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 physical or physiological traits of an organism, determined by its genotype and environment.

• Genotype: The genetic makeup of an organism; the combination of alleles present.

• Dominant: An allele that masks the effect of a recessive allele in heterozygotes; expressed in the phenotype if present.

• Recessive: An allele whose effect is masked by a dominant allele; only expressed in the phenotype when homozygous.

• Homozygous: Having two identical alleles for a gene ("true-breeding" or "pure-bred").

• Heterozygous: Having two different alleles for a gene ("carrier" or "hybrid").

• X-linked: Refers to genes located on the X chromosome; often associated with sex-linked inheritance patterns.

Choosing Letters for Genotypes

When representing genotypes, letters are used to symbolize alleles. Typically, a capital letter represents the dominant allele, and a lowercase letter represents the recessive allele.

• If a cross between pure-breeding purple (PP) and pure-breeding white (pp) flowers produces all purple offspring, purple is dominant (P), and white is recessive (p).

• Genotypes are written as pairs: PP (homozygous dominant), Pp (heterozygous), pp (homozygous recessive).

Making Gametes and Punnett Squares

Punnett squares are tools used to predict the possible genotypes and phenotypes of offspring from a genetic cross.

• Step 1: Determine the genotypes of the parents.

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

• Step 3: Fill in the Punnett square to determine possible offspring genotypes and phenotypes.

• Example: Crossing pure-breeding purple (PP) with pure-breeding white (pp) yields all Pp (purple) offspring.

Monohybrid Cross

A monohybrid cross examines the inheritance of a single gene with two alleles.

• Example: Crossing two heterozygous purple-flowered plants (Pp x Pp).

• Possible offspring genotypes: PP, Pp, pp.

• Phenotypic ratio: 3 purple : 1 white.

Inheritance of Multiple Genes

Dihybrid Cross

A dihybrid cross examines the inheritance of two different genes simultaneously, each with two alleles.

• Example: Crossing PpTt x PpTt, where P = purple, p = white, T = tall, t = short.

• Step 1: Determine genotypes of parents (both PpTt).

• Step 2: Determine all possible gametes (PT, Pt, pT, pt for each parent).

• Step 3: Use a 4x4 Punnett square to determine possible offspring genotypes and phenotypes.

Phenotypic ratio for a dihybrid cross (when both parents are heterozygous for both traits):

• 9:3:3:1 ratio (9 with both dominant traits, 3 with one dominant and one recessive, 3 with the other dominant and one recessive, 1 with both recessive traits).

Probability in Genetics

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

• Rule of Addition (OR): The probability of either of two mutually exclusive events occurring is the sum of their individual probabilities.

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

• All probabilities must add up to 1.

Example: The probability of getting a purple, tall plant from a PpTt x PpTt cross can be calculated by multiplying the probabilities for each trait.

Meiosis Recap

Connection to Mendelian Genetics

Meiosis is the process by which gametes (sperm and egg cells) are produced, ensuring genetic diversity and the correct number of chromosomes in offspring.

• During meiosis, homologous chromosomes (and thus alleles) are separated, leading to the formation of gametes with different combinations of alleles.

• This separation explains Mendel's Law of Segregation and Law of Independent Assortment.

Key Point: The random assortment of chromosomes during meiosis leads to genetic variation in offspring, which is the basis for Mendelian inheritance patterns.