• Introduction to Dolly the Sheep Cloning Mistake

    • Correction: The DNA used for cloning Dolly was from a sheep, not a goat.
    • Interbreeding between Species
    • Goats and sheep can interbreed despite chromosomal differences, producing chimeras (hybrid organisms).
    • Ethical concerns: experiments from the 80s combining sheep and goat embryos would unlikely pass ethical review today.

  • Aneuploidy and Polyploidy

    • Objectives: Understanding aneuploidy, polyploidy, effects, and causes (nondisjunction during meiosis).
    • Key terms to learn, including X inactivation.

  • Chromosomal Basics in Humans

    • Humans: 2 Xs = female, XY = male.
    • Variations: 3 Xs (XXX), 2 Xs and 1 Y (XXY), and 1 X (Turner syndrome).
    • At least one X chromosome is required for viability; absence leads to non-viable embryos.

  • Importance of the X Chromosome

    • Contains over 1,000 genes unrelated to sex determination; essential for cell viability and organ development.
    • Why single or extra X chromosomes can be viable due to X inactivation.

  • X Inactivation

    • Only one X chromosome is active in each cell.
    • Results vary in individuals with multiple X chromosomes: some genes escape repression (especially near telomeres).
    • X inactivation does not occur in gametes or germ cells, only early embryonic stage.

  • Calico Cats

    • Color patterns (calico) result from X inactivation.
    • Female cats often express colors based on which X allele is inactivated in embryonic development.
    • Males cannot be calico because they have one X and one Y chromosome.

  • Red-Green Color Blindness

    • X-linked recessive trait; more prevalent in males (8%) than females (0.5%).
    • Heterozygous females (XX): Some cells express the dominant allele preventing color blindness.

  • Sex Chromosome Differences

    • In flies: 1 X is lethal; they lack X inactivation (compensation via doubling gene expression).
    • In humans: More tolerance for variations in sex chromosomes (e.g., XXY, XXX), although health issues arise.

  • Nondisjunction in Meiosis

    • Nondisjunction during meiosis can lead to aneuploidy, resulting in conditions like Turner syndrome and Klinefelter syndrome.
    • Inadequate separation of chromosomes in meiosis I or II can yield gametes with too many or too few chromosomes.
    • Klinefelter syndrome: an XXY male resulting from nondisjunction; can occur during both male and female meiosis.

  • Mosaic Turner Syndrome

    • Cells in one individual may have differing numbers of sex chromosomes; some may have normal (XX), and some may have only one (X).
    • Nondisjunction at early development stages results in varying cellular compositions.

  • Aneuploidy from Mitosis

    • Can also result from errors in mitosis after zygote formation, leading to mixed cell types in a single organism (e.g., some diploid, some monosomic).
    • The chance of having viable offspring or certain health issues depends on the proportion of affected cells.

  • Conclusion

    • Understanding these chromosomal alterations, their effects on health, and inheritance patterns is crucial in genetics studies.
    • Further discussion will continue regarding non-human organisms in the next class session.