Biology Exam - Genetics

Key Definitions

• Allele: A variant form of a gene.

• Autosome: A chromosome that is not a sex chromosome.

• Autosomal dominance: A pattern of inheritance where one copy of a dominant allele is sufficient to express the trait.

• Autosomal inheritance: Inheritance of genes located on autosomes.

• Carrier: An individual who has one copy of a recessive allele but does not express the trait.

• Chromatids: Two identical copies of a chromosome joined together.

• Chromosome: A structure containing DNA and proteins, found in the nucleus of eukaryotic cells.

• Codominance: A pattern of inheritance where both alleles are expressed equally.

• Dihybrid cross: A cross between individuals heterozygous for two traits.

• DNA: Deoxyribonucleic acid, the molecule that carries genetic information.

• Dominant: An allele that masks the expression of a recessive allele.

• Gene: A unit of heredity that is transferred from a parent to offspring and determines some characteristic of the offspring.

• Genotype: The genetic makeup of an individual.  

• Heterozygous: Having two different alleles for a particular gene.

• Homologous chromosomes: Pairs of chromosomes that have the same genes but may have different alleles.

• Homozygous: Having two identical alleles for a particular gene.

• Incomplete dominance: A pattern of inheritance where the heterozygote phenotype is a blend of the two homozygous phenotypes.

• Law of independent assortment: During gamete formation, alleles for different genes segregate independently of each other.

• Law of segregation: During gamete formation, each pair of alleles segregates so that each gamete receives only one allele for each gene.

• Meiosis: A type of cell division that produces gametes (sex cells) with half the number of chromosomes.

• Mitosis: A type of cell division that produces two identical daughter cells.

• Monohybrid cross: A cross between individuals heterozygous for one trait.

• Nondisjunction: The failure of chromosomes to separate properly during meiosis.

• Pedigree: A diagram showing the inheritance of a trait through a family.

• Phenotype: The observable characteristics of an individual.

• Punnett square: A diagram used to predict the genotypes and phenotypes of offspring from a genetic cross.

• Recessive: An allele that is masked by a dominant allele.

• Recombination: The process of exchanging genetic material between homologous chromosomes during meiosis.

• Sex chromosome: A chromosome that determines an individual's sex (e.g., X and Y chromosomes in humans).

• Sex-linked inheritance: Inheritance of genes located on sex chromosomes.

• Trait: A characteristic or feature of an organism.

Test cross: A cross between an individual with an unknown genotype and a homozygous recessive individual.

Key Concepts:

•  DNA Structure:

  • Double helix structure composed of nucleotides.

  • Nucleotide components: Deoxyribose sugar, phosphate group, nitrogenous base (A, T, C, G).

  • Base pairing: A with T, C with G.

  • Chromosomes: Structures made of DNA and proteins (histones).

    • Parts: Centromere (constriction point), chromatids (identical copies of a chromosome), telomeres (protective ends).

• 2. Cell Cycle:

  • Interphase: G1 (growth), S (DNA replication), G2 (preparation for division).

  • Mitosis: Cell division resulting in two identical daughter cells. Phases: Prophase, Metaphase, Anaphase, Telophase.

  • Cytokinesis: Division of the cytoplasm.

  • Meiosis: Cell division resulting in four haploid gametes (sex cells). Two rounds of division (Meiosis I and Meiosis II). Importance: Genetic variation through crossing over (prophase I) and independent assortment (metaphase I).

  • Haploid Cells: Cells with half the number of chromosomes (n).

  • Errors in Meiosis:

    • Nondisjunction: Failure of chromosomes to separate properly.

    • Trisomy: Having an extra chromosome (e.g., Trisomy 21 – Down syndrome).

    • Monosomy: Missing a chromosome (e.g., Turner syndrome – XO).

    • Klinefelter's Syndrome: XXY

    • Karyotypes: Visual representation of an individual's chromosomes.

• 3. Mendelian Genetics:

  • Law of Segregation: Each individual has two alleles for each gene, and these alleles separate during gamete formation.

  • Law of Independent Assortment: Alleles for different genes assort independently of each other during gamete formation (if on different chromosomes).

  • Terms:

    • Phenotype: Observable traits.

    • Genotype: Genetic makeup (e.g., AA, Aa, aa).

    • Dominant: Allele that masks the recessive allele.

    • Recessive: Allele that is masked by the dominant allele.

    • Allele: Variant form of a gene.

    • Gene: A unit of heredity.

  • Punnett Squares:

    • Monohybrid Cross: Tracks one trait (e.g., Aa x Aa).

    • Dihybrid Cross: Tracks two traits (e.g., AaBb x AaBb).

    • Test Cross: Used to determine the genotype of an unknown dominant phenotype (e.g., A? x aa).

• 4. Non-Mendelian Genetics:

  • Incomplete Dominance: Heterozygote shows a blended phenotype (e.g., red flower x white flower = pink flower).

  • Codominance: Both alleles are fully expressed (e.g., blood type AB).

  • Multiple Alleles: More than two alleles exist for a gene (e.g., blood types A, B, O).

• 5. Patterns of Inheritance:

  • X-linked: Genes located on the X chromosome. Affects males more often.

  • Autosomal Dominant: One copy of the dominant allele is enough for expression.

  • Autosomal Recessive: Two copies of the recessive allele are needed for expression.

  • Genetic Diseases:

    • Hemophilia: X-linked recessive, blood clotting disorder.

    • Color Blindness: X-linked recessive, difficulty distinguishing colors.

    • Sickle Cell Anemia: Autosomal recessive, abnormal hemoglobin.

  • Pedigree Chart: A chart showing the inheritance of a trait through generations.