Lecture 2 - Chapter 15 - The Chromosomal Basis of Inheritance

Lecture 2 - Chapter 15 - The Chromosomal Basis of Inheritance

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Unformatted text preview: Lecture 2: TH 8/28/08 I. Announcements by class visitors II. Quick tutorial of BB III. Completion of Chapter 14 IV. Start on Chromosomal Basis of Inheritance Dr. K. Heidelberg 2 Some terms Homologous pair of chromosomes Purple allele White allele Flower color gene locus Fig 14.4 The set of genes an organism inherits is known as its genotype . Genes of most multicellular organisms are carried on chromosome pairs in the cell nucleus - one set inherited from each parent. Chromosome sets (except for X and Y) are virtually identical, and they match up to form pairs. Each pair has to 2 copies, or alleles , of almost every gene - one from each parent. Alleles may be identical (same sequence of nucleotides ), or slightly different. These sequence differences may lead to different forms of the same trait (e.g. brown vs. blue eyes; smooth vs. wrinkled pea texture). 3 A dominant allele does not subdue a recessive allele; alleles dont interact (no blending!) Alleles are simply variations in a genes nucleotide sequence For any character, dominance/recessiveness relationships of alleles depend on the level at which we examine the phenotype The Relationship Between Dominance and Phenotype 4 Mendels Model 1) Trait variation is due to alternative versions (alleles) of heritable factors (genes) 2) For each character, an organism inherits two alleles- one from each parent 3) Dominant alleles mask recessive alleles 4) Law of Segregation Two alleles for a single heritable character separate (segregate) during gamete formation and end up in different gametes Appearance: P Generation Genetic makeup: Gametes F 1 Generation Appearance: Genetic makeup: Gametes: F 2 Generation Purple flowers Pp P p 1 2 1 2 P p F 1 sperm F 1 eggs PP Pp Pp pp P p 3 : 1 Purple flowers PP White flowers pp P p Explains both genotype and phenotype Fig. 14.5 Mendels Law of Segregation Punnett square explains 3:1 ratio in the F 2 generation (Concept of homozygous vs. heterozygous ) F1 = 100% products that have 1 dominant and 1 recessive gene. 6 As confirmation, Mendel inferred dominant and recessive traits for 6 other characters All showed 3:1 pattern Fig 14.4 7 Dominant phenotype, unknown genotype: PP or Pp ? Predictions Recessive phenotype, known genotype: pp ! If PP If Pp or Sperm Sperm p p p p P P P p Eggs Eggs Pp Pp Pp Pp Pp Pp pp pp TECHNIQUE Fig. 14-7 What can planned experiments tell us about an unknown parent? The Test Cross RESULTS All offspring purple 1 / 2 offspring purple and 1 / 2 offspring white or 8 Mendelian Genetics is often more complex when additional genes are involved Examples of deviation characteristics of inheritance by a single gene: When a gene has more than two alleles When alleles are not completely dominant or recessive When a gene produces multiple phenotypes 9 (seed color + shape) Law of Independent Assortment Phenotypic ratio ~ 9:3:3:1 When a gene has linked alleles 10 Fig. 14-UN11 11 Law of Independent Assortment...
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Lecture 2 - Chapter 15 - The Chromosomal Basis of Inheritance

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