Mendelian Inheritance and Probability

Manual: General · Subject: Genetics

Use Mendel’s laws and probability tools to predict inheritance patterns in simple crosses.

Mendel’s Laws

Segregation and Independent Assortment

Mendel’s law of segregation states that the two alleles for a gene separate during gamete formation. His law of independent assortment states that alleles of different genes assort independently when they are on different chromosomes or far apart on the same chromosome.

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Important Caveat

Independent assortment is a useful model, but linked genes violate it when they are close together on the same chromosome.

Solving a Monohybrid Cross

1

Identify the parental genotypes.
2

Determine the possible gametes from each parent.
3

Combine gametes in a Punnett square or probability tree.
4

Count genotype and phenotype outcomes.
5

State ratios and probabilities clearly.

In a heterozygous self-cross $Aa \times Aa$ , what is the expected genotype ratio?

1:1 3:1 1:2:1 9:3:3:1

What phenotype ratio is expected from $Aa \times Aa$ if $A$ is completely dominant?

Probability Rules

For independent events, use the product rule: $P(A \text{ and } B)=P(A)\cdot P(B)$ . For mutually exclusive outcomes, use the sum rule: $P(A \text{ or } B)=P(A)+P(B)$ .

If the probability of one child being affected is $\frac{1}{4}$ , what is the probability that two independent children are both affected?

\frac{1}{2}

\frac{1}{8}

\frac{1}{16}

\frac{1}{4}

State Mendel’s law of segregation in one sentence.

Genotype vs Phenotype

Genotype

Allelic makeup of an organism
Examples: $AA$ , $Aa$ , $aa$

Phenotype

Observable trait or measurement
Examples: tall, short, affected, unaffected

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Memory Deck

Match the Pairs

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