gen2041 notes lecture 1: patterns of inheritance fields of
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GEN2041 Notes Lecture 1: Patterns of Inheritance Fields of Genetics
• Transmission Genetics: How traits are passed from parents to offspring
• Molecular Genetics: How the hereditary material controls the expression of genes and thus traits
• Population Genetics: Genetic variation and its role in evolution Key Model Organisms
• Mice • Nematode worm (C. elegans) • Bacteria (E. coli) • Mustard grass (Arabidopsis) • Zebra fish • Vinegar fly (Drosophila)
Key Terms
• Trait: A characteristic of an organism • Phenotype: The appearance of an organism • Genotype: The genetic composition of the individual • Gene: Unit of heredity (region of DNA) influencing a trait • Allele: Alternate version of the same gene • Locus: Specific place on a chromosome occupied by a gene • Homozygote: An organism with two copies of the same allele at
a locus • Heterozygote: An organism with two different alleles at a locus
Autosomal Inheritance (Discontinuous Variants – Mendelian)
• Discovered by Mendel • Uses a good model organism that can self or cross fertilise • Isolated true-breeding lines • Devised a hypothesis and then rigorously tested it
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Law of Segregation (Monohybrid Crosses) • Performed reciprocal crosses (phenotype of male and female
parent is reversed) • Parental phenotype expressed in
the F1: dominant • Parental phenotype not
expressed in the F1: recessive • Each gamete only carries one
member of each gene pair, where every allele has a 50% chance of being expressed in the offspring
Law of Independent Assortment (Dihybrid Crosses)
• Studied simultaneous inheritance of two traits
• Gives a result of 9:3:3:1 phenotypic ratio
• Gene pairs were assorting independently during gamete formation
Sex-Linked Inheritance (Discontinuous Variants)
• Discovered that some genes can be X linked
• Son: Receives X from mother and Y from father
• Daughter: Receives X from mother and X from father
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Lecture 2: Cell Division and Chromosome Theory Accidents in Cell Division
• Abnormal chromosome segregation (aneuploidy) • Developmental defects and mental retardation due to errors in
meiosis • Errors in mitosis can contribute to cancer
Chromosomes
• Humans have 46 chromosomes • 22 pairs of autosomes • One pair of sex chromosomes (XX in female and XY in male) • Most eukaryotes are diploid; each chromosome is a member of a
pair called homologues • In a diploid organism most cells are diploid (2n), some are
haploid (n)
Chromosome Theory of Inheritance
• Genes are in pairs; as are chromosomes • The alleles of a gene segregate equally into gametes, as do
members of a pair of homologous chromosomes • Different genes act independently; as do different chromosome
pairs
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Cell Division • Each process produces two daughter cells • Mitosis: The same number and types of chromosomes as the
original mother cell (genetically identical) [diploid] • Meiosis: Half the number of chromosomes as the mother cell,
one member of each chromosome pair [haploid] Mitosis (Somatic Cells)
• Interphase § G1: Material required for survival and growth is made § S: Chromosomes replicated to form sister chromatids § G2: Synthesis of proteins needed for division
• Prophase § Chromosomes contract and shorten (held together by
cohesion) § Spindle forms (made of protein fibres called microtubules)
v Kinetochore MTs: Attach to and guide the chromosomes
v Polar MTs: Form the framework, running pole to pole § Nuclear membrane dissolves
• Metaphase § Both sister chromatids attached to kinetochore MTs and
chromosomes line up on metaphase plate • Anaphase
§ Division of the centromeric region into two, cohesion breaks down, each sister chromatid attached to only one pole
§ Chromosomes migrate to opposite poles, involves shortening of the kinetochore MTs by kinesins
§ Poles move apart due to lengthening of polar MTs • Telophase
§ Chromosomes at poles begin to decondense § Spindle fibres disperse § Nuclear membrane reforms (2 nuclei)
• Cytokinesis (cytoplasm divides)