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).
- 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.