Explain genotype, phenotype, dominance, and other inheritance patterns

Ontario Grade 11 Biology

Genetic Processes

How allele combinations relate to traits

In SNC2D, you learned that cells are the basic units of life and that organisms inherit information from their parents. In this lesson, we will use a simple model to describe how inherited versions of a gene can relate to an observable trait. For example, a plant may have purple or white flowers. The visible flower colour is its phenotype. The allele combination used to describe that colour is its genotype. Genetic models help explain patterns in families, but a probability for a child or offspring is not a guarantee of what will happen in one particular case.

What you will learn

Distinguish genotype from phenotype.
Explain how dominance affects the phenotype linked to a genotype.
Use a Punnett square to predict possible offspring genotypes and phenotypes.
Compare complete dominance, incomplete dominance, codominance, and inheritance involving multiple alleles.

From inherited information to a trait

A gene is a unit of inherited information associated with a trait. An allele is one version of a gene. For a basic inheritance model, an organism receives one allele from each parent. The two alleles together make its genotype for that gene.

A genotype is the allele combination an organism has. A phenotype is the observable form of a trait, such as a flower's colour. Some phenotypes are visible; others can be described in different ways. Phenotype refers to the trait, not the allele combination.

For a simple example, imagine a flower-colour gene with a purple allele and a white allele. We can use

P

for the purple allele and

p

for the white allele. These letters are a model's labels; they do not show how a gene works inside a cell.

An organism with two matching alleles is homozygous for that gene. An organism with two different alleles is heterozygous. Thus,

PP

and

pp

are homozygous genotypes, while

Pp

is heterozygous.

Genotype means the allele combination.
Phenotype means the observable trait.
Homozygous means two matching alleles; heterozygous means two different alleles.

Dominance and a simple inheritance model

In complete dominance, one allele is dominant over another for a particular trait. The dominant allele determines the phenotype in a heterozygous organism. The other allele is called recessive. A recessive phenotype appears only when the organism has two recessive alleles in this simple model.

For the flower example, suppose purple is completely dominant to white. Then

PP

and

Pp

have purple flowers, while

pp

has white flowers. Dominant does not mean more common, stronger, or better. It describes how the alleles relate to a phenotype in this model.

A Punnett square is a grid used to list possible allele combinations in offspring. Each parent contributes one allele. If both parents have genotype

Pp

, each can contribute

P

or

p

. The four equally likely combinations in the model are

PP

,

Pp

,

Pp

, and

pp

.

The square predicts a 1 in 4 chance of

PP

, a 2 in 4 chance of

Pp

, and a 1 in 4 chance of

pp

. With complete dominance, that corresponds to a 3 in 4 chance of purple and a 1 in 4 chance of white. These are probabilities across possible offspring, not a promise about a particular offspring or a small group.

Pp \times Pp \rightarrow 1PP:2Pp:1pp

Dominance describes the phenotype relationship between alleles.
Punnett squares list possible genotypes from parental alleles.
A predicted probability is not certainty for an individual offspring.

Other inheritance patterns

Not every trait follows complete dominance. In incomplete dominance, neither allele completely masks the other in a heterozygote. The heterozygote has an intermediate phenotype. For instance, if a red-flowered plant and a white-flowered plant produce pink-flowered heterozygotes, pink is intermediate between red and white.

In codominance, both alleles are expressed in the heterozygote. The phenotype shows both forms rather than blending them into an intermediate. Human AB blood type is an example: the A and B alleles are both expressed in a person with blood type AB.

Some genes have more than two alleles in a population. This is called multiple-allele inheritance. Each individual still has only two alleles for that gene in the simple model. The ABO blood group system has three common alleles:

I^A

,

I^B

, and

i

. The A and B alleles are codominant to each other, and each is dominant over

i

.

These patterns are models for describing observed inheritance. The same labels do not mean every trait follows the same pattern. To choose a pattern, look at the relationship between genotype and phenotype given in the question or supported by observations.

