Chromosomal Inheritance

Foundations of Chromosomal Inheritance

  • The Chromosome Theory

    • Genes are physically located on specific loci on chromosomes.

    • The behavior of chromosomes during meiosis (segregation and independent assortment) explains Mendel's laws of inheritance.

    • Thomas Hunt Morgan provided the first direct evidence by linking a specific gene (eye color in fruit flies) to the X chromosome.

Morgan’s Experimental Evidence

  • Drosophila melanogaster as a Model Organism

    • Fruit flies are ideal due to their short generation time, high offspring count, and simple genome containing only 4 pairs of chromosomes.

    • The discovery of a white-eyed mutant male revealed sex-linked inheritance patterns.

  • Sex-Linkage Discovery

    • Morgan's experiments showed the white-eye trait was passed to all daughters but no sons in the first cross, establishing the chromosomal basis of inheritance.

Chromosomal Basis of Sex

  • Sex Determination in Mammals

    • Sex is determined by sex chromosomes: XX typically develops as female, and XY typically develops as male.

    • The X chromosome is significantly larger and contains more genes than the Y chromosome.

    • The SRY (sex-determining region Y) gene on the Y chromosome triggers testis development in embryos.

    • Short homologous (pseudoautosomal) regions allow the X and Y chromosomes to pair during meiosis.

  • Sex-Linked Genes

    • Humans possess approximately 78 Y-linked genes and about 1,100 X-linked genes.

    • X-Linked Recessive Traits: Traits like color blindness, Duchenne muscular dystrophy, and hemophilia are expressed in males who are hemizygous (having only one X chromosome).

    • Females require two mutant alleles to express these traits, making these disorders far more common in males.

  • X Inactivation and Barr Bodies

    • In female mammals, one X chromosome is randomly inactivated in each somatic cell early in development to balance gene dosage.

    • The inactivated X condenses into a Barr body.

    • This leads to mosaic expression of X-linked genes, such as the fur pattern in calico cats.

    • Inactivation is initiated by the expression of the XIST gene.

Linkage and Recombination

  • Linked Genes

    • Genes located on the same chromosome tend to be inherited together and violate the Law of Independent Assortment.

    • Morgan’s dihybrid crosses showed a higher frequency of parental phenotypes than predicted, indicating linkage.

  • Genetic Recombination

    • Parental types: Offspring phenotypes matching the original parents.

    • Recombinants: Offspring with new combinations of traits.

    • Unlinked genes (on different chromosomes) show a 50\% recombination frequency.

    • Linked genes can still recombine through crossing over during prophase I of meiosis.

  • Linkage Mapping

    • Alfred Sturtevant used recombination frequencies to map gene order and relative distance on chromosomes.

    • One map unit (centimorgan, cM) is defined as a 1\% recombination frequency.

    • Genes far apart on the same chromosome can have recombination frequencies approaching 50\%, making them genetically unlinked despite being physically linked.

Large-Scale Chromosomal Alterations

  • Abnormal Chromosome Number

    • Nondisjunction: The failure of chromosomes to separate properly during meiosis, resulting in gametes with abnormal chromosome counts.

    • Aneuploidy: A condition of having an abnormal number of a specific chromosome.

      • Monosomy: One copy of a chromosome (e.g., Turner syndrome, XO).

      • Trisomy: Three copies of a chromosome (e.g., Down syndrome, trisomy 21).

    • Polyploidy: Organisms with more than two complete sets of chromosomes (3n, 4n); common in plants but rare in animals.

  • Structural Changes

    • Deletion: Loss of a chromosomal segment.

    • Duplication: Repetition of a segment.

    • Inversion: Reversal of a segment's orientation.

    • Translocation: Movement of a segment to a non-homologous chromosome.

  • Specific Human Disorders

    • Down syndrome: Caused by trisomy 21; incidence increases with maternal age.

    • Sex chromosome aneuploidies: Includes Klinefelter syndrome (XXY), XYY males, triple X syndrome (XXX), and Turner syndrome (XO).

    • Cri du chat syndrome: Results from a deletion on chromosome 5.

    • Chronic myelogenous leukemia (CML): Caused by a specific translocation known as the Philadelphia chromosome.

Epigenetics and Extranuclear Inheritance

  • Genomic Imprinting

    • An epigenetic phenomenon where the expression of an allele depends on which parent it was inherited from.

    • Typically involves DNA methylation to silence one of the parental alleles.

    • Example: The mouse Igf2 gene is expressed only from the paternal allele.

  • Inheritance of Organelle Genes

    • Genes in mitochondria and chloroplasts are located outside the nucleus and are inherited maternally via the egg's cytoplasm.

    • Mutations in mitochondrial DNA can cause energy-related disorders, such as mitochondrial myopathy.

    • Mitochondrial replacement techniques are used to prevent the transmission of these inherited diseases.