ERRORS IN MEIOSIS, MITOSIS

Basic Human Chromosome Numbers

• Humans have $22$ pairs of autosomes (somatic cells; responsible for genes passed down) and 1 pair of sex chromosomes (chromosome 23; gametes).

• Sex chromosome configurations: $XX$ (female), $XY$ (male).

• Somatic cells carry $46$ chromosomes; gametes carry $23$.

Nondisjunction and Aneuploidy

• Nondisjunction: homologous chromosomes fail to separate during Anaphase I; results in gametes with $n+1$ or $n-1$ chromosomes.

• Aneuploidy: abnormal chromosome number; in humans typically $45$ or $47$ chromosomes per cell.

Trisomy Syndromes

• Trisomy = presence of an extra chromosome in some or all cells.

Trisomy 13 (Patau)

• Extra copy of chromosome $13$; Patau syndrome.

• Severe term disorders leading to miscarriage or death after birth in many cases.

• Features may include:

  • anophthalmia: one eye

  • holoprosencephaly,

  • severe heart defects,

  • nasal development problems,

  • cleft lip/palate,

  • microphthalmia: small eyes.

  • hypotelorisms: decrease distance in organs (too far, too close)

• Some less severe mosaic/partial cases may live longer.

Trisomy 18 (Edwards)

• Three copies of chromosome $18$; Edwards syndrome.

• Most babies die before or shortly after birth.

• A minority (about $13\%$) live past the first birthday.

• Features can include short sternum, club feet, heart/kidney defects, neurodevelopmental delays.

Trisomy 21 (Down Syndrome)

• Trisomy of chromosome $21$; Down syndrome.

• Development varies; life expectancy historically around $60$ years (variable by context).

• Individuals have diverse talents and can thrive with support.

• Features:

  • almond eyes

  • small lip

  • delayed development

  • wide and short neck

  • weak feet

Pallister-Killian Syndrome (PKS)

• Rare mosaic disorder due to a duplication/tetrasomy of the short arm of chromosome $12$ (12p).

• Mosaicism: only a subset of cells carry the abnormality.

Turner Syndrome (Monosomy X)(XO)

• Females with only one X chromosome (monosomy X).

• High rate of miscarriage (about $98\%$).

• Short stature, infertility, lack of functional ovaries, webbed neck, possible congenital heart defects (constricted aorta), reduced secondary sexual characteristics.

• Females that look like males, incomplete female gene.

XYY Syndrome

• Males with karyotype $47, XYY$.

• Often taller than average; usually normal development and sexual characteristics.

• Some may have mild learning difficulties.

• Historically over-credited with criminal behavior; this is not supported by current evidence.

• Excessive male genes

Klinefelter Syndrome (XXY)

• Males with karyotype $XXY$.

• Typically tall; sterile or reduced fertility.

• May have feminine-like traits (sparse facial hair, breast development) and require testosterone therapy.

• Other physical features: broader hips, longer limbs, reduced body hair.

• Males with female genes, characteristics.

Monosomy X and Other Monosomies

• Monosomy disorders involve the loss of an entire chromosome or part of one.

• Turner syndrome is the primary human example in females.

Errors in Mitosis

• Mitosis enables repair, growth, and replacement of cells.

• Dangers include cancer: uncontrolled cell division due to mutations in DNA.

• Tumors may be benign or malignant.

Tumors

• Benign tumor: abnormal cells remain at the original site; usually removable by surgery.

• Malignant tumor: cancer cells invade surrounding tissue and can metastasize.

• Metastasis: cancer cells spread via lymph or blood vessels to distant sites.

• Cancer progression often described as:
  1) A tumor grows from a single cancer cell.
  2) Cancer cells invade neighboring tissue.
  3) Cancer cells spread through lymph and blood vessels to other parts of the body.