Meiosis and Genetic Variation

Meiosis and Crossing Over

8.11 Chromosomes Are Matched in Homologous Pairs

• Definition of somatic cells:

  • The body cells of each species contain a specific number of chromosomes.

  • Example: Human somatic cells have 46 chromosomes, which consist of 23 pairs of homologous chromosomes.

• Homologous chromosomes:

  • Chromosomes of a homologous pair carry genes for the same characteristics at the same locus.

• Checkpoint question:

  • Are all of your chromosomes fully homologous?

8.12 Gametes Have a Single Set of Chromosomes

• Diploid and haploid cells:

  • Cells with two sets of homologous chromosomes are termed diploid (2n).

  • Gametes, consisting of eggs and sperm, are haploid cells (1n) and possess a single set of chromosomes.

• Sexual life cycles:

  • Involve alternation between haploid and diploid stages.

8.13 Meiosis Reduces the Chromosome Number from Diploid to Haploid

1 of 3

• Process of meiosis:

  • Like mitosis, meiosis precedes chromosome duplication but entails two rounds of division to produce four daughter cells.

  • The first division, meiosis I, begins with homologous chromosome pairing.

  • In crossing over, homologous chromosomes exchange corresponding segments.

2 of 3

• Meiosis I:

  • Separates members of each homologous pair, resulting in two daughter cells, each with one set of chromosomes.

• Meiosis II:

  • Similar to mitosis, where sister chromatids of each chromosome separate.

  • Result: A total of four haploid cells.

3 of 3

• Checkpoint question:

  • If a cell has the haploid number of chromosomes and each chromosome has two chromatids arranged singly at the center of the spindle, what meiotic stage is it in?

8.14 Visualizing the Concept: Mitosis and Meiosis Have Important Similarities and Differences

• Both processes initiate with diploid parent cells with chromosome duplication occurring during interphase.

• Outcomes of each process:

  • Mitosis produces two genetically identical diploid somatic daughter cells.

  • Meiosis converts into four genetically unique haploid gametes.

8.15 Independent Orientation of Chromosomes in Meiosis and Random Fertilization Lead to Varied Offspring

• Variation among homologous pairs:

  • Each chromosome of a homologous pair differs at various loci.

  • Random arrangements of chromosome pairs at metaphase I create diverse combinations in eggs and sperm.

  • Random fertilization increases variation.

8.16 Homologous Chromosomes May Carry Different Versions of Genes

• Genetic diversity:

  • Differences between homologous chromosomes arise from varying versions of genes at corresponding loci.

  • Crossing over:

  • Exchange of corresponding segments occurs between nonsister chromatids of homologous chromosomes.

8.17 Visualizing the Concept: Crossing Over Further Increases Genetic Variability

• Genetic recombination:

  • Results from crossing over during prophase I of meiosis, further augmenting genetic variation.

• Checkpoint question:

  • In examining a chromosome from a gamete, is it likely to resemble the same chromosome from a skin cell?

8.18 Accidents During Meiosis Can Alter Chromosome Number

• Nondisjunction:

  • Leads to an abnormal chromosome count.

  • Can occur due to:

  • Failure of homologous chromosomes to separate during meiosis I.

  • Failure of sister chromatids to separate during meiosis II.

• Checkpoint question:

  • How could nondisjunction result in a diploid gamete?

8.19 A Karyotype Is a Photographic Inventory of an Individual’s Chromosomes

1 of 2

• Steps to prepare a karyotype:

  1. Isolate white blood cells.

  2. Stimulate cells to grow.

  3. Arrest cells at metaphase.

  4. Photograph under a microscope.

• Arranging chromosomes:

  • Order chromosomes into pairs to detect chromosomal abnormalities.

2 of 2

• Checkpoint question:

  • How would a female karyotype differ from a male karyotype?

8.20 Connection: An Extra Copy of Chromosome 21 Causes Down Syndrome

• Trisomy 21:

  • The most prevalent chromosome abnormality resulting in Down syndrome.

8.21 Connection: Abnormal Numbers of Sex Chromosomes Do Not Usually Affect Survival

• Nondisjunction of sex chromosomes:

  • Can lead to individuals with abnormal numbers of X or Y chromosomes.

  • Effects vary:

  • Some conditions (e.g., XYY) may not impact health.

  • Other conditions (e.g., XXX) typically present normal phenotypes.

Table 8.21 Abnormalities of Sex Chromosome Number in Humans