screen. The original plants are the P generation and their offspring are the F1 generation. The T and t symbols represent dominant and recessive alleles of a single gene. we breed the offspring to one another in the cross Tt x Tt, and wish to predict the results we need to follow the production of gametes during meiosis. These individuals make gametes with either T or t in equal numbers. The chance of gametes of different types encountering one another is represented on a Punnett square. The genotypic ratio of homozygous dominant: heterozygous: homozygous recessive individuals is 1:2:1. Many genetics problems will fall in the category of a monohybrid cross. They all can be approached using the model above Testcross. a heterozygote was crossed with a homozygous recessive individual. A situation in which an individual of dominant phenotype, but of unknown genotype, is crossed with one or more recessive individuals. This can provide information on the unknown genotype. Dihybrid cross Dihybrid cross Dihybrid cross The inheritance of two contrasting characters were considered at the same time Law of independent assortment Mendel’s nd 2 law Mendel’s 2nd law explained by observing movement of chromosomes at meiosis Cell division The cell cycle Interphase Interphase Mitosis – division of the nucleus Division of the nucleus Prophase metaphase anaphase telophase Division of the cytoplasm Produce identical daughter cells Mitosis and cell cycle Meiosis • Reduction division • Generate variability in gametes Meiosis-Interphase Meiosis-prophase I Meiosis-metaphase I Meiosis-anaphase I Meiosis-telophase I Meiosis I -animated Meiosis II prophase II metaphase II anaphase II telophase II Meiosis II -animated Meiosisoverall Independent assortment Crossing over Anaphase I and II note new gene combinations as a result of crossing over Comparing Mitosis and Meiosis Comparing Mitosis and Meiosis Incomplete dominance In some cases, alleles may not interact in a dominant/recessive pattern. Heterozygotes have an intermediate phenotype or express both alleles. The genotypic ratio will match the phenotypic ratio. Snapdragons demonstrate incomplete dominance. Incomplete dominance II Multiple allele Linkage and crossing-over Recombinant frequency Skin colour_a pair of twin sisters Continuous or discontinuous Polygenic effects on genetic variation Polygenic inheritance_skin colour Skin colour is a continuous variation because… How would U explain the very different skin colour of the twin sisters? X-inactivation X-inactivation