BIOL1020 Module 4 - Eukaryotic Genetics

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Dr Paul Ebert

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Eukaryotic Genetics Reproduction & genetics Asexual:  Limited variation  Common in plants  Rare in higher animals Sexual:  Lots of variation  Common in plants and animals Genetics is the study of heredity (how offspring resemble their parents) and variation (why offspring are not identical) Genetic variation  Inherited characteristics determined by genes  All individuals of a species have the same genes (variation due to gene mutation)  Variants of the same gene (alleles) which account for variation  Different combinations of alleles account for variation (30 000 genes and 2 alleles per genes) Sutton’s hypothesis (1902)  Genes located on chromosomes  Alleles are on homologous chromosomes  Alleles segregate during meiosis I  Fertilisation restores diploid condition Eukaryotic chromosomes Chromosomes composed of DNA and proteins called histones  Unreplicated chromosome contains continuous DNA molecule wrapped around histones  After replication DNA composed of two identical chromatids connected by a centromere  Genes located on chromosomes at a particular locus  Size and spread called karotype Most eukaryotes are diploid (2n)(two copies of each chromosome)  Homologous - 1 from egg and 1 from the sperm Gametes are haploid (1n)  Human gametes have 23 chromosomes  Human somatic (non-gamete) cells have 46 chromosomes (2 sets) Sexual reproduction 1. Meiosis: Haploid gametes produced 2. Sex or pollination: Gametes are brought together 3. Fertilisation: Gametes fuse and makes diploid zygote 4. Organism develops: mitosis and gene expression Meiosis I: Reduction division  Homologous chromosome – mother and father into different cells  Diploid becomes haploid Meiosis II: separation of chromatids  Four haploid gametes per starting cell  Resulting cells not genetically identical Genetic variation Chromosomes assort independently during gamete formation Recombination: gene links broken by ‘crossing over’, resulting in a mix of paternal and maternal regions between homologous chromosomes  Roughly 2-3 crossover events per chromosome pair  Increases genetic diversity Mutations Chromosomes sometimes fail to separate properly  Gametes +/- 1 chromosome  Aneuploid zygote (2n +/- 1)  Down syndrome – trisomic chromosome 21 Heredity Inherited traits are determined by genes Genotype: allelic construction of an individual Phenotype: appearance of an individual (combined influence of genotype + environment) Gregor Mendel (1866) Discovered ‘particulate’ inheritance  P – parents from two ‘pure breeding’ lines crossed  F –1first filial generation (self-fertilised) – one trait lost  F –2second generation – 3:1 ratio of traits  Included ‘test crosses’ Mendel’s Laws 1. Inheritance is particulate (heritable factors = gens)  Alternative versions of genes account for variations 2. Organisms inherit 2 alleles for each character (one from each parent)  Only one allele is passed on to gamete (gamete is haploid)  Alleles are segregated during gamete formation 3. If the 2 alleles differ, one may be dominant (other is recessive)  Dominance is an effect of the gene product (does not reflect re
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