hereditary

Genetics Study Notes

Basics of Genetics

  • Character and Trait:

    • Character refers to a heritable feature that varies among individuals. Traits are the specific variants of a character (e.g., purple or white flower color in pea plants).

  • True-Breeding:

    • A true-breeding organism is one that, when mated with another true-breeding organism of the same phenotype, will produce offspring that have the same phenotype.

    • Genotype Association:

    • Genotypes of true-breeding plants are homozygous (e.g., AA or aa).

  • Generations in Genetics:

    • Parental Generation (P): The initial set of parents in a genetic cross.

    • Filial Generations (F1, F2, etc.): The offspring of the parental generation; F1 is the first generation, F2 is the second generation, etc.

  • Traits in Genetics:

    • Dominant Trait: A trait that is expressed in the phenotype even when only one allele is present (e.g., allele A).

    • Recessive Trait: A trait that is expressed in the phenotype only when both alleles are present (e.g., allele a).

  • Chromosomes:

    • Sex Chromosomes: Chromosomes that determine the sex of an individual (XX for females, XY for males).

    • Autosomes: Non-sex chromosomes that determine traits other than sex.

  • Genetic Terminology:

    • Gene: A segment of DNA that codes for a protein.

    • Locus (plural: loci): The specific location of a gene on a chromosome.

    • Allele: Different versions of a gene.

    • Wildtype alleles: The typical form of a gene found in the natural population.

    • Mutant alleles: Variants of a gene that differ from the wildtype and may be associated with a specific phenotype.

    • Genotype: The genetic constitution of an individual, often represented by alleles (e.g., AA, Aa, aa).

    • Phenotype: The observable traits of an individual, resulting from the interaction of genotype and environment.

    • Homozygous: Having two identical alleles for a gene (e.g., AA or aa).

    • Heterozygous: Having two different alleles for a gene (e.g., Aa).

    • Hemizygous: Having only one allele for a gene in a diploid organism (e.g., males for genes located on the X chromosome).

Mendelian Patterns of Inheritance

  • Mendelian Laws of Inheritance:

    • These foundational principles explain how traits are passed from parents to offspring and relate to the process of meiosis.

    • Law of Dominance: In a heterozygote, the dominant allele masks the effect of the recessive allele.

    • Law of Segregation: During gamete formation, the alleles for each gene segregate from each other so that each gamete carries only one allele for each gene.

    • Law of Independent Assortment: Genes for different traits assort independently of one another in the formation of gametes.

  • Punnett Square:

    • A grid system used to predict the genotypes and phenotypes of offspring from a genetic cross, particularly useful for single-character resolution.

  • Monohybrid Terminology:

    • Monohybrid: An organism heterozygous for one specific trait (e.g., Aa).

    • Monohybrid Cross: A genetic cross between parents who are both heterozygous for one trait (e.g., Aa x Aa).

    • Genotypic and Phenotypic Outcomes:

    • Outcomes of a monohybrid cross typically yield a ratio of 1:2:1 for genotypes (1 homozygous dominant: 2 heterozygous: 1 homozygous recessive) and a ratio of 3:1 for phenotypes (3 dominant: 1 recessive).

  • Test Cross:

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

    • Used to determine the genotype of the unknown individual based on the offspring's phenotypes.

  • Dihybrid Cross:

    • A genetic cross between parents that differ in two traits (e.g., AaBb x AaBb).

    • Use of the product rule applies to calculate probabilities, allowing for predictions of genotypic and phenotypic ratios (e.g., 9:3:3:1).

Non-Mendelian Patterns of Inheritance

  • Multiple Alleles:

    • Determine if the pattern of inheritance involves multiple alleles by evaluating the phenotype observed.

    • Ability to resolve outcomes such as blood types (A, B, O) using the concept of codominance.

  • Incomplete Dominance:

    • A pattern where a heterozygous phenotype is intermediate between the two homozygous phenotypes (e.g., red and white flowers producing pink flowers).

    • Recognize incomplete dominance in specific genetic crosses and resolve outcomes accordingly.

  • Codominance:

    • A situation where both alleles in a heterozygote contribute to the phenotype (e.g., AB blood type).

    • Understand human blood types' exhibit both Mendelian inheritance and codominant patterns.

  • Epistasis:

    • Interaction between genes where one gene can hide or modify the expression of another gene.

    • Identify scenarios of epistasis and resolve phenotypic outcomes.

  • Multiple Gene Phenotypic Expressions:

    • Recognize when multiple genes contribute to a single phenotype and how this can create a range of phenotypes.

    • Example: Polygenic inheritance such as skin color determined by several genes, resulting in a spectrum, explained through the concept of additive effect.

  • Environmental Effects:

    • Acknowledge that phenotypes can be influenced not only by genotype but also by environmental factors.

    • Be able to cite examples showing environmental impacts (e.g., hydrangea flower color varying with soil pH).

    • Multifactorial: Refers to traits controlled by multiple genes and environmental factors, leading to complex phenotypic outcomes.

Sex-Linked Traits

  • Sex Chromosomes and Trait Determination:

    • Understanding the XY system informs the determination of offspring sex and associated phenotypes.

    • Genes located on sex chromosomes can produce sex-linked traits (e.g., color blindness, hemophilia).

  • Genetic Nomenclature:

    • Specific nomenclature for sex-linked genes must be understood for both coding and phenotypic interpretation.

  • Phenotypic Outcomes from Sex-Linked Traits:

    • Resolve phenotypic outcomes and patterns noted in sex-linked inheritance, distinguishing between male and female inheritances.

Extra-Chromosomal Patterns of Inheritance

  • Inheritance Mechanisms:

    • Mitochondria and Chloroplasts: These organelles contain their own DNA, leading to unique inheritance patterns.

    • mtDNA (mitochondrial DNA) and cpDNA (chloroplast DNA): Patterns of inheritance differ from nuclear DNA, being maternally inherited.

    • Conduct analysis on genetic outcomes derived from mtDNA and cpDNA inheritance patterns.

Pedigrees

  • Symbols in Pedigrees:

    • Understanding the standardized symbols (squares for males, circles for females, filled shapes for affected individuals, etc.) used to depict relationships and traits within pedigrees.

  • Interpretation of Pedigrees:

    • Ability to deduce genotypes from pedigree charts, determining carriers vs. affected individuals.

    • Assess inheritance patterns: distinguish autosomal dominant vs. autosomal recessive traits based on pedigree information.