Monohybrid inheritance
Use alleles and probability to predict possible offspring without treating ratios as guarantees.
What you will learn
- Distinguish genotype from phenotype.
- Construct a one-gene cross.
Separate genetic information from its expression
A genotype is an organism's allele combination for the gene considered. A phenotype is the expressed characteristic, influenced by genotype and sometimes the environment. Under complete dominance, a heterozygote shows the dominant phenotype.
Homozygous means two identical alleles; heterozygous means two different alleles. A dominant allele is not necessarily more common in a population.
List gametes before combining them
Each gamete receives one allele for the gene. A heterozygote can produce either allele. Combine each parent's possible gametes in a Punnett square and count the relevant genotype or phenotype.
Ratios describe probabilities across offspring, not a promise that every group of four contains exactly one recessive individual.
Worked example
Two heterozygous plants Aa and Aa are crossed. A is completely dominant. Find the probability of the recessive phenotype.
Show the worked solution
- Each parent produces A or a gametes.
- The four equally likely combinations are AA, Aa, aA and aa.
- Only aa expresses the recessive phenotype under complete dominance.
Answer 1/4, or 25%
Common mistakes
- A genotype ratio and a phenotype ratio can differ.
- Dominance does not mean an allele is stronger or more frequent.
What can you explain now?
Cross Aa with aa. What fraction of offspring is expected to be Aa?
Compare with the explanation
1/2
The first parent contributes A or a with equal probability; the second always contributes a.
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