Human Genetics (three)

Genetic material determines phenotypes (pigmentation, behavior, physiological traits).
Phenotypes are influenced by both genetic and environmental factors, with interactions between them.
Variation exists due to DNA replication errors, contributing to phenotypic diversity (observable traits, disease susceptibility).

Mitosis is crucial for homeostasis, cell replenishment, growth, and development.
Mitosis involves a diploid cell replicating into identical diploid cells, unless mutations occur.
Checkpoints within mitosis ensure proper cell division; preventing uncontrolled replication (e.g., cancer).
DNA polymerase can generate mutations somatically (affecting individual cells) or germline (affecting future generations).

Meiosis is essential for sexual reproduction, enabling the production of sex cells.
Variation during meiosis is transferred to the next generation.
Unlike mitosis (diploid to diploid), meiosis goes from a diploid cell to haploid cells.
Errors in meiosis can result in extra or missing chromosomes, leading to phenotypic consequences.

Meiosis involves separating maternal and paternal chromosome pairs.
Reductional division (Meiosis I): Diploid cell becomes haploid, homologous chromosomes separate.
Equational division (Meiosis II): Sister chromatids separate, similar to mitosis; maintains haploid state.
Random assortment during meiosis leads to different combinations of maternal and paternal chromosomes in daughter cells.
Recombination (crossing over) generates additional diversity by mixing chromosomes.

Replicated chromosomes become visible, homologous chromosomes line up, and crossing over occurs in prophase I.
Homologous chromosomes separate in metaphase I, reducing the cell from diploid to haploid.
Random assortment of chromosomes occurs.

Spermatogonium (diploid, undifferentiated) progresses to primary spermatocyte (diploid, meiosis I).
Primary spermatocyte (diploid, prophase I) undergoes meiosis I to form secondary spermatocytes (haploid).
Secondary spermatocytes undergo meiosis II to form spermatids, which mature into sperm cells (haploid) released through the lumen.

Oogonium progresses to primary oocyte (diploid).
Primary oocyte undergoes meiosis I to form a secondary oocyte (haploid) and a polar body.
Meiosis II occurs upon fertilization, producing a second polar body and a mature egg (haploid).
The egg contains mitochondrial DNA and other materials necessary for early development.

Genetic imprinting is a phenomenon where only one copy of a gene (either maternal or paternal) is expressed, the other is silenced.
Occurs early in development and can involve epigenetic mechanisms like DNA methylation.
Disruption of imprinting can lead to significant issues.
Example: $IGF-2$ (insulin growth factor 2) gene, where the paternal copy is expressed and the maternal copy is silenced via methylation. Aberrant expression of both maternal and paternal $IGF-2$ leads to dysregulation of growth.

Non-disjunction occurs when chromosomes fail to separate properly during meiosis.

Meiosis I: Homologous chromosomes do not separate, resulting in both copies ending up in a single cell.
Meiosis II: Sister chromatids do not separate, leading to an unequal distribution of chromosomes.
Consequences: Can result in gametes with an extra chromosome (trisomy) or a missing chromosome (monosomy) after fertilization.
Karyotyping can detect major chromosomal shifts caused by non-disjunction, aiding in prenatal diagnostics.
The risk of non-disjunction increases with parental age.

Autosomes: Aneuploidy (abnormal number) is often lethal during embryonic development.
Sex Chromosomes: Variations in the number of sex chromosomes can result in viable offspring with specific syndromes (e.g., Turner syndrome $45, X$ , Klinefelter syndrome $XXY$ ).

Turner Syndrome ( $45, X$ ): Single X chromosome due to non-disjunction in either meiosis I or II in either parent.
Klinefelter Syndrome ( $XXY$ ): Extra X chromosome due to non-disjunction in meiosis I or II in the female or meiosis I in the male.