Announcements Colloquium sessions for which you can get credit posted on web site: Feb 20, 27 Mar 6, 13, 20 Apr 17, 24 May 15. Review study CD that came with text for lab this week (especially mitosis and meiosis). Objectives Compare mitosis and meiosis. Recognize how chromosome movement during meiosis results in Mendel's laws of Segregation and Independent Assortment. Define Chromosomal Theory of Inheritance. Understand sex-linkage and why it supports the Chromosomal Theory of Inheritance. Learn how to use pedigrees to track Mendelian inheritance. Figure 13.6 Overview of meiosis: how meiosis reduces chromosome number Comparison of mitosis and meiosis Event Mitosis Meiosis # divisions One Two Homologue pairing None Yes, Prophase # daughter cells Two Four Genetic composition identical differs from to parent parent Chromosome number same 1/2 that of as parent parent Role in life cycle asexual gamete reproduction, formation etc. 1
Recombination How does recombination occur? Independent orientation of chromosomes # possibilities = 2 n, where n is the haploid chromosome number crossing over adds to possibilities Each offspring receives different genetic material from its parent Figure 13.10 The results of alternative arrangements of two homologous chromosome pairs on the metaphase plate in meiosis I Figure 15.1 The chromosomal basis of Mendel s laws Meiosis and inheritance (15.1) Each locus on a different chromosome Rule of Independent Assortment follows from independent orientation at Metaphase I Rule of Segregation follows from separation of homologues at Anaphase I 2
Meiosis and inheritance (15.1) Each locus on a different chromosome Rule of Independent Assortment follows from independent orientation at Metaphase I Rule of Segregation follows from separation of homologues at Anaphase I Figure 15.1 The chromosomal basis of Mendel s laws Chromosomal theory of inheritance Traits inherited according to Mendel's laws are on chromosomes Work on sea urchins supported this theory Scramble up chromosomes in eggs Misshapen sea urchins result Discovery of sex linkage, using fruit flies, confirmed this theory Drosophila as a model organism for genetics Flies have short generation times (2 weeks) Easy to rear large numbers of flies Drosophila species have only four large chromosomes Sex is determined genetically (females XX, males XY) 3
Introduction to sex linkage Some traits are linked to genes that determine sex Sex linked traits may occur in either sex This is different than sex-limited traits, which are only found in one sex. The white eye locus in fruit flies At the turn of the 20th century, Morgan and coworkers bred thousands of flies searching for ones that differed from the wild-type. A white eyed mutant male was discovered and crossed with a red-eyed female. Figure 15.2 Morgan s first mutant Allele naming in flies When a mutant is discovered, the locus is named after the mutant phenotype (e.g. w, the white eye locus) The typical phenotype is called wild-type mutant allele = w wild-type allele = w + 4
Results of first cross F 1 generation females all red eyes males all red eyes F 2 generation females all red eyes 1/2 males white eyes 1/2 males red eyes Figure 15.3 results of parental cross P generation Females homozygous for wild-type allele Males have one copy of mutant allele F 1 generation Females heterozygous Males have X chromosome with wild-type allele Y chromosome Males are heterogametic and hemizygous Figure 15.3 Naming genotypes for sex-linked alleles P generation X w+ X w+ X X w Y F 1 generation X w X w+ X X w+ Y Figure 15.3 Naming genotypes for sex-linked alleles P generation X w+ X w+ X X w Y F 1 generation X w X w+ X X w+ Y X w X w+ X w+ X w+ X w Y X w+ Y 5
Reciprocal cross of white eye females with red eye males F 1 generation females all red eyes males all white eyes F 2 generation 1/2 females white eyes 1/2 females red eyes 1/2 males white eyes 1/2 males red eyes Diagram of reciprocal cross P generation X w X w X X w+ Y F 1 generation X w+ X w X X w Y F 2 generation X w+ X w X w X w X w+ Y X w Y Conclusions Reciprocal crosses yield differing results Sex linked traits show criss-cross inheritance The Y chromosome was associated with males and not found in females The gene for eye color was on the X chromosome This constitutes proof of the chromosomal theory of inheritance 6