Predicting outcomes with genetic crosses and probability

Inheritance, variation and evolution • Reproduction

Flashcards

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Define a Punnett square limitation.

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Punnett squares assume single-gene control and random mating; they do not model polygenic traits or environmental effects.

Key concepts

What you'll likely be quizzed about

Probability in single-gene crosses

Probability expresses the chance of a specific genotype or phenotype arising from a cross. Probabilities convert to percentages, fractions or ratios (for example 1/4, 25%, or 1:3). Each offspring represents an independent event so probabilities apply to each conception rather than sequencing of previous children. The coin-toss analogy helps: a 50% chance of an allele from one parent remains 50% for every independent offspring.

Punnett square construction and use

A Punnett square places one parent’s gamete alleles along the top and the other parent’s along the side; the grid cells show all possible genotype combinations of offspring. Counting the resulting genotypes gives direct proportions and simple ratios (for example BB : Bb : bb = 1 : 2 : 1 for two heterozygotes). Punnett squares model single-gene, two-allele inheritance and provide expected ratios for genotype and phenotype. Limitations include assumptions of independent assortment, no selection and single-gene control of the trait.

Expressing outcomes as proportions and ratios

Direct proportion expresses the part-to-whole relationship (for example 2 of 4 = 2/4 = 1/2). Simple ratios compare counts of genotypes or phenotypes (for example 1:3 means one of four shows a genotype and three of four show other outcomes). Converting between fraction, percentage and ratio ensures clarity when comparing predicted outcomes with observed data. Ratios derive from counting Punnett square cells or calculated probabilities.

Sex inheritance and sex chromosomes

Sex is determined by the pair of sex chromosomes inherited from the parents. Typical notation uses X and Y: females have two X chromosomes (XX) and males have one X and one Y (XY). Gametes from a female all carry X; male gametes carry either X or Y, so each conception has a 50% chance of producing XX (female) or XY (male). Sex-linked traits on the X chromosome show different inheritance patterns because males have only one X chromosome and therefore express recessive X-linked alleles without a second allele to mask them.

Interpreting family trees (pedigrees)

Family trees use standard symbols (squares for males, circles for females) and shading to show affected individuals. Horizontal lines connect parents; vertical lines descend to their children. Patterns of inheritance across generations reveal whether a trait is dominant or recessive and whether carriers exist. A recessive disorder appearing in children of unaffected parents indicates carrier parents; a dominant disorder typically appears in successive generations. Family trees help deduce genotypes for given individuals and predict offspring risks.

Limitations and biological factors

Punnett squares model single-gene inheritance and assume random mating, no selection and single-locus control. Many traits are polygenic or influenced by the environment; such traits do not follow simple Mendelian ratios. Crossover during meiosis and mutations alter allele combinations but do not change the basic probabilistic predictions for independent gamete formation. Observed ratios in small sample sizes often deviate from expected values; probability predicts expected frequencies in large numbers of offspring.

Key notes

Important points to keep in mind

Alleles use letters: dominant capital, recessive lowercase.

Punnett squares show possible offspring genotypes by combining gametes.

Probabilities apply to each independent offspring, not sequences of previous children.

Express outcomes as fraction, percentage or ratio for clarity.

Family trees use squares (male), circles (female) and shading for affected individuals.

Two unaffected parents with an affected child usually indicate recessive inheritance.

Punnett squares assume single-gene inheritance and random mating; many traits are polygenic.

X-linked traits often show higher male expression because males have one X chromosome.

Convert grid counts into proportions by dividing by total grid cells (usually four for a single-gene cross).

Mendel’s ratios arise from repeating crosses and counting large numbers of offspring.