Chapter 15: The Chromosomal Basis of Inheritance – Study Notes

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.

• 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.

• Meiosis Phases:

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

  • Metaphase I: Homologous pairs align randomly.

  • Anaphase I: Homologs separate, segregating alleles.

• 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 confirming 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, especially in humans.

• Genetic Determination of Sex in Humans: Sex is determined by the presence of sex chromosomes: XX (female) and XY (male). The SRY gene (Sex-determining Region Y) on the Y chromosome triggers 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. If a male inherits a recessive allele on the X chromosome, he will express the trait.

• 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.

Concept 15.3: Linked Genes and Genetic Recombination

Genes located near each other on the same chromosome tend to be inherited together, but crossing over can separate them.

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

• Parental vs. Recombinant Phenotypes:

  • Parental phenotypes match the original parental combinations of traits.

  • Recombinant phenotypes show new combinations due to crossing over.

• Crossing Over: During prophase I of meiosis, homologous chromosomes exchange segments, producing recombinant chromosomes and increasing genetic diversity.

Concept 15.4: Chromosomal Alterations and Genetic Disorders

Changes in chromosome number or structure can lead to genetic disorders with significant consequences for offspring.

• Errors in Meiosis: Nondisjunction can result in gametes with abnormal chromosome numbers, leading to disorders in offspring.

• 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 segment from one chromosome to another, nonhomologous chromosome.

Table: Chromosomal Alterations and Associated Human Disorders

Disorder	Type of Chromosomal Alteration	Description
Down syndrome	Trisomy 21	Extra chromosome 21; causes intellectual disability and characteristic facial features
Klinefelter syndrome	XXY (aneuploidy)	Male with extra X chromosome; tall stature, reduced fertility
Extra Y	XYY (aneuploidy)	Male with extra Y chromosome; often taller than average
Trisomy X syndrome	XXX (aneuploidy)	Female with extra X chromosome; usually healthy, may be taller
Turner syndrome	Monosomy X (XO)	Female with only one X chromosome; short stature, infertility
cri du chat syndrome	Deletion	Deletion on chromosome 5; intellectual disability, cat-like cry
Chronic myelogenous leukemia	Translocation	Reciprocal translocation between chromosomes 9 and 22 (Philadelphia chromosome)

Additional info: Polyploidy is more common in plants than in animals and can lead to the formation of new species. Nondisjunction can occur during either meiosis I or II, affecting the resulting gametes differently.