Lecture 5 April 17, 2012 “The Chromosome Theory of Inheritance” ****Continuation of Presentation 2**** Duplicate GenesSuppression Penetrace Penetrance: % with expected phenotype for a given genotype - Complete penetrance: When individuals always show the phenotype for a given genotype. i.e. Mendel’s garden pea T: tall, t: short all tt plants are short - Incomplete penetrance: When individuals do not show a trait even though they have the appropriate genotype. i.e. Polydactyly in humans…the presence of extra fingers and toes. Caused by a dominant mutation (P_) - not all P_ individuals have extra digitsVariable Expressivity - The degree to which a genotype is phenotypically expressed in an individual o Example: neurofibromatosis - Dominant Trait can manifest itself as mildly as pigmented areas of the skin or as tumor-like growths - Expressivity depends on the: o Individual’s genetic background o Environment of the Individual The Role of Environment - Some characteristics, such as eye color, appear to be almost entirely genetically determined. - Other human characteristics result from combination of heredity and environment - Examples: skin color, height, heart disease, cancer, alcoholism, etc.Environmental Effect of PhenotypesVariation on Mendelian Genetics Sample Problems**** Presentation 3 **** Eukaryotic Chromosomes - Chromosomes Chromatin = a combination of DNA and protein molecules - Are not visible in a cell until cell division - The DNA in a cell is packed into an elaborate, multilevel system of coiling and folding - Heterochromatin: tightly packed DNA. - Genes are not expressed - Euchromatin: loosely packed DNA--regions where genes are expressed.Homologous ChromosomesPloidy - Diploid chromosome number (2n): having two sets of homologous chromosomes (i.e. One copy from mom and one copy from dad). - Somatic cells are usually diploid, and therefore each gene exists in pairs (i.e. AA or Aa or aa) - Haploid chromosome number (n): Having only one set of a homologous chromosomes. - Sex cells or gametes are haploid, and therefore only have one copy of each gene (i.e. A or a). - Fertilization restores the diploid chromosome number and produces a 2n zygote. The Cell Cycle - Before a cell divides, it duplicates all of its chromosomes, resulting in two copies called sister chromatids - When the cell divides, the sister chromatids separate from each other Mitosis and Cytokinesis Mitosis -Asexual cell division -Is the division of the chromosomes -Is preceded by interphaseMitosis consists of four distinct phases – Prophase Metaphase Anaphase TelophaseMitosis: Summary - The main events of mitosis… - Replication and sister chromatid adhesion, followed by segregation of the sister chromatids into each daughter cell. - In short…you start with a 2n cell and end up with two 2n