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.