LECTURE CONNECTIONS

9 | Chromosome Variation

© 2009 W. H. Freeman and Company

Chapter 9 Outline

• 9.1 Chromosome Mutations Include Rearrangements, Aneuploids, and Polyploids

• 9.2 Chromosome Rearrangements Alter Chromosome Structure

• 9.3 Aneuploidy Is an Increase or Decrease in the Number of Individual Chromosomes

• 9.4 Polyploidy Is the Presence of More than Two Sets of Chromosomes

• 9.5 Chromosome Variation Plays an Important Role in Evolution

9.3 Aneuploidy Is an Increase or Decrease in the Number of Individual Chromosomes

• Causes of Aneuploidy

• Deletion of centromere during mitosis and meiosis

• Robertsonian translocation

• Nondisjunction during meiosis and mitosis

Types of Aneuploidy

• Nullisomy: loss of both members of a homologous pair of chromosomes. 2n − 2

• Monosomy: loss of a single chromosome. 2n − 1

• Trisomy: gain of a single chromosome. 2n + 1

• Tetrasomy: gain of two homologous chromosomes. 2n + 2

Concept Check 3

A diploid organism has 2n = 36 chromosomes. How many chromosomes will be found in

A) a trisomic member of this species?

B) a monosomic member of this species?

C) a nullisomic member of this species?

D) a tetrasomic member of this species?

A) 2n + 1 = 36 + 1 = 37B) 2n – 1 = 36 – 1 = 35C) 2n – 2 = 36 – 2 = 34D) 2n + 2 + 36 + 2 = 38

Effects of Aneuploidy in Humans

• High percentage of all human embryos that are conceived posses chromosomes abnormalities.

• Aneuploidy in human sex chromosomes is better tolerated that aneuploidy of autosomal chromosomes.

• Sex- chromosome aneuploids:

- Turner syndrome XO

- Klinefelter syndrome XXY

Effects of Aneuploidy in Humans

• Autosomal aneuploids:

• Trisomy 21 – Down syndrome

• Primary Down syndrome, 75% random nondisjunction in egg formation in meiosis I

• Familial Down syndrome, Robertsonian translocation between chromosomes 14 and 21

Concept Check 4

Briefly explain why in humans and mammals, sex-chromosome aneuploids are more common than autosomal aneuploids.

No mechanism of dosage compensation for autosomal chromatids. Autosomes carry more genes. Most autosomal aneuploids are spontaneously aborted.

Effects of Aneuploidy in Humans

Autosomal aneuploids:

• Aneuploidy and maternal age

• Possible interpretation of this connection

9.4 Polyploidy Is the Presence of More than Two Sets of Chromosomes

• Autopolyploidy:

From single species

• Allopolyploidy:

From two species

Allopolyploidy – between 2 species that are sufficiently related that hybridization can occur.

The Significance of Polyploidy

Hexaploid genome of wheat contains chromosomes from three different wild species.

9.5 Chromosome Variation Plays an Important Role in Evolution

• New and extra copies of genes give rise to new functions.

• New and extra sets of genes may give rise to new species.