Probability in Genetic Crosses Flashcards

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In genetics, what does the word "probability" mean?

It means the chance that a certain genotype or phenotype will appear in the offspring.

How are probabilities from genetic crosses usually written?

They are written as fractions, ratios or percentages, such as 1/4, 3:1 or 25%.

What is the probability that a child from a Bb × Bb cross will have the recessive phenotype?

The probability is 1/4 or 25%.

In a Bb × Bb cross, what is the expected ratio of dominant to recessive phenotypes?

The expected ratio is 3 dominant : 1 recessive (3:1).

How does a Punnett square help you work out probability in a genetic cross?

It shows all the possible allele combinations, so you can count them and work out the chance for each outcome.

Does a probability of 1/4 mean that every family will have exactly one child in four with that trait?

No. It means each child has a 1/4 chance, but real families may not match the fraction exactly.

Why is each fertilisation event in humans like a new roll of a dice?

Because each child is an independent event, so the probability is the same every time and past births do not change it.

Why do observed results in many offspring usually get closer to the predicted probabilities?

Because with a larger number of offspring, the effects of random chance even out and ratios get closer to the expected values.

If there is a 1/2 chance of a child being a carrier of a recessive allele, what percentage is this?

A probability of 1/2 is the same as 50%.

Two carrier parents for a recessive disorder have genotype Aa × Aa. What is the probability of a child who is not a carrier and does not have the disorder?

The probability of genotype AA (unaffected and not a carrier) is 1/4 or 25%.

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