Chapter 15: The Chromosomal Basis of Inheritance

Concept 15.1: Mendelian Inheritance and Chromosome Behavior

This section explores how Mendel’s principles of inheritance are explained by the behavior of chromosomes during meiosis, and highlights key experiments that established the chromosomal theory of inheritance.

• Principle of Segregation: During meiosis I, homologous chromosomes separate, ensuring that each gamete receives only one allele of each gene. This explains Mendel’s observation that alleles segregate during gamete formation.

• Principle of Independent Assortment: Nonhomologous chromosomes align independently at the metaphase plate during meiosis I, leading to the random assortment of maternal and paternal chromosomes into gametes. This underlies Mendel’s law of independent assortment.

• Key Experiment – T.H. Morgan and Drosophila melanogaster: Morgan used fruit flies to demonstrate that genes are located on chromosomes. He observed that certain traits (such as eye color) were inherited together, providing evidence that genes reside on specific chromosomes.

• Example: Morgan’s discovery of white-eyed male flies led to the conclusion that the gene for eye color was located on 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. This section discusses genetic determination of sex, the significance of the SRY gene, and the inheritance of sex-linked diseases.

• Genetic Determination of Sex in Humans: Humans have two sex chromosomes: X and Y. Females are XX; males are XY. The presence of the SRY gene (Sex-determining Region Y) on the Y chromosome triggers male development.

• Linked Genes vs. Sex-Linked Genes: Linked genes are genes 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: Diseases such as color blindness, Duchenne muscular dystrophy, and hemophilia are more common in males because males have only one X chromosome. A single recessive allele on the X chromosome will result in the disease in males, while females require two copies.

• Punnett Square Example: For X-linked recessive diseases, a carrier female (XAXa) and a normal male (XAY) can produce affected sons (XaY) and carrier daughters (XAXa).

• X Inactivation: In female mammals, one of the two X chromosomes 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 patches of fur color correspond to cells with different active X chromosomes.

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 during meiosis can produce new combinations of alleles.

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

• Parental vs. Recombinant Phenotypes: Parental phenotypes are offspring with combinations of traits matching the parental generation. Recombinant phenotypes have new combinations due to crossing over.

• Crossing Over: During prophase I of meiosis, homologous chromosomes exchange segments, which can separate linked genes and produce recombinant offspring.

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

Concept 15.4: Chromosomal Alterations and Genetic Disorders

Changes in chromosome number or structure can lead to genetic disorders. This section covers the types of chromosomal alterations and their consequences.

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

• Trisomy, Triploidy, and Polyploidy:

  • Trisomy: Presence of an extra chromosome (2n+1), e.g., Down syndrome (trisomy 21).

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

  • Polyploidy: More than two complete sets of chromosomes (common in plants).

• 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

Additional info: Polyploidy is rare in animals but common in plants, where it can lead to new species formation. Nondisjunction can occur during either meiosis I or II, affecting the resulting gametes differently.