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Genetics and Inheritance Patterns

Codominance Pattern

  • In codominance, there are no dominant or recessive alleles.
  • All alleles are expressed together to determine the phenotype.
  • The ABO blood typing system follows the codominance pattern.

ABO Blood Typing System

  • Red blood cells have identity flags (antigens) on their surface.
  • A flag represents the A antigen, indicating blood type A.
  • B flag represents the B antigen, indicating blood type B.
  • AB blood type: Both A and B antigens are present on the red blood cells simultaneously; this is codominance in action.
  • If there are no antigens, what is the blood type? The transcript does not specify, but outside knowledge would tell us that it is type O.

Monogenic vs. Polygenic Human Traits

  • Monogenic traits: Determined by a single gene (e.g., hair color, skin color).
  • Polygenic traits: Determined by multiple genes interacting (e.g., metabolism, weight).

Genes and Metabolism

  • Obesity often has a strong family history but is not determined by a single gene.
  • Many genes interact to influence metabolism and weight.
  • The complexity of these interactions makes it difficult to find long-lasting solutions for weight loss.

X-Linked Inheritance Patterns

  • Hemophilia: A blood clotting disorder due to a deficiency in clotting factors (e.g., factors seven and eight in hemophilia A or B).
  • Hemophilia is inherited in an X-linked recessive manner.
  • The X chromosome carries the gene, while the Y chromosome does not.

X-Linked Recessive Traits

  • Males are more susceptible to X-linked recessive disorders because they have only one X chromosome (XY).
  • If a male inherits the mutated X chromosome, there is no other X chromosome to compensate.
  • Family history may show that only males tend to have the disorder.

Mutations

  • Mutations: Changes in the genetic code.
  • The more changes, the more severe the mutation, leading to the patient being more symptomatic.

Epigenetics

  • Epigenetics: The study of how the environment changes gene expression.
  • Environmental factors can influence estrogen production.

Mitosis vs. Meiosis

  • Mitosis goes through PMAT (prophase, metaphase, anaphase, telophase) once.
  • Meiosis goes through PMAT twice because it needs to create a haploid count.
  • Mitosis results in a diploid count.
  • In mitosis, all resulting cells look exactly the same.
  • Cell cycle: G1 phase, S phase (DNA replication), G2 phase, and then mitosis.
  • Mitosis involves two sets of DNA separating into two cells, each with a diploid count.
  • There is no genetic variability in mitosis.
  • There is genetic variability in meiosis due to crossover events.

Meiosis and Genetic Variability

  • In metaphase I of meiosis, crossing over occurs where there is an exchange of genetic material.
  • Crossing over explains why sperm cells and egg cells are not all genetically identical.
  • Meiosis explains why siblings from the same parents have differences.

Law of Independent Assortment

  • The law of independent assortment occurs in meiosis I.
  • During meiosis I, pairs of chromosomes line up at the equator of the cell.
  • How they line up determines which genes end up in each cell.
  • Different arrangements lead to different gene combinations in the resulting cells.

Summary of Meiosis

  • Only meiosis offers genetic variability through crossover events and the law of independent assortment.

Aneuploidy

  • Aneuploidy: Results from nondisjunction in meiosis or mitosis.
  • Nondisjunction: Failure of chromosomes to separate properly, leading to an incorrect number of chromosomes in the product (e.g., not 23).
  • Aneuploid sperm or egg can result in an offspring with an abnormal chromosome count (e.g., one extra chromosome).
  • This explains trisomy diseases.

Trisomy Diseases

  • Trisomy 21 (Down syndrome): The most common trisomy.
  • Individuals with Down syndrome have three copies of chromosome 21.
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