Predicting Inherited Traits
Explain allele inheritance in sexual reproduction and use one-gene Punnett-square models to predict offspring genotypes and phenotypes under complete dominance, recognizing that many traits have more complex inheritance
What a learner can do afterwards
- Defines allele, dominant and recessive within a two-allele, complete-dominance model
- Constructs a Punnett square for a monohybrid cross and calculates probability ratios
- Explains the difference between genotype and phenotype
- Uses cystic fibrosis as an example of autosomal recessive inheritance and recognizes that human eye color is not fully predicted by a one-gene model
1 · Read
Genes are sections of DNA carried on chromosomes. Alleles are versions of a gene. For the genes studied here, an individual has two copies, one from each parent. The copies can be the same allele or different alleles. More than two alleles can exist in a population. Many traits, including human eye color, involve several genes rather than this one-gene model.
Genotype means the allele combination for a gene; phenotype means the observable trait. In a complete-dominance model, a dominant allele determines the phenotype in a mixed pair. For pea height, T is dominant for tall and t is recessive for short. TT and Tt give tall plants; tt gives short plants. Dominant does not mean better or more common. Other inheritance patterns can work differently.
Eggs and sperm carry one allele for the gene. A parent with two different alleles passes either with equal chance in this model. Draw a Punnett square by writing one parent's two alleles across the top and the other's down the side. Keep repeated letters. In each box, join its top allele with its side allele. Write the capital first in a mixed pair.
For pea seed color, Y is dominant for yellow and y is recessive for green. Cross Yy with Yy:
| Alleles | Y | y |
|---|---|---|
| Y | YY | Yy |
| y | Yy | yy |
Each box combines one allele from each parent. The two Yy boxes are separate pairings. The genotype ratio is 1 YY : 2 Yy : 1 yy.
The four pairings have equal chances in this model. Chance = matching boxes / total boxes. In the seed cross, 3/4 predict yellow and 1/4 green, a 3:1 phenotype ratio. Multiply a chance by 100 for a percent, or by a group size for an expected count: 3/4 x 20 = 15. Actual counts can differ. A ratio does not promise a fixed result in every group.
Three quarters is a predicted chance, not a required pattern for each group of four offspring.
Cystic fibrosis follows autosomal recessive inheritance. In this simplified same-variant model, C is a working CFTR allele and c is CF-causing. A Cc carrier does not have the condition but can pass C or c. Two carriers can therefore have a cc child with the condition. Human eye color involves several genes, so this one-gene model cannot fully predict it.
Punnett squares combine parental alleles to predict genotype and phenotype chances. Predictions depend on the inheritance model and do not guarantee actual counts.
Lesson, questions and flashcards written for this dataset, with scientific background and question-design study from OpenStax and Khan Academy.
Readings written for this dataset. Video references credit the Khan Academy collection; their titles and source identities are retained.
More to read
What passes from parent to offspring?
Alleles are versions of a gene. In the two-copy model studied here, an individual receives one copy from each parent. The copies may be identical or different. A population can contain more than two versions even though each individual in this model carries two.
An egg or sperm contributes one allele for the gene. A parent with two different alleles can pass either one. The model assigns equal chances to those contributions.
Dominance describes the phenotype of an allele combination. It does not decide which allele a parent is allowed to pass. If B is completely dominant over b, a Bb parent shows the dominant trait but can still pass b.
Keep inheritance and appearance separate. Observing a dominant phenotype may not distinguish BB from Bb. Both have the same phenotype under complete dominance, but only one of those genotypes contains b.
Counting possible pairings
Consider a model plant with flower-color alleles P and p. P is completely dominant. One parent is Pp and the other is pp. The Pp parent can contribute P or p; the pp parent contributes p.
Write P and p across the top of a four-box square. Write p and p down the side. Joining each top allele with its side allele gives Pp, pp, Pp and pp. Keep repeated results because they represent separate pairings.
Two of the four boxes predict the dominant phenotype. Its chance is 2/4, or 1/2. The recessive phenotype also has a half chance. This calculation uses the model's equal contribution and pairing probabilities.
Each box represents a possible pairing, not a particular offspring waiting to be born. A half chance does not require half of every small group to show that phenotype. Actual counts can vary from the expected proportions.
Choosing an inheritance model
A Punnett square predicts the inheritance of specified alleles. Predicting a phenotype also requires knowing how those alleles affect the characteristic. Complete dominance is one possible relationship, not a rule for every trait.
Cystic fibrosis is an example of autosomal recessive inheritance. In a simplified model, a carrier has one working CFTR allele and one recessive CF-causing allele. Two parents carrying the same variant can each pass it to a child. A square for those specified alleles predicts a one-quarter chance of two copies for each child.
Human eye color involves several genes. A single brown-versus-blue allele pair does not fully explain it. Earlobe shape, tongue rolling and hairline shape are also not dependable examples of single-gene dominant/recessive inheritance.
Before using a square, state the alleles, parental genotypes and inheritance assumptions. Without those details, a neat grid can give a misleading answer.
Explore more
2 · Watch
En anglaisTake it off screen
Where it sits
Learn first
Optional connections
This opens up
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.