Genetics and Post-Term Pregnancies

Post-Term Pregnancies and Associated Risks

  • A scheduled due date is considered close to term, but approximately 7% of pregnancies go beyond that date.
  • Post-term pregnancies become riskier for both the mother and the fetus.

Risks to the Mother

  • Prolonged stress on the mother's body.
  • Increased physical strain and discomfort in the last trimester due to limited space.

Risks to the Fetus

  • Meconium Emission:
    • Late in pregnancy, the fetus may emit meconium (essentially fetal poop, composed of ingested amniotic fluid).
    • The fetus practices lip movements and swallowing, ingesting amniotic fluid, which is normally not harmful.
  • Meconium Aspiration:
    • After meconium is released, the fetus may swallow it, and aspiration (inhalation into the lungs) can occur.
    • After birth, the respiratory system rapidly develops, but the presence of meconium in the lungs can cause toxicity.
  • Cerebral Palsy:
    • Meconium aspiration is one of several causes of cerebral palsy, a condition similar to a stroke, causing damage to the central nervous system.
    • This damage occurs prenatally (before birth) or perinatally (around the time of birth).
    • Unlike strokes in adults, cerebral palsy often affects both legs due to incomplete development of pyramidal tract decussation in the medulla.

Management of Post-Term Pregnancies

  • Induction of Labor:
    • Healthcare providers may induce labor using artificial or synthetic oxytocin (e.g., Pitocin) to stimulate contractions.
    • However, induction is not always successful, and a Cesarean section (C-section) may still be necessary.
  • Postpartum Recovery:
    • After delivery, the mother's body requires about six weeks to heal.
    • Fluid and swelling can cause the abdomen to remain enlarged for some time after delivery, even after a C-section.
    • The uterus reduces in size but remains slightly larger than pre-pregnancy.
    • The cervix returns to a firmer state.
    • Significant discharge is normal after delivery, regardless of whether it was a vaginal delivery or C-section.

Genetics Overview

  • Genetics involves the passing of hereditary traits.
  • Visible physical traits (e.g., skin tone, eye color) account for only about 4% of our genetics.
  • Much more of our genetic makeup is not visible.

Basic Genetic Concepts

  • Haploid Cells: Sperm and oocytes (egg cells) are haploid cells.
  • Zygote: Sperm and oocyte combine to form a zygote.
  • Cleavage: The zygote undergoes cleavage, a series of cell divisions.
  • Genetic Counseling:
    • Genetic counseling is available for individuals or couples planning to have children.
    • It involves blood tests and DNA analysis to determine the risk of passing on genetic conditions.

Chromosomes and Genes

  • Somatic Cells:
    • Any cell except for sex cells (oogonia, spermatogonia) contains 23 pairs of chromosomes (46 total).
    • These cells are diploid (2n).
  • Chromosome Structure:
    • During most of the cell's life cycle, genetic material (chromatin) is not condensed into organized chromosomes.
    • Chromosomes become visible during replication and mitosis.
  • Homologous Chromosomes:
    • Pairs of chromosomes (one from the mother, one from the father) that belong to the same chromosome number (e.g., both are chromosome 1).
    • Chromosomes from different numbers are not homologous (e.g maternal chromosome 1 and paternal chromosome 17).
  • Genes:
    • Sections of DNA (series of nucleotides) that code for specific traits.
    • Each chromosome contains multiple genes.
  • Alleles:
    • Alternate versions of a gene at the same location on a chromosome.
    • For example, a gene for eye color can have alleles for brown or blue eyes.
  • Mutations:
    • Permanent changes in a gene or allele, caused by random events, exposure to chemicals, toxins, or radiation.

Genetic Inheritance

  • Phenylketonuria (PKU):
    • A condition where the liver lacks the enzyme phenylalanine hydroxylase, which is needed to process phenylalanine (found in certain foods and artificial sweeteners like aspartame).
    • Without this enzyme, phenylalanine becomes toxic.
  • Allele Notation:
    • P: represents a functional gene that produces phenylalanine hydroxylase. This is the dominant allele.
    • p: represents a non-functional gene that does not produce the enzyme. This is the recessive allele.
  • Punnett Square:
    • Used to determine the probability of offspring inheriting specific allele combinations (genotypes).
  • Genotype:
    • The genetic makeup of an individual, including allele combinations (e.g., PP, Pp, pp).
  • Dominant Allele:
    • An allele that expresses its trait even when paired with a recessive allele.
    • In this case, at least one P allele results in normal enzyme function.
  • Recessive Allele:
    • An allele whose trait is only expressed when paired with another recessive allele.
    • Two p alleles are required to cause phenylketonuria.
  • Homozygous:
    • Having two identical alleles for a gene (e.g., PP or pp).
  • Heterozygous:
    • Having two different alleles for a gene (e.g., Pp).
  • Phenotype:
    • The physical expression of a trait (what we observe in the real world).
    • Individuals with PP or Pp genotypes have a normal functioning enzyme and do not have PKU, while individuals with pp genotype have PKU.

More Complex Inheritance Patterns

  • Incomplete Dominance:
    • A situation where the heterozygous genotype results in a phenotype that is intermediate between the two homozygous phenotypes.
  • Sickle Cell Anemia:
    • An example of incomplete dominance.
    • AA: Homozygous dominant individuals have normal red blood cells.
    • AS: Heterozygous individuals have some level of anemia, but are generally functional. S represents the sickle cell allele and A represents the normal allele.
    • SS: Homozygous recessive individuals have sickle cell disease or anemia.
  • Multiple Alleles:
    • When there are more than two possible alleles for a gene.
    • For example, blood type.

Blood Type Inheritance

  • Alleles:
    • I^A: Codes for type A blood.
    • I^B: Codes for type B blood.
    • i: Codes for type O blood (lack of markers).
  • Genotypes and Phenotypes:
    • I^AI^A or I^Ai: Type A blood phenotype.
    • I^BI^B or I^Bi: Type B blood phenotype.
    • I^AI^B: Type AB blood phenotype.
    • ii: Type O blood phenotype.
      *Example: If both parents are type A (I^Ai), their child could be O (ii), meaning both parents must carry the recessive i allele. This is because that is the only combination of blood type alleles that will produce type O blood.

Sex Chromosomes and Sex-Linked Inheritance

  • Sex Chromosomes:
    • Humans have 22 pairs of autosomes and one pair of sex chromosomes (XX or XY).
    • XX typically results in female development, while XY results in male development.
    • The sperm determines the sex of the offspring because it can carry either an X or a Y chromosome, while the oocyte always carries an X chromosome.
  • Sex-Linked Inheritance:
    • Genes located on the sex chromosomes (X or Y) exhibit sex-linked inheritance.
  • Red-Green Color Blindness:
    • A common example of a sex-linked trait carried on the X chromosome.
    • A dominant allele (C) on the X chromosome results in normal color vision, while a recessive allele (c) results in red-green color blindness.
  • Inheritance Patterns:
    • Females (XX): A female with one C allele has normal color vision, even if she also has one c allele (carrier).
    • Males (XY): A male with a c allele on his X chromosome will be color blind because he does not have another X chromosome with a dominant C allele to mask it.
      *Example: If the mother, has a genotype of (X^C X^c) and the father has a genotype of (X^cY); the son has a 50% chance of being colorblind.
      *Sex linked inheritance patterns often skip a generation.