Molecular Genetics: Mendelian Genetics and Inheritance

Molecular Genetics

• Definition of Genetics: The study of inherited variation.
• Definition of Gene: A sequence of DNA that instructs a cell to produce a particular protein.
• Definition of Allele: An alternate form of a gene that occurs at a given locus on a chromosome.
• Definition of Homozygous: Having two identical alleles of a gene (e.g. TT or tt).
• Definition of Heterozygous: Having two different alleles of a gene (e.g. Tt).
• Definition of Genotype: The alleles combination in an individual that cause particular traits or disorders.
• Definition of Phenotype: The expression of a gene in traits or symptoms.
• Definition of Dominant: A gene variant expressed when present in even one copy (e.g. TT, Tt).
• Definition of Recessive: An allele whose expression is masked by another allele (e.g. tt).
• Definition of Diploid: A cell containing two sets of chromosomes.
• Definition of Haploid: A cell containing one set of chromosomes.
• Definition of Gamete: A sex cell (sperm or ovum).

Mendelian Genetics

• Gregor Mendel is considered the Father of Genetics (1822-1884).
• Mendel's Experiments:
  • First Experiment: Crossed tall and dwarf plants, resulting in F1 offspring that were all tall (Tt).
  • Second Experiment: Crossed plants with different traits, resulting in an F2 generation with a 9:3:3:1 ratio.
• Laws of Inheritance:
  • Law of Segregation: The distribution of alleles of a gene into separate gametes during meiosis.
  • Law of Independent Assortment: The random arrangement of homologous chromosome pairs during metaphase 1 of meiosis.

Crossing Over

• Definition: An event during prophase 1 of meiosis when homologous chromosomes exchange parts, mixing up maternal and paternal gene combinations.
• Features of Crossing Over:
  • Probability of crossing over increases with increasing distance between two loci on the chromosome.
  • Involves breakage of each of two homologous chromosomes by endonuclease enzymes, followed by exchange of parts and reunion by ligase enzymes.
  • Occurs during prophase 1 of meiosis in the post-replication tetrad stage.

Sex Determination and X-Linked Inheritance

• Definition of Sex Chromosome: A chromosome containing genes that specify sex (X and Y chromosomes).
• Definition of Autosome: A chromosome that does not have a gene that determines sex.
• XX-XY Mechanism of Sex Determination:
  • XX zygote develops into a female, and XY zygote develops into a male.
• X-Linked Inheritance:
  • Refers to a trait determined by a gene located on the X chromosome.
  • X-Linked diseases are inherited through one of the sex chromosomes (X or Y chromosomes).
  • Examples of X-Linked recessive diseases include Hemophilia and Colorblindness.

X-Linked Inheritance Patterns

• When the mother is a carrier of an X-Linked recessive mutation:
  • Sons have a 50% chance of being affected, and daughters have a 50% chance of being carriers.
• When the father is affected by an X-Linked recessive mutation:
  • None of his sons will be affected, and all of his daughters will be carriers.
• X-Linked dominant inheritance:
  • Examples include incontinentia pigmenti.
  • Much more severe effects in males.

Sex-Linked Recessive Traits in Humans

• Colorblindness:
  • Inherited and more common in men (1 in 20), affecting fewer women (1 in 200).
  • The gene for red-green colorblindness lies on the X-chromosome.
  • Follows a pattern of X-Linked recessive inheritance.
  • Diagnosis: Uses a test with colored spots to diagnose red-green color deficiencies.

This quiz covers the basics of Mendelian genetics, including transmission genetics, crossing over in meiosis, and X-linked inheritance. Test your knowledge of how traits are passed down from generation to generation.