Chromosomes

I. Chromosomal Theory of Inheritance

  • A. Morgan’s Experiments

    • Scientists in 1902 began to develop the “Chromosome Theory of Inheritance.”
    • Key aspects of the Chromosomal Theory:
    1. Genes have specific loci on chromosomes.
    2. Chromosomes undergo segregation and independent assortment.

  • B. Thomas Morgan and D. Melanogaster

    • Early 20th century experimental embryologist.
    • Investigated organisms such as fish, flies, and other animals.
    • Transitioned from cytologist to focusing on chromosomes behaving like Mendel's “heritable factors.”
    • Initially skeptical of Mendel's work and Chromosome Theory.
    • Significance:
    • Provided the first experimental support for the concept of genes residing on chromosomes.
    • The Chromosomal Theory conformed with Mendel's notions of “heritable factors.”

  • Drosophila melanogaster (Fruit Fly) as a Model Organism

    • Advantages of using Drosophila:
    • High fecundity: Produces numerous offspring quickly.
    • Short generation time (approximately 2 weeks).
    • Contains 4 pairs of chromosomes (3 pairs of autosomes, 1 pair of sex chromosomes).

  • Key Genetic Terminology

    • Wild Type:
    • The phenotype most commonly found in the population.
    • Example: Red eyes, represented as $w^{+}$.
    • Mutant Phenotype:
    • The alternative phenotypes to the wild type.
    • Example: White eyes, represented as $w$.
    • In both fruit flies and humans, there are 2 versions of the sex chromosomes:
    • X and Y chromosomes:
      • Females: XX
      • Males: XY
    • Notably, the X and Y chromosomes are not homologous; they possess different genes.
    • Genes located on sex chromosomes are termed sex-linked genes.

II. Inheritance Patterns on Sex Chromosomes

  • A. Understanding the Crosses

    • P Generation: Red-eyed female cross with a white-eyed male.
    • F1 Generation: All offspring exhibit red eyes, displaying dominance of red over white.
    • F2 Generation: Conducting a cross of a red-eyed F1 female with a red-eyed F1 male yields a 3:1 phenotype ratio in eye color.

  • B. Sex Determination

    • The Y chromosome's presence does not solely determine male status.
    • The Y chromosome's role is predominantly in sperm production.
    • The ratio of X chromosomes to sets of autosomes (A) directly determines the sex of the offspring:
    • XX:AA = 1:1 → Female
    • XY:AA = 1:2 → Male
    • XO:AA = 1:2 → Sterile Male
    • XXX:AAA = 1:1 → Female
    • XXY:AA = 1:1 → Female

  • C. Explanation for Absence of White-eyed F2 Females

    • The eye color gene resides on the X chromosome; thus, there is no corresponding locus on the Y chromosome.
    • This explains why F2 females cannot express the white-eyed phenotype.

III. Eye-color Monohybrid Cross

  • Conducting the Cross:

    • P Generation: $X^{w+}X^{w+}$ (female) x $X^{w}Y$ (male).
    • F1 Generation: Results in $X^{w+}X^{w}$ females (Red-eyed) and $X^{w}Y$ males (White-eyed).
    • F1 Cross: $X^{w+}X^{w} x X^{w}Y$.

  • D. Morgan’s Findings

    • Identified that specific genes are located on designated chromosomes.
    • Discovered unique inheritance patterns for genes situated on sex chromosomes.
    • Provided strong support for the Chromosomal Theory of Inheritance.

IV. Inheritance Patterns of Sex Chromosomes

  • A. Characteristics of Sex Chromosomes

    • One pair of sex chromosomes present.
    • Act homologously during meiosis.
    • Contain sex-determining genes and other unrelated genes.
    • Sex-linked Genes:
    • Genes located on either the X or Y chromosomes.

  • B. Human Sex Determination

    • Males: Typically XY (heterogametic), resulting in half of the sperm carrying an X and half a Y chromosome.
    • Females: Typically XX (homogametic), such that all eggs carry an X chromosome.
    • Development of female phenotype arises due to absence of Y chromosome.

  • C. Variation in Sex Determination Mechanisms:

    • Various systems exist in different organisms:
    • The X-Y system in humans
    • The X-O system in grasshoppers
    • The Z-W system in chickens
    • The haplo-diploid system in bees

V. X-linked Genes

  • A. Understanding X-linked Traits

    • Controlled exclusively by genes found on the X chromosome.
    • Exhibit unique inheritance patterns.

  • B. Examples of X-linked Traits

    • Red-Green Color Blindness:
    • A recessive trait.
    • Female can be:
      • Homozygous dominant: $XX$ → typical vision
      • Heterozygous: $XX^{*}$ → typical vision but a carrier
      • Homozygous recessive: $X^{}X^{}$ → exhibits red-green color blindness.
    • Male can be:
      • Hemizygous dominant: $XY$ → typical vision
      • Hemizygous recessive: $X^{*}Y$ → exhibits red-green color blindness.

  • C. Calculation of Probabilities in Genetic Crosses

    • For a cross between a carrier female ($XX^{*}$) and an unaffected male (XY):
    • Gametes produced: $X^{*}$, $X$ for female; $X$, $Y$ for male.
    • Offspring probabilities:
      • Probability (male) = 0.5
      • Probability (colorblind male) = 0.25
      • Probability (female) = 0.5
      • Probability (colorblind female) = 0.

  • D. Example of Hemophilia as an X-linked Recessive Trait

    • Exploration of inheritance:
    • A father with hemophilia has a daughter with hemophilia.
    • Inquiry into how this inheritance pattern may manifest.

VI. X Inactivation

  • A. Expression of X-linked Genes

    • Genes located on the X chromosome are expressed in both males and females.
    • Males possess only 1 copy of X, while females have 2 copies.
    • The expression of X-linked genes does not result in double the protein production in females.

  • B. Barr Bodies

    • Definition:
    • A Barr body is a dense structure of DNA and protein that forms from the inactivation of one X chromosome in females during development.
    • Location:
    • Found along the inside of the nuclear envelope.
    • The process involves modifications, often through methylation, among DNA and histone proteins.

  • C. Inactivation Process

    • X inactivation is random; different cells may inactivate different X chromosomes.
    • This randomness leads to varied expression patterns among cells, resulting in phenomena such as heterozygous female tortoiseshell cats.

  • D. Example of Tortoiseshell Cats

    • Traits for fur color may be X-linked, leading to mosaic expression patterns in heterozygous females.
    • Observable outcomes include a patchy appearance in fur color due to the variable expression of proteins from X-linked genes.
    • A similar pattern can be seen in human eye color, particularly among females, given that the genes controlling eye color may also be prone to Barr body formation.