Lecture 4. The Chromosome Theory of Inheritance

I. Importance

First step in transforming genes from abstract concept to a concrete biological phenomena (location,

composition, etc.)

Necessary for the advancement of all fields of genetics

II. History

A. Genes reside in nucleus- With microscope observed egg and sperm nuclei are the only elements contributed

equally by male and female gametes

1. Boveri and Sutton, early 1900’s after the rediscovery of Mendel

Observations: the behavior of chromosomes during meiosis exactly parallels the behavior of Mendel’s genes

genes are in pairs- so are chromosomes

genes are transmitted seemingly unchanged from generation to generation, so are chromosomes

the alleles of a gene segregate equally into gametes-so do members of a pair of homologous chromosomes

during meiosis

alternative alleles of unrelated genes assort indep., homologous chromosomes go to opposite poles without

regard to parental origin

½ of the genes are of maternal and ½ are of paternal origin, same is true with chromosomes

Early examples where chromosome behavior corresponded with different phenotypes

Review of Meiosis AGAIN- homologous chromosomes, chromatids etc.

III. Definitive Proof

A. Morgan’s experiments with white color gene, sex linked

Males are hemizygous for X-linked genes (hemizygous- having only 1 copy of the chromosome and hence 1 copy of the allele)

B. Bridges work with nondisjunction X chromomsomes

Sex determination

C. Conclusions: Specific genes do reside on chromosomes, Phenotype corresponds to genotype, which corresponds to behavior of chromosomes

IV. X-linked traits in humans

Sex limited trait only found in 1 sex

Sex-influenced- genotype only expressed in 1 sex

Pedigree of hemophilia

V. Conclusions

Genes are on chromosomes

VI. Terms/Concepts to know

Chromosomes, homologous chromosomes, Meiosis, haploid, diploid, sex chromosomes, Chromatids, X-linked, hemizygous, familiarity with Morgan’s and Bridges’ experiments, also early evidence (Sutton and Boveri) for genes on chromosomes, relate chromosome behavior during meiosis with transmission of genes, homologous chromosomes, chromatids

Tables and Figures from Chapter 4: Tables 1 and 3, Figs 3, 5, 12, 17, 17, 20, 21, 23