Genetics: Hybridization and Alleles

Genetics

• Hybridization: The mating of two true-breeding varieties (P generation) differing in one or more characters, resulting in hybrid offspring (F1 generation).
• F2 generation: The offspring of the F1 generation.
• Alleles: Alternative versions of a gene.
• Locus: The specific spot where a gene resides on a chromosome.
• Law of segregation: Two alleles for a heritable character separate during gamete formation and end up in different gametes.
• Homozygote: An organism with two identical alleles for a character.
• Heterozygote: An organism with two different alleles for a gene.
• Testcross: Breeding an individual with an unknown genotype with a homozygous recessive individual to determine the unknown genotype.
• Dihybrids: Organisms heterozygous for both characters in a cross between two true-breeding parents differing in two characters.
• Dihybrid cross: A cross between F1 dihybrids that can determine whether two characters are transmitted to offspring as a package or independently.
• Law of independent assortment: Each pair of alleles segregates independently of any other pair of alleles during gamete formation.
• Complete dominance: The phenotype of F1 hybrids is identical to the phenotype of the dominant homozygote.
• Incomplete dominance: The phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties.
• Codominance: Two dominant alleles affect the phenotype in separate, distinguishable ways.
• Pleiotropy: When one gene has multiple phenotypic effects.
• Cystic fibrosis and sickle-cell disease: Examples of hereditary diseases caused by pleiotropic alleles.
• Epistasis: The expression of a gene at one locus alters the phenotypic expression of a gene at a second locus.
• Polygenic inheritance: When multiple genes independently affect a single trait.
• Carriers: Heterozygous individuals who carry the recessive allele but are phenotypically normal.
• Recessive disorders: Show up only in individuals homozygous for the allele.
• Albinism: Example of a recessive disorder.
• Achondroplasia: Example of a dominant disorder.
• Huntington's disease: A degenerative disease of the nervous system with no effect until the individual is 40 years of age, and an example of a dominant disorder.
• Sex-linked gene: A gene located on either sex chromosome (X or Y).
• X chromosome: The larger sex chromosome.
• Color blindness and Duchenne muscular dystrophy: Disorders caused by recessive alleles on the X chromosome in humans.

DNA Replication

• Replication bubble: A region at the end of each replication bubble where new DNA strands are elongating, Y-shaped.
• Replication fork: The location where the parental strands of the DNA are separated, opening up a replication bubble.
• Helicases: Enzymes that untwist the double helix at the replication forks.
• Single-strand binding proteins: Bind to and stabilize single-stranded DNA.
• Topoisomerase: Relieves the strain of twisting of the double helix by breaking, swiveling, and rejoining DNA strands.
• RNA primer: The initial nucleotide strand needed to start the elongation of new DNA strands.
• Primase: Synthesizes the RNA primers.
• DNA polymerase: Catalyzes the synthesis of new DNA at a replication fork.
• Nucleoside triphosphate: Each nucleotide that is added to a growing DNA strand.
• dATP: Supplies adenine to DNA and is similar to the ATP of energy metabolism.
• 5' to 3': Direction of new strand synthesis.
• Okazaki fragments: The lagging strand is synthesized as a series of short segments called Okazaki fragments.
• DNA ligase: Joins Okazaki fragments.

DNA Repair

• Mismatch repair: DNA repair enzymes correct errors in base pairing.
• Nucleotide excision repair: Nuclease cuts out and replaces damaged stretches of DNA.

Gene Expression

• Gene expression: The process by which DNA directs protein synthesis, including transcription and translation.
• Transcription: The synthesis of RNA using information in DNA; creates mRNA.
• Translation: The synthesis of a polypeptide, using information in the mRNA.
• Ribosomes: The site of translation.
• Nuclear envelope: Separates transcription from translation in eukaryotes.
• RNA processing: RNA transcripts are modified through RNA processing to yield the finished mRNA.
• Primary transcript: The initial RNA transcript from any gene prior to processing.
• Central dogma: The concept that all cells are governed by a cellular chain of command: DNA->RNA->protein.
• Codons: Triplets of nucleotides that specify amino acids.
• Amino acids and stop signals: Of the 64 triplets, 61 code for amino acids and 3 are stop signals.
• RNA polymerase: Catalyzes RNA synthesis.
• Complementary: RNA is complementary to the DNA template strand.
• Uracil: The base found on RNA in place of thymine.
• 5' to 3': Direction of RNA transcribed.
• Terminator: In bacteria, the polymerase stops transcription at the end of the terminator, and the mRNA can be translated without further modification.
• Polyadenylation signal sequence: In eukaryotes, RNA polymerase II transcribes the polyadenylation signal sequence, and the RNA transcript is released 10-35 nucleotides past this sequence.
• RNA processing: After transcription, pre-mRNA is modified through ___RNA processing.
• 5' cap and poly-A tail: The 5' end receives a 5' cap and the 3' end gets a poly-A tail during RNA modification.
• Facilitate the export of mRNA to the cytoplasm, protect mRNA from hydrolytic enzymes, and help ribosomes attach to the 5′ end: Functions of the 5' cap and the poly-A tail.
• Introns: Long noncoding stretches of nucleotides that lie between coding regions on RNA transcripts.
• Exons: The parts of the transcripts that are actually expressed.
• RNA splicing: Removes introns and joins exons, creating an mRNA molecule with a continuous coding sequence.
• Spliceosomes: Mostly carry out RNA splicing.
• Ribozymes: Catalytic RNA molecules that function as enzymes and can splice RNA.
• Form a three-dimensional structure, contain functional groups that may participate in catalysis, and can hydrogen-bond with other nucleic acid molecules: Three properties of RNA that enable it to function as an enzyme.
• Alternative RNA splicing: Genes can encode more than one kind of polypeptide, depending on which segments are treated as exons during splicing.
• tRNA: Transfers amino acids to the growing polypeptide in a ribosome; aids in translation.
• Aminoacyl-tRNA synthetase: An enzyme that makes a correct match between a tRNA and an amino acid, the first step of transcription.
• Correct match between the tRNA anticodon and an mRNA codon: Second step of transcription.
• Protein and rRNA: What the large and small ribosomal subunits are made of.
• P, A, E: The three binding sites for tRNA on the ribosome.
• P site: Holds the tRNA that carries the growing polypeptide chain.
• A site: Holds the tRNA that carries the next amino acid to be added to the chain.
• E site: The exit site, where discharged tRNAs leave the ribosome.
• Initiation, elongation, and termination: The three stages of translation.
• Codon recognition, peptide bond formation, and translocation: The three steps of elongation.
• Free ribosomes: Synthesize proteins that function in the cytosol.
• Bound ribosomes: Make proteins of the endomembrane system and proteins that are secreted from the cell.

Test your knowledge on key genetics concepts such as hybridization, alleles, and the law of segregation. Understand the differences between homozygotes and heterozygotes, and explore the significance of testcrosses and dihybrid crosses. This quiz is designed for students looking to strengthen their grasp of fundamental genetic principles.