Mendelian Genetics

Genetics Vocabulary

Understanding genetics requires familiarity with several key terms that describe the structure and function of genes, as well as patterns of inheritance.

• Gene: A segment of DNA that encodes information for a specific trait.

• Allele: Different forms of a gene found at the same locus on homologous chromosomes.

• Phenotype: The observable characteristics or traits of an organism, such as flower color or height.

• Genotype: The genetic makeup of an organism; the combination of alleles present (e.g., PP, Pp, or pp).

• Dominant: An allele that masks the effect of a recessive allele in heterozygotes; represented by a capital letter (e.g., P).

• Recessive: An allele whose effect is masked by a dominant allele; only expressed in homozygous individuals; represented by a lowercase letter (e.g., p).

• Homozygous: Having two identical alleles for a gene (e.g., PP or pp); also called "true-breeding" or "pure-bred".

• Heterozygous: Having two different alleles for a gene (e.g., Pp); also called "carrier" or "hybrid".

• X-linked: Refers to genes located on the X chromosome, often showing unique inheritance patterns.

Choosing Letters for Genotypes

When representing genotypes, dominant alleles are typically assigned a capital letter, while recessive alleles use the corresponding lowercase letter. For example, if crossing pure-breeding purple (PP) with pure-breeding white (pp) flowers results in all purple offspring, purple is dominant (P) and white is recessive (p).

• Example: Pure-breeding purple (PP) x pure-breeding white (pp) yields all purple (Pp) hybrids.

Making Gametes and Punnett Squares

Punnett squares are used to predict the possible genotypes of offspring from parental crosses. The process involves:

• Step 1: Determine the genotypes of the parents.

• Step 2: Determine the possible gametes each parent can produce.

• Step 3: Use a Punnett square to determine the possible genotypes of the offspring.

• Example: Crossing PP x pp yields all Pp offspring (all purple if P is dominant).

Monohybrid Cross

A monohybrid cross examines the inheritance of a single gene. For example, crossing two heterozygous purple-flowered plants (Pp x Pp):

• Step 1: Parent genotypes: Pp x Pp

• Step 2: Gametes: P or p from each parent

• Step 3: Offspring genotypes: PP, Pp, Pp, pp (1:2:1 ratio)

• Phenotypic ratio: 3 purple : 1 white (if purple is dominant)

More Than One Gene: Dihybrid Crosses

When studying two genes at once (e.g., flower color and plant height), a dihybrid cross is used. For example, crossing pure-breeding purple, tall (PPTT) with pure-breeding white, short (pptt) plants:

• Step 1: Parent genotypes: PPTT x pptt

• Step 2: Gametes: PT from PPTT, pt from pptt

• Step 3: All offspring are PpTt (hybrid for both traits)

When crossing two dihybrid individuals (PpTt x PpTt), use a 4x4 Punnett square to determine all possible genotype combinations for the offspring.

Punnett Squares for Dihybrid Crosses

A dihybrid Punnett square allows visualization of the inheritance of two genes simultaneously. Each parent can produce four types of gametes (e.g., PT, Pt, pT, pt), resulting in 16 possible genotype combinations in the offspring.

• Phenotypic ratio for two heterozygotes (PpTt x PpTt): 9:3:3:1 (if both traits show complete dominance)

Probability in Genetics

Probability is used to predict the likelihood of specific genotypes or phenotypes appearing in offspring.

• Sum Rule (OR): The probability of either of two mutually exclusive events occurring is the sum of their individual probabilities. Probabilities must add up to 1.

• Product Rule (AND): The probability of two independent events both occurring is the product of their individual probabilities.

• Example: Probability of getting a purple, tall plant from a dihybrid cross = probability of purple x probability of tall.

Meiosis Recap

Overview of Meiosis

Meiosis is the process by which gametes (sperm and egg cells) are produced, reducing the chromosome number by half and introducing genetic variation.

• Key Features: Two rounds of cell division (meiosis I and II), resulting in four non-identical haploid cells.

• Importance: Ensures genetic diversity through independent assortment and crossing over.

Additional info: Meiosis is essential for sexual reproduction and underlies Mendel's laws of segregation and independent assortment.