Chapter 15: The Chromosomal Basis of Inheritance

Concept 15.1: Mendelian Inheritance and Chromosome Behavior

Mendelian inheritance is explained by the physical behavior of chromosomes during meiosis. The principles of segregation and independent assortment are rooted in chromosomal movements and arrangements.

• Principle of Segregation: During meiosis I, homologous chromosomes separate, ensuring that each gamete receives only one allele of each gene.

• Principle of Independent Assortment: Nonhomologous chromosomes align independently at the metaphase plate, leading to genetic variation in gametes.

• Meiotic Phases:

  • Prophase I: Homologous chromosomes pair and exchange segments (crossing over).

  • Metaphase I: Homologous pairs align randomly at the metaphase plate.

  • Anaphase I: Homologous chromosomes separate to different gametes.

• T.H. Morgan's Drosophila Experiments: Morgan used fruit flies (Drosophila melanogaster) to demonstrate that genes are located on chromosomes. He observed inheritance patterns of eye color, linking specific traits to the X chromosome and providing evidence for the chromosomal theory of inheritance.

• Example: Morgan's cross between red-eyed and white-eyed flies showed that the white-eye trait was sex-linked and associated with the X chromosome.

Concept 15.2: Sex-Linked Genes and Patterns of Inheritance

Sex-linked genes are located on sex chromosomes and exhibit unique inheritance patterns. In humans, the X and Y chromosomes determine sex, and the SRY gene on the Y chromosome triggers male development.

• Genetic Determination of Sex: Females have two X chromosomes (XX), and males have one X and one Y chromosome (XY). The SRY gene (Sex-determining Region Y) on the Y chromosome initiates male development.

• Linked Genes vs. Sex-Linked Genes:

  • Linked genes are located close together on the same chromosome and tend to be inherited together.

  • Sex-linked genes are specifically located on sex chromosomes (usually the X chromosome in humans).

• Sex-Linked Diseases: More common in males because males have only one X chromosome. A single recessive allele on the X chromosome will cause the disease in males, while females require two copies.

• Punnett Square Example: For X-linked color blindness:

  • Let XN = normal vision, Xn = color blindness.

  • Cross: XNXn (carrier female) × XNY (normal male)

  • Possible offspring: XNXN (normal female), XNXn (carrier female), XNY (normal male), XnY (color-blind male)

• X Inactivation: In female mammals, one X chromosome in each cell is randomly inactivated during early development, forming a Barr body. This leads to mosaic expression of X-linked genes, as seen in tortoiseshell cats, where different patches of fur express different alleles.

• Example: Tortoiseshell coloration in cats results from X inactivation, with different fur color genes active in different cells.

Concept 15.3: Linked Genes and Genetic Recombination

Genes located near each other on the same chromosome are called linked genes and tend to be inherited together. However, crossing over during meiosis can separate linked genes, producing recombinant phenotypes.

• Linked Genes: Do not assort independently because they are physically connected on the same chromosome.

• Parental vs. Recombinant Phenotypes:

  • Parental phenotypes match the original parental combinations of traits.

  • Recombinant phenotypes show new combinations of traits due to crossing over.

• Crossing Over: During prophase I of meiosis, homologous chromosomes exchange segments, which can separate linked genes and increase genetic diversity.

• Example: If two genes are close together, most offspring will have parental phenotypes, but some will be recombinants due to crossing over.

Concept 15.4: Chromosomal Alterations and Genetic Disorders

Changes in chromosome number or structure can lead to genetic disorders. These alterations can occur during meiosis due to errors in chromosome segregation or recombination.

• Errors in Meiosis: Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly, leading to gametes with abnormal chromosome numbers.

• Consequences: Fertilization involving abnormal gametes can result in offspring with genetic disorders.

• Definitions:

  • Trisomy: Presence of an extra chromosome (2n + 1).

  • Triploidy: Three complete sets of chromosomes (3n).

  • Polyploidy: More than two complete sets of chromosomes.

• Structural Alterations:

  • Deletion: Loss of a chromosome segment.

  • Duplication: Repetition of a chromosome segment.

  • Inversion: Reversal of a chromosome segment within the chromosome.

  • Translocation: Movement of a chromosome segment to a nonhomologous chromosome.

Table: Chromosomal Alterations and Associated Human Disorders

• Example: Nondisjunction during meiosis can produce gametes with n+1 or n-1 chromosomes, leading to disorders such as Down syndrome (trisomy 21).

Additional info: Polyploidy is more common in plants and can result in new species, while most aneuploidies in humans are lethal except for certain cases like trisomy 21 or sex chromosome aneuploidies.