genetics

• DNA replication requires the enzyme DNA polymerase to add new nucleotides to the growing DNA strand.
• 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
• For a diploid organism with two alleles in a gene pair, genotypes can be homozygous dominant (DD), heterozygous (Dd), or homozygous recessive (dd)
• Phenotypes can be dominant or recessive, with dominant phenotypes not always more common than recessive phenotypes
• Mendel's Law of Segregation states that alleles for each trait separate during meiosis
• A Punnett Square is used to solve genetics problems
• 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 parents pure 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, following Mendel's Law of Independent Assortment
• Dihybrid crosses involve the segregation of alleles for two different traits and can be solved using the formula 2n (n = number of heterozygotes)
• In a dihybrid cross, each pair of alleles segregates independently during gamete formation
• Pedigrees can show the inheritance of a trait over several generations
• Recessive genetic disorders are autosomal recessive traits like cystic fibrosis and albinism
• Dominant genetic disorders are autosomal dominant traits like Huntington's disease and achondroplasia
• Complex patterns of inheritance include incomplete dominance, where the phenotype of a heterozygote is intermediate between the two homozygotes
• Complex inheritance of traits does not follow inheritance patterns described by Mendel
• Incomplete Dominance:

  • When an organism is heterozygous for a trait, its phenotype will be that of the dominant trait
  • Example: Red flowered snapdragons (RR) crossed with white-flowered snapdragons (WW) produce heterozygous offspring with pink flowers (RW)
• In incomplete dominance, the heterozygous phenotype is an intermediate phenotype between the two homozygous phenotypes
• Codominance:

  • When an organism is heterozygous for a trait, both alleles are expressed in the heterozygous condition
  • Example: Sickle-cell Disease, where individuals heterozygous for the trait have both normal and sickle-shaped cells
• Multiple Alleles:

  • Not all traits are determined by two alleles; some are determined by more than two alleles
  • Example: Human blood group, which has three forms of alleles (IA, IB, i) determining blood types A, B, and O
• Epistasis:

  • One allele hides the effects of another allele
  • Example: Labrador coat color controlled by two sets of alleles, where the presence of the e allele masks the effects of the dominant B allele
• Sex Determination & Sex-Linked Traits:

  • Humans have 23 pairs of chromosomes, with the 23rd pair determining sex
  • Sex-linked traits are where the gene or allele for the trait is found on the X or Y chromosome
  • Examples: Red-green color blindness (X-linked) and Hemophilia (X-linked)
• Gene Linkage:

  • Genes located close on the same chromosome are linked and usually travel together during gamete formation
  • Linked genes do not segregate independently, an 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 but common in plants like bread wheat and oats
• Pedigree:

  • A diagram tracing the inheritance of a particular trait through several generations
  • Used for inferring genotypes and predicting disorders in future offspring