Probability and Punnett Squares (Genetics)

Vocabulary flashcards covering probability concepts, independent and mutually exclusive events, Punnett squares, Mendel's laws, and allele frequencies.

Total probability

The sum of probabilities of all mutually exclusive outcomes; for a fair coin, p(head) + p(tail) = 1.

Independent events

Events whose outcomes do not affect one another; the joint probability equals the product of the individual probabilities.

Mutually exclusive events

Events that cannot occur at the same time; their probabilities are added when considering alternatives.

Joint probability for two coin tosses (same face)

Probability of both coins showing the same face: p(head, head) = 0.25 and p(tail, tail) = 0.25.

Mixed outcome for two coin tosses

One head and one tail can occur in two ways (head then tail) or (tail then head); combined probability is 0.5.

Punnett square

A grid used to predict all possible genetic (or other) outcomes from parental gametes, based on Mendel's laws.

Genotype

The genetic makeup at a locus, described by two alleles (e.g., DD, Dd, dd).

Allele

Different forms of a gene (e.g., D and d); there are two alleles per gene in diploids.

Diploid

Organisms with two alleles per gene, one on each homologous chromosome.

Gamete

A haploid reproductive cell carrying one allele for each gene; produced by meiosis.

Mendel's Law of Segregation

Alleles for a gene separate into gametes so each gamete carries only one allele.

Mendel's Law of Independent Assortment

Alleles of one gene assort independently of alleles of other genes in gametes.

Homozygous dominant

Genotype with two dominant alleles (DD).

Homozygous recessive

Genotype with two recessive alleles (dd).

Heterozygous

Genotype with one dominant and one recessive allele (Dd).

Genotypic distribution from Dd x Dd

Cross yields DD: 1/4, Dd: 1/2, dd: 1/4.

Punnett square with unequal allele frequencies

When allele frequencies are p and q, genotype probabilities under random mating are p^2 (BB), 2pq (Bb), q^2 (bb).

Allele frequency example (0.7 and 0.3)

If B=0.7 and b=0.3, genotype probabilities are BB=0.49, Bb=0.42, bb=0.09.

Combination probability (any order)

For outcomes like 1, 2, 5 in any order, sum the probabilities for all permutations (e.g., 6 permutations of 1/216 each → 6/216 = 0.02778).

Three-roll independence (order matters)

Probability of a specific sequence (e.g., 1 then 2 then 5) is 1/6 × 1/6 × 1/6 = 1/216.

Genotype frequencies check

In a two-allele system, the sum BB + Bb + bb equals 1.0.

Pedigree trees

Graphs used with Punnett squares to trace inheritance and predict trait passage across generations.