9.17- 9.20 The Chromosomal Basis of Inheritance

1. C’some Behavior Accounts for Mendel’s Principles

• Chromosomal Theory of Inheritance- Genes are located on c’somes and the behavior of c’somes during meiosis & fertilization accounts for inheritance patterns.

2. Genes on the Same C’some are Likely to be Inherited Together

• There are more genes in a cell the chromosomes• Each chromosome has thousands of

genes on it• Linked Genes- genes that are

located close together on a the same chromosome.•Are more likely to be inherited together •Linked genes do not follow Mendel’s principle of independent assortment. Why? Discuss w/ partner.

• The farther apart genes are from each other on a c’some, the less likely the will be inherited together.

2. Crossing Over Produces New Combinations of Alleles

• A scientist named Thomas Hunt used Drosophila (fruit flies) to study genetics.• Hunt discovered

recombination frequency• Recombination

Frequency- % of recombinants that results from crossing over

• The farther apart a gene is on a chromosome the less likely they will crossover together during meiosis

Alleles “A” and “E” are less likely to crossover together b/c they are farther apart from each other

3. Genetics Use Crossover Data to Map Genes

Copyright Pearson Prentice Hall

Gene Maps

Aristaless (no bristles on antenna)

Chromosome 2Exact location on chromosome

13.0 Dumpy wing

0.0

48.5

Black body

54.5

Purple eye

67.0

Vestigial (small) wing

99.2

Arc (bent wings)

107.0

Speck wing

• Gene map- shows the location of a gene on a chromosome.

• Gene maps can be used to calculate recombination frequencies.

• Genes that are close together have a low recombination frequency (crossovers are rare).

• Genes that are farther apart have a high recombination frequency (crossovers are likely).