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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 labthis week (especially mitosis and meiosis).

Objectives• Compare mitosis and meiosis.• Recognize how chromosome movement during

meiosis results in Mendel's laws of Segregationand 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 TwoHomologue pairing None Yes, Prophase# daughter cells Two FourGenetic composition identical differs from

to parent parentChromosome number same 1/2 that of

as parent parentRole in life cycle asexual gamete

reproduction, formationetc.

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Recombination• How does recombination occur?

– Independent orientation of chromosomes# possibilities = 2n, where n is the haploid

chromosome number– crossing over adds to possibilities

• Each offspring receives different geneticmaterial from its parent

Figure 13.10 The results of alternative arrangements of two homologouschromosome 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

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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 modelorganism 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)

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Introduction to sex linkage

• Some traits are linked to genes thatdetermine sex

• Sex linked traits may occur in eithersex

• 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 fliessearching for ones that differed from the‘wild-type.’

• A white eyed ‘mutant’ male was discoveredand 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+

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Results of first cross

• F1 generation– females all red eyes– males all red eyes

• F2 generation– females all red eyes– 1/2 males white eyes– 1/2 males red eyes

Figure 15.3 results of parental cross

P generationFemales homozygousfor ‘wild-type’ alleleMales have one copyof ‘mutant’ alleleF1 generationFemales heterozygousMales have

X chromosome with‘wild-type’ alleleY chromosome

Males are‘heterogametic’ and‘hemizygous’

Figure 15.3 Naming genotypes for sex-linked alleles

P generation Xw+Xw+ X XwY

F1 generation XwXw+ X Xw+Y

Figure 15.3 Naming genotypes for sex-linked alleles

P generation Xw+Xw+ X XwY

F1 generation XwXw+ X Xw+Y

Xw+YXwY

Xw+Xw+XwXw+

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Reciprocal cross of white eye femaleswith red eye males

• F1 generation– females all red eyes– males all white eyes

• F2 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 Xw Xw X Xw+Y

F1 generation Xw+Xw X XwYF2 generation

XwYXw+Y

XwXwXw+Xw

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