Mendelian Genetics

Mendelian Genetics

• Genetics is the study of heredity and variation in living organisms.
• Chromosomes carry hereditary information in the form of genes.
• A gene is a unit of heredity, a section of DNA that encodes a single protein.
• Alleles are alternative forms of a gene, located at the same position on homologous chromosomes, and control the same trait.
• Homozygous: having identical alleles for a particular trait.
• Heterozygous: having two different alleles for a particular trait.
• Dominant: an allele that masks the expression of a recessive allele; the dominant trait is expressed in the heterozygous condition.
• Recessive: an allele that is masked by a dominant allele.
• Genotype: the genetic makeup of an organism.
• Phenotype: the physical appearance of an organism, resulting from the interaction of the genotype and the environment.
• A monohybrid cross is a genetic cross involving a single pair of genes (one trait), where the parents differ in that trait.
• The P generation is the parental generation in a genetic cross.
• In a monohybrid cross with true-breeding parents (e.g., TT × tt), the F1 generation is heterozygous (e.g., Tt) and typically shows the dominant phenotype.
• A Punnett square is used to predict the possible outcomes of genetic crosses by showing all possible combinations of gametes.
• When using a Punnett square:
  • Determine the genotypes of the parents
  • Write down the cross (mating)
  • Draw the Punnett square
  • Split the letters of the genotype for each parent and place them outside the square (representing the gametes)
  • Determine the possible genotypes of the offspring by filling in the Punnett square
  • Summarize the results (genotypes and phenotypes of offspring)
• In the F2 generation (resulting from self-fertilization of the heterozygous F1 generation), the genotypic ratio is 1:2:1 (homozygous dominant : heterozygous : homozygous recessive), and the phenotypic ratio is 3:1 (dominant : recessive).
• The principle of dominance states that one allele can mask the expression of another allele in the F1 generation.
• The principle of segregation states that during gamete formation, the pairs of hereditary factors (genes) separate.

DNA and RNA

• Genes carry information from one generation to the next.
• Genes use information to produce the traits of an organism.
• There exists a mechanism for copying genes every time a cell divides.
• Genes carry information, put it to work, and are easily copied.
• Chromosomes consist of about half nucleic acid and half protein.
• The model for the structure of DNA proposed by Watson and Crick in 1953 revealed that genes are composed of DNA.
• DNA molecules are made up of nucleotides.
• A nucleotide is composed of a phosphate group, a 5-carbon sugar called deoxyribose, and a nitrogen base.
• The sugar in DNA is deoxyribose (5-carbon sugar).
• The nitrogen bases in DNA are adenine (A) and guanine (G) (purines, double-ringed) and cytosine (C) and thymine (T) (pyrimidines, single-ringed).
• The DNA backbone is formed by alternating sugar and phosphate groups, with the nitrogen bases sticking out sideways.
• Chargaff's rules state that the amount of guanine (G) equals the amount of cytosine (C), and the amount of adenine (A) equals the amount of thymine (T).
• Rosalind Franklin's used X-ray diffraction to study DNA, showing that DNA strands are twisted in a helix, there are two strands, and the nitrogen bases are likely in the center of the molecule.
• Watson and Crick used Franklin's data to build a model of DNA as a double helix, which explained how DNA could carry information and be copied.
• The sides of the DNA ladder consist of alternating sugar and phosphate groups.
• The rungs of the DNA ladder are formed by two nitrogen bases paired across the center.
• The two DNA strands are joined by weak hydrogen bonds between the base pairs.
• Complementary base pairing: adenine (A) always bonds with thymine (T), and guanine (G) always bonds with cytosine (C).
• Adenine pairs with thymine using two hydrogen bonds, while guanine pairs with cytosine using three hydrogen bonds.
• The sequence of nitrogen bases carries the genetic information, and any sequence is possible.
• DNA can be easily copied or replicated because each strand can serve as a template to construct the other half due to base pairing; the strands are complementary.
• Replication is the process where a cell copies or duplicates its DNA.
• During DNA replication, the two strands separate, and each serves as a template for producing a new complementary strand, following base pairing rules.
• The parent molecule has two complementary strands of DNA held together by hydrogen bonds, where adenine pairs with thymine and cytosine pairs with guanine.
• The first step in replication is the separation of the two DNA strands by breaking the hydrogen bonds between the bases; each parental strand then serves as a template.
• New nucleotides are inserted along both template strands, where each nitrogen base pairs with its complement to form a new strand just like the old one, following the base-pairing rules.
• The nucleotides are connected by covalent bonds to form the sugar-phosphate backbones of the new strands, and hydrogen bonds form between the complementary bases, resulting in two double-stranded DNA molecules.
• DNA replication occurs at hundreds of points along large eukaryotic chromosomes, proceeding in both directions until the entire molecule is replicated.
• Replication begins at origins of replication, where proteins initiate replication by separating the two strands and creating a replication bubble.
• A replication fork is a "Y" shaped region at the end of each replication bubble where new DNA strands are elongating; two replication forks are formed when the strands separate.
• New bases are added according to base pairing rules: if there is adenine on the template strand, a nucleotide with thymine is added to the new strand.
• The end result of replication is two identical DNA molecules, each with one original strand and one new strand.
• Helicases are enzymes that untwist the double helix at the replication forks, breaking hydrogen bonds and separating the parental strands to serve as templates.
• DNA polymerase is a major enzyme in DNA replication that adds new nucleotides to the existing chain at a rate of about 50 nucleotides per second in human cells.
• DNA polymerase also proofreads each new DNA strand to ensure an exact copy is made.