lec 24

Key Concepts of Chromosomal Variation

  • Terminology:

    • Euploidy: Normal chromosome number for a species

    • Aneuploidy: Loss or gain of one or more chromosomes

    • Polyploidy: Gain of entire haploid sets of chromosomes

    • Auto- and Allopolyploidy: Multiple chromosome sets from the same or different species, respectively

    • Deletion, Duplication, Inversion, Translocation: Types of chromosomal alterations

Learning Objectives

  • LO47: Explain gene dose balance; autosomal aneuploidy less tolerated than polyploidy.

  • LO48: Discuss challenges of increased ploidy during meiosis and chromosome pairing; gene duplications/deletions mechanism.

  • LO49: Analyze consequences of ploidy and chromosomal variation on traits, agriculture, and speciation.

Chromosome Structure and Function

  • Chromosomal variation affects gene dosage and expression; significant in evolution and medical contexts.

  • Aneuploidies arise from nondisjunction during meiosis; maternal age affects rates of nondisjunction.

Aneuploidy Effects

  • Monosomy: One chromosome missing (not viable in humans).

  • Trisomy: One extra chromosome (examples: Trisomy 21 - Down syndrome).

  • X-linked disorders can result from aneuploid gametes (e.g., Klinefelter Syndrome - XXY).

Polyploidy

  • More than two copies of the haploid set of chromosomes (e.g., Diploid 2n, Triploid 3n, Tetraploid 4n).

  • Common in plants and allows for greater cell size and agricultural benefits; detrimental in mammals.

Chromosome Rearrangements

  • Deletions: Loss of chromosomal segments.

  • Duplications: Accidental repeat of chromosome segments.

  • Inversions: Rearrangements within the chromosome without loss/gain of genes.

  • Translocations: Rearrangement between non-homologous chromosomes.

Dosage Compensation

  • Mechanism to equalize gene expression between sexes with different chromosome configurations.

  • In mammals, X inactivation leads to distinct phenotypes in XX and XY individuals; 15% of X-linked genes escape inactivation.

Consequences of Inversions

  • Suppresses recombination, leading to low fertility in inversion heterozygotes.

  • Different types of inversions (pericentric and paracentric) affect recombination outcomes.