I^A I^B \rightarrow \text{AB blood type}

Incomplete dominance produces an intermediate heterozygous phenotype.
Codominance means both alleles are expressed in the heterozygote.
Multiple alleles means more than two allele forms exist in a population, not that one individual has more than two.

Using and evaluating inheritance models

Begin with the information given about a trait. Identify the alleles and determine the stated inheritance pattern. Then connect each genotype to its phenotype. If parents' genotypes are known, list the allele each parent could pass on and combine the possibilities in a Punnett square.

An inheritance model is useful because it makes predictions clear and testable. Still, a model is not the same as an observation. A family may show a pattern that fits a model, but a small number of observations cannot prove that every future case will match it. Use the information in the question and avoid claiming more than the model supports.

Keep the key terms separate. A genotype is not a phenotype. A dominant allele is not necessarily common. And a probability describes possible outcomes across repeated cases; it does not tell with certainty which outcome one offspring will have.

Use the stated genotype-to-phenotype rules before predicting outcomes.
Treat a Punnett square as a probability model, not a guarantee.
Distinguish an observed pattern from the model used to explain it.

How common inheritance patterns differ

Pattern	Heterozygous phenotype	Example used here
Complete dominance	Shows the dominant allele's phenotype	Pp is purple
Incomplete dominance	Intermediate between the two forms	RW is pink
Codominance	Both allele forms are expressed	I^A I^B is type AB
Multiple alleles	More than two allele forms occur in the population	ABO has I^A, I^B, and i

Worked example

Complete dominance in flowers

In a plant species, purple flowers (

P

) are completely dominant to white flowers (

p

). Predict the possible genotypes and phenotypes for offspring of two heterozygous plants.

Identify parental alleles
Each parent has genotype $Pp$ , so each can pass on either $P$ or $p$ . Each offspring receives one allele from each parent.
$Pp \times Pp$
List combinations
Pair each possible allele from one parent with each possible allele from the other. The combinations are $PP$ , $Pp$ , $Pp$ , and $pp$ . PP,\ Pp,\ Pp,\ pp
Connect genotype to phenotype
The complete-dominance rule makes both $PP$ and $Pp$ purple. Only $pp$ is white. Therefore, three of the four equally likely outcomes are purple and one is white.
$\frac{3}{4}\text{ purple},\quad \frac{1}{4}\text{ white}$

Answer: The genotype ratio is 1

PP

:2

Pp

:1

pp

. The phenotype probability is 3 in 4 purple and 1 in 4 white.

Check: A single offspring is not guaranteed to have purple flowers; the fractions describe the model's probabilities.

Worked example

Incomplete dominance

For a flower trait, suppose

R

is associated with red and

W

with white. Heterozygotes are pink. Predict the offspring of a pink plant crossed with a white plant.

Represent the parents
Under the stated pattern, pink is the heterozygous genotype $RW$ , and white is $WW$ . The pink parent can pass on $R$ or $W$ ; the white parent can pass on only $W$ .
$RW \times WW$
Combine possible alleles
Combining the options gives $RW$ and $WW$ in equal proportions. There is no $RR$ combination in this cross. RW,\ RW,\ WW,\ WW
Interpret the outcomes
The problem states that $RW$ is pink and $WW$ is white. The predicted probabilities are therefore one-half pink and one-half white.
$\frac{1}{2}\text{ pink},\quad \frac{1}{2}\text{ white}$

Answer: The offspring have a 1 in 2 probability of being pink and a 1 in 2 probability of being white.

Check: Pink is intermediate here because the question specifies incomplete dominance; do not apply the complete-dominance rule.

Worked example

ABO blood group alleles

A parent with genotype

I^A i

and a parent with genotype

I^B i

have a child. Find the possible blood-group phenotypes and their probabilities in the model.

