Lecture 39: Changes in Chromosome Number

(version 31 July 2002)

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Changes in Chromosome Numbers

Ploidy Levels

let x = number of chromosomes in basic set

x = monoploid

2x = diploid

3x = triploid

4x = tetraploid

6x = hexaploid

Polyploidy

Very common in plants (many crop species are polyploids)

Autopolyploids

Self-polyploids

Multiple chromosome sets from the same parent

Allopolyploids

Chromosome sets from different parents

Problems with polyploids

If chromosomes do not occur in pairs, problems at meiosis

Such individuals are usually sterile

If double basic chromosome set, normal meiosis

Suppose basic chromosome set is doubled

Autopolyploid

If normal chromosome set were to double, meiosis would still be normal

Allopolyploid

Chromosomes from two (or more) different parents (Species)

The Rabbage

Why are polyploids most common in plants?

Few animal examples (some fish families)

asexual reproduction in plants

Aneuploidy in animals

euploid = contains all chromosomes

aneuploid = not euploid

suppose normally a diploid

2n -1 = monosomic

2n + 1 = trisomic

2n - 2 = nullisomic

Human examples

Sex chromosomes

X0 = Turner's syndrome

monosomic

XXY = Klinefelter's syndrome

trisomic

Most autosomal aneuploids are lethal and not seen

Down's -- trisomy 21

mean life expectancy 17 years

Patau's -- trisomy 13

mean life expectancy 130 days

Edward's --- trisomy 18

mean life expectancy a few weeks

Related details from Online Mendelian Inheritance in Man on Down's --- trisomy 13 --- trisomy 18

Causes of aneuploidy

Meiotic Nondisjunction

1st division nondisjunction

half gametes n+1, rest n-1

2nd division nondisjunction

half gametes normal
1/4 n+1
1/4 n-1

Somatic aneuploids

Mitotic nondisjunction

Bilateral gynandromorphs are a striking example