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Mendelian Genetics and Inheritance Patterns

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  • Homozygous dominant, heterozygous, homozygous recessive
    Homozygous dominant: two identical dominant alleles; heterozygous: one dominant and one recessive allele; homozygous recessive: two identical recessive alleles.
  • Phenotypic vs. genotypic ratio
    Phenotypic ratio: ratio of observable traits (e.g., 3 purple:1 white flowers); genotypic ratio: ratio of allele combinations (e.g., 1 PP:2 Pp:1 pp).
  • Law of Segregation
    Allele pairs separate during gamete formation, so each gamete carries only one allele for each gene.
  • Law of Independent Assortment
    Alleles of different genes are inherited independently of each other during gamete formation.
  • Monohybrid cross
    Cross between organisms heterozygous for one character (e.g., Pp x Pp) producing a 3:1 phenotypic ratio.
  • Dihybrid cross
    Cross between organisms heterozygous for two characters (e.g., RrYy x RrYy) producing a 9:3:3:1 phenotypic ratio.
  • Testcross
    Mating an individual with unknown genotype with a homozygous recessive to determine the unknown genotype.
  • Dominant vs. recessive disorders
    Dominant disorders: caused by dominant alleles, often lethal in homozygous form; recessive disorders: caused by homozygous recessive alleles, carriers are phenotypically normal.
  • Probability of inheritance
    Use the rule of multiplication for independent events and rule of addition for alternative ways an event can occur.
  • Incomplete dominance
    Heterozygous phenotype is intermediate between the two homozygous phenotypes.
  • Codominance
    Both alleles are fully expressed in heterozygotes (e.g., AB blood type).
  • Pleiotropy
    One gene affects multiple phenotypic traits (e.g., sickle-cell anemia affects red blood cells and malaria resistance).
  • Polygenic inheritance
    Multiple genes contribute additively to a single phenotypic trait (e.g., human height, skin color).
  • Linked genes
    Genes located close together on the same chromosome tend to be inherited together, not following independent assortment.
  • Crossing over
    Exchange of chromosome segments during meiosis that produces recombinant gametes with new allele combinations.
  • Recombination frequency
    Percentage of recombinant offspring; used to map gene loci on chromosomes.
  • Sex chromosomes and sex determination
    Females are XX, males XY; Y chromosome contains SRY gene triggering testis development.
  • Sex-linked genes
    Genes located on sex chromosomes, mostly on X chromosome; recessive X-linked traits more common in males.
  • Pedigree analysis
    Chart showing inheritance of traits across generations to determine genotype and mode of inheritance.
  • Fetal genetic testing methods
    Amniocentesis and chorionic villus sampling collect fetal cells for karyotyping and biochemical tests to detect genetic disorders.
  • Newborn screening
    Routine tests at birth for genetic disorders like phenylketonuria (PKU) to enable early treatment.