List parental contributions
The first parent can pass on $I^A$ or $i$ . The second can pass on $I^B$ or $i$ . Each combination is equally likely in this model.
$I^A i \times I^B i$
Form possible genotypes
Combining the contributions gives $I^A I^B$ , $I^A i$ , $I^B i$ , and $ii$ . Each occurs in one of the four equally likely combinations.
$I^A I^B,\ I^A i,\ I^B i,\ ii$
Use the allele relationships
$I^A$ and $I^B$ are codominant, so $I^A I^B$ gives type AB. Each is dominant over $i$ , so $I^A i$ is type A and $I^B i$ is type B. The genotype $ii$ gives type O.
$\frac{1}{4}\text{ A},\quad \frac{1}{4}\text{ B},\quad \frac{1}{4}\text{ AB},\quad \frac{1}{4}\text{ O}$

Answer: The model predicts a one-quarter probability for each of blood types A, B, AB, and O.

Check: An individual has two alleles in this example, even though three allele forms occur in the population.

Common mistakes and how to avoid them

Calling the visible trait a genotype.

Correction: The visible trait is the phenotype. The allele combination is the genotype.

Assuming a dominant allele must be the most common allele.

Correction: Dominance describes how an allele affects the heterozygous phenotype in a model. It does not state how common the allele is.

Assuming every heterozygote shows complete dominance.

Correction: Check the inheritance pattern. A heterozygote may show an intermediate phenotype or both allele forms.

Treating a Punnett-square probability as a guaranteed outcome.

Correction: A probability describes possible outcomes. It cannot determine with certainty the genotype or phenotype of one offspring.

Lesson summary

Genotype is an allele combination; phenotype is an observable trait.
In complete dominance, a dominant allele determines the heterozygous phenotype.
Incomplete dominance gives an intermediate heterozygous phenotype; codominance expresses both allele forms.
Multiple alleles can exist in a population, while an individual in the simple model has two alleles for the gene.
Punnett squares organize possible outcomes and probabilities, not certainties.

Check your understanding

Question 1

In a complete-dominance model,

T

is dominant to

t

. Which genotype is heterozygous?

$TT$
$Tt$
$tt$
The phenotype

Show answer and explanation

Tt

Heterozygous means the two alleles differ, so

Tt

is heterozygous.

Question 2

In an incomplete-dominance example, red and white alleles produce pink heterozygotes. What pattern does pink represent?

Complete dominance
Codominance
Incomplete dominance
Multiple alleles

Show answer and explanation

Incomplete dominance

Pink is intermediate between red and white, which is the described incomplete-dominance pattern.

Question 3

A cross predicts a one-quarter chance of a recessive phenotype. What can you conclude about one offspring?

It will certainly have the recessive phenotype.
It cannot have the recessive phenotype.
The model gives a probability, not certainty for that individual.
Every group of four offspring must include exactly one with the phenotype.

Show answer and explanation

The model gives a probability, not certainty for that individual.

The fraction describes a chance across possible outcomes. It does not guarantee an individual result or an exact count in a small group.

Key terms

Allele: One version of a gene.
Genotype: The allele combination an organism has for a gene.
Phenotype: An observable form of a trait.
Dominant allele: An allele that determines the heterozygous phenotype in a complete-dominance model.
Recessive allele: An allele whose phenotype is masked by a dominant allele in a heterozygote in a complete-dominance model.
Homozygous: Having two matching alleles for a gene.
Heterozygous: Having two different alleles for a gene.
Punnett square: A grid used to list possible allele combinations in offspring.

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About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 11 Biology (SBI3U), expectation D3.3. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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D3.3 · Explain genotype, phenotype, dominance, and other inheritance patterns

Ontario Grade 11 Biology

How allele combinations relate to traits

What you will learn

From inherited information to a trait

Dominance and a simple inheritance model

Other inheritance patterns

Using and evaluating inheritance models

How common inheritance patterns differ

Complete dominance in flowers

Incomplete dominance

ABO blood group alleles

Common mistakes and how to avoid them

Lesson summary

Check your understanding

Question 1

Question 2

Question 3

Key terms

Continue through SBI3U

About this lesson and its review