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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)