Biology 210
GENETICS
25 February, 1998

Chapter 7b:
Variation in chromosome number

OUTLINE:
I. Background

II.Types of chromosomal Mutations: III.Position Effects

IV.Chromosomal abnormalities and CANCER

I. Background

cp. Chromosomal mutations, gene mutations

usually gene mutations are not detected by microscopes N.B. - exceptions:

Topography of chromosomes:

Size
chromosomes are numbered from the longest to the shortest (more or less - as best as can be determined by light microscopy). This is figure 7.12 from your text (page 272).
figure 7_12 from Hartl & Jones

Centromere position

position of nucleolar organizers

chromomere patterns

heterochromatin patterns
figure 7_15 (page 277) - in females, one of the X-chromosomes can be inactivated, and can be readily seen as forming a stable heterochromatin throughout the cell cycle.

figure 7_15 from Hartl & Jones, 1998

banding patterns


II. Types of chromosomal Mutations:

A. Deletions
figure 7-18 (page 282) Deletions can be mapped by using a testcross.
figure 7-18


figure 7.19 Mapping deletions in part of the X-chromosome in polytene salivary gland nuclei of Drosophila melanogaster.

figure 7-19

Unequal crossing over can cause deletions & duplications
figure 7-20figure 7-20figure 7-20

figure 7-20
B. Duplications
figure 7-21

Genetic basis of absent or impaired red-gree colour vision.
figure 7-22 (page 285 in text)
figure 7-22


The double-bar mutant in Drosophila is another case of duplication (and deletion) from unequal crossovers.




C. Inversions

figure 7-23 (page 286 in text)
figure 7-23

Chromosome inversions can pass through mitosis by forming loops:

figure 7-24

figure 7-25 (page 287).

D. Translocations

figure 7-29 (page 290)
figure 7-29
The difference between HETEROzygous and HOMOzygous translocationfigure 7-29figure 7-29

figure 7.30 (page 291) - different possible products from meiosis with a chromosomal translocation.
figure 7-30

Only SOME of the gametes are fertile - this is an example of SEMISTERILITY

figure 7.31a (page 293) - Two sets of chromosomes - one is "normal", and the other has undergone a translocation.
figure 7-31

at Metaphase I in meiosis, the chromosomes line up, and can be seperated in two different ways.
figure 7-31b


A ROBERTSONIAN translocation is formed by the fusion of two acrocentric chromosomes.
figure 7-32

figure 7.34 (page 295) the mechanism for formation of a Robertsonian translocation.
figure 7-34

Robertsonian translocations result in the merging (and loss of a small bit) of chromosomes.

It is likely that human chromosome # 2 came from a translocation between 2 different chromosomes found in chimps.


III.Position Effects

figure 7-35

figure 7-36

IV.Chromosomal abnormalities

and CANCER

figure 7-37

Deletions can cause many human diseases

figure 7-39

Chromosome Icon Back to the Genetics syllabus

Leaf bar # 16



Last modified on: 2 February, 2000 by Dave Ussery