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Lecture 2

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Human Biology
Stephen Wright

Week 2 Summary: Structural Molecular Differences between alleles: - Alleles usually differ at one or several nucleotides out of thousands - Alleles are replicated during the S-phase - Alleles: different versions of the same gene - Mutations: nucleotide sequence of an allele changes as a result of a rare chemical accident. Chromosome Segregation at eh Molecular Level: 1. Restriction Fragment Length Polymorphism:  Restriction enzymes: bacterial enzymes that cut DNA at specific base sequences (by chance)  Southern Hybridization is used to reveal RFLP. If for example, albinism allele has an extra restriction site, the probe detects one extra fragment 2. Direct Sequencing: most comprehensive picture but is time consuming and expensive 3. Polymerase Chain Reaction (PCR): used when alleles differ in number of times a sequence is repeated Alleles at the Molecular Level: - Allelic series: multiple alleles with the wild type and a known mutant. - Full/Complete Dominance: expressed when at least one copy is present - Alternate Allele: fully recessive - Null Mutations: produces non-functional proteins. o Do not produce a functional protein o Many genes are haplosufficient so null mutations have no effect on phenotype o Happloinsufficiency makes null alleles dominant and affects phenotype - Dominant negative: interference between alleles. The mutant allele interferes with the wildtype, blocking it. - Incomplete Dominance: occurrence of intermediate phenotype (Black + white = gray) - Codominance: expression of BOTH alleles of the heterozygote (black + white = spotted) - Recessive Lethal Alleles: capable of causing the death of an organism - Pleiotropic: allele that affects several properties in an organism. If lethal, only so when homozygous. - Temperature Sensitive Mutations (ts): heat-sensitive alleles can be maintained as heterozygous: o Permissive temperature: wild-type phenotype o Restrictive temperature (higher): mutant phenotype - Knock-out technique: To test if a gne is essential. A null allele is tested for lethality - Phenylketonuria: defective live phenylalanine hydroxylase (PAH). - PAH normally helps convert phenylalanine into tyrosine. - Phenylalanine buildup leads to phenyl pyruvic acid which leads to mental retardation - Plethora of mutations : o Null alleles: mutant, completely lacking PAH functions o Leaky Mutations: reduced PAH function - Haplosufficient: one functional gene is enough for wild-type phenotype. The mutations are recessive. - Complementation test: crossing two individuals homozygous for recessive traits  If the wildtype phenotype is produced when 2 haploid genomes bearing different recessive mutations are united in the same cell. Then it’s a multiple gene phenotype wth complementation. Interfering Gene Interactions: 1. Obtain single-gene mutants and test for dominance 2. Test the mutants for allelism (one or several loci) 3. Combine mutants in pairs (double mutant) – see if they interact If they interact  phenotype will differ from the single allele phenotype complementation test Epistasis: overriding mutation which masks the allele of other gene Double Mutant for Random Mutations 9:3:3:1 – no interaction 9:7 – absence of gene function leads to absence
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