Genetics - Monohybrid and Dihybrid Crosses

Vocabulary and principles from Genetics lecture notes covering Mendelian inheritance, probability rules, and statistical testing.

Monohybrid cross

A cross between two parents that differ in a single characteristic.

Principle of segregation (Mendel’s first law)

The principle stating that each individual diploid organism possesses two alleles for any particular characteristic; these alleles separate when gametes are formed, with one allele going into each gamete.

Concept of dominance

The concept that when two different alleles are present in a genotype, only the trait encoded by the dominant allele is observed in the phenotype.

Probability

The likelihood of the occurrence of a particular event, used in genetics to predict the outcome of a genetic cross.

Multiplication rule

A rule used to find the probability of two or more independent events occurring together (this outcome AND that outcome); calculated as $P(A ext{ and } B) = P(A) imes P(B)$..

Addition rule

A rule used to find the probability of any one of two or more mutually exclusive events occurring (this outcome OR that outcome); calculated as $P(A ext{ or } B) = P(A) + P(B)$..

Chi-square ($\chi^2$) Goodness-of-Fit Test

A statistical test used to determine if the probability that the difference between observed and expected values is due to chance, calculated using the formula $\chi^2 = \sum \frac{(O-E)^2}{E}$.

Degrees of freedom

Used in statistical tests and calculated as $n-1$, where $n$ is the number of classes or possible outcomes.

Dihybrid cross

A cross between two individuals that differ in two characteristics used by Mendel to conclude that alleles encoding different traits separate independently.

$9:3:3:1$ ratio

The phenotypic ratio observed in the $F_2$ progeny of a dihybrid cross ($RrYy \times RrYy$) resulting from the independent separation of alleles.

Branch diagram

A method used to determine the expected proportions of both characteristics in a dihybrid cross by breaking it into two monohybrid crosses and multiplying the phenotypic probabilities.

Mutually exclusive

A condition required to use the addition rule ($P(A) + P(B)$) where outcomes cannot happen at the same time.