genetics

• Gregor Mendel, an Austrian monk and plant breeder, published findings on inheritance in garden pea plants in 1866
• Inheritance in pea plants involves true-breeding, self-fertilization, and cross-pollination
• The P generation refers to the parent generation, the F1 generation is the offspring of the P cross, and the F2 generation is the offspring from the F1 cross
• Mendel studied 7 different traits in pea plants, observing a 3:1 ratio in the F2 generation
• Alleles are alternative forms of a single gene passed from generation to generation
• Dominant alleles are represented by capital letters (e.g., Y), while recessive alleles are represented by lowercase letters (e.g., y)
• Genotype refers to the gene combination for a trait, while phenotype is the observable trait based on the genotype
• Genotypes for a diploid organism with two alleles in a gene pair can be homozygous dominant (DD), heterozygous (Dd), or homozygous recessive (dd)
• Phenotypes for Mendelian traits can be dominant or recessive, depending on the alleles present
• Mendel's Law of Segregation states that alleles for each trait separate during meiosis
• A Punnett square is used to solve genetics problems and predict offspring genotypes and phenotypes
• Hybrids are heterozygous organisms resulting from a cross involving hybrids for a single trait
• Mendel's Law of Dominance states that in a cross of pure parents for contrasting traits, only one form of the trait will appear in the next generation
• Mendel's Law of Segregation explains how alleles for a trait separate during gamete formation and recombine at fertilization
• Mendel's Law of Independent Assortment states that alleles for different traits are distributed independently to sex cells and offspring
• A dihybrid cross tracks the inheritance of two traits and follows Mendel's Law of Independent Assortment
• In a dihybrid cross, the number of possible gametes produced can be calculated using the formula 2n, where n is the number of heterozygotes
• Recessive genetic disorders are autosomal recessive traits, while dominant genetic disorders are autosomal dominant traits
• Complex patterns of inheritance, like incomplete dominance, do not follow Mendelian inheritance patterns
• Complex inheritance of traits does not follow inheritance patterns described by Mendel
• Incomplete Dominance:

  • Heterozygous phenotype is an intermediate phenotype between the two homozygous phenotypes
  • Example: Red flowered snapdragons (RR) crossed with white-flowered snapdragons (WW) result in pink flowers (RW)
• Codominance:

  • In the heterozygous condition, both alleles are expressed
  • Example: Sickle-cell Disease provides a case study of codominant inheritance
• Multiple Alleles:

  • Some traits are determined by more than two alleles
  • Example: Human blood group (ABO Blood Group) has three forms of alleles: IA (blood type A), IB (blood type B), i (blood type O)
• Coat color of rabbits:

  • Four alleles code for coat color: C, cch, ch, and c
  • Hierarchy of dominance: C > cch > ch > c
  • Multiple alleles demonstrate a hierarchy of dominance
• Epistasis:

  • One allele hides the effects of another allele
  • Example: Labrador’s coat color controlled by two sets of alleles (E and e)
• Sex Determination & Sex-Linked Traits:

  • Humans have 23 pairs of chromosomes, with the 23rd pair determining sex
  • Sex-linked traits are found on the X or Y chromosome
  • Examples: Red-green color blindness, Hemophilia
• Gene Linkage:

  • Genes located close on the same chromosome are linked and usually travel together during gamete formation
  • Exception to Mendel’s law of independent assortment
• Polyploidy:

  • Occurrence of one or more extra sets of all chromosomes in an organism
  • Rare in animals, lethal in humans, common in plants like bread wheat and oats
• Pedigree:

  • Diagram tracing the inheritance of a particular trait through several generations
  • Used to infer genotypes and predict disorders