Genotype and Phenotype

Genotype and Phenotype

Middle School

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Learning Objectives

• Define genotype and phenotype and explain the relationship between them.

• Use a Punnett square to predict the probable genotypes and phenotypes of offspring.

• Explain the difference between homozygous and heterozygous genotypes.

• Describe patterns of inheritance that are non-Mendelian, such as codominance and incomplete dominance.

Key Vocabulary

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Genotype

The genetic makeup of an organism, composed of the alleles inherited from its parents.

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Phenotype

The physical appearance or observable expression of an organism's inherited genotype.

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Allele

A specific variant or form of a gene that is responsible for determining a particular trait.

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Dominant Allele

The allele that is expressed as long as at least one copy is present in the genotype.

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Recessive Allele

The allele that is only expressed when two copies of it are present in the genotype.

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Genotype vs. Phenotype

• An organism's genotype is its genetic makeup of two inherited alleles.

• The phenotype is the physical expression of an organism's genotype.

• A capital letter represents a dominant allele (B); a lowercase is recessive (b).

• Dominant alleles (BB, Bb) will mask the recessive allele (bb).

Multiple Choice

What is the term for the physical appearance or observable expression of an inherited genotype?

• Phenotype

• Genotype

• Allele

• Homozygote

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Homozygous and Heterozygous Genotypes

• An organism is homozygous when it has two identical alleles (BB or bb).

• This can be homozygous dominant (BB) or homozygous recessive (bb).

• An organism is heterozygous when it has two different alleles (Bb).

• In a heterozygous pair, the dominant allele's trait is expressed.

Multiple Choice

Which of the following genotypes is heterozygous?

• Bb

• BB

• bb

• All of the above

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Predicting Traits with Punnett Squares

• A Punnett square predicts the probability of offspring genotypes and phenotypes.

• Place one parent's alleles on top and the other parent's on the side.

• A monohybrid cross (Bb x Bb) shows a 3:1 phenotypic ratio.

• A dihybrid cross can produce a more complex 9:3:3:1 phenotypic ratio.

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Multiple Choice

In a monohybrid cross between two heterozygous parents (Bb), what is the probability of the offspring having the dominant phenotype?

• 75%

• 25%

• 50%

• 100%

Non-Mendelian Inheritance

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Codominance

• ​Both alleles for a trait are equally and distinctly expressed.

• ​​The phenotype shows both of the traits at the same time.

• ​For example, a flower that has both red and white petals.

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Incomplete Dominance

• ​The dominant allele does not completely mask the recessive allele.

• ​​This results in a blended or an intermediate physical trait.

• ​For example, a red and white flower creating a pink flower.

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Polygenic Traits

• ​These traits are controlled by more than one gene together.

• ​​This creates a wide range of continuous variation in traits.

• ​Human height and skin color are examples of this inheritance.

Multiple Choice

What type of inheritance results in a blended or intermediate phenotype, such as pink flowers from red and white parents?

• Incomplete dominance

• Codominance

• Polygenic inheritance

• Mendelian inheritance

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Common Misconceptions

Misconception	Correction
Dominant traits are always the most common in a population.	Dominance only describes which allele is expressed in a heterozygote.
All traits are determined by a single gene.	Many traits are polygenic, controlled by multiple genes.
An organism's traits are determined only by its genes.	The environment can also influence how genes are expressed.
You can always know the genotype by looking at the phenotype.	A dominant phenotype can have a homozygous (BB) or heterozygous (Bb) genotype.

Multiple Choice

If a pea plant with a heterozygous genotype for purple flowers (Pp) is crossed with one that has white flowers (pp), what percentage of their offspring would you expect to have white flowers?

• 50%

• 25%

• 75%

• 0%

Multiple Choice

How does the expression of alleles in codominance differ from incomplete dominance?

• In codominance both alleles are expressed fully, while in incomplete dominance the alleles blend to create an intermediate phenotype.

• In codominance one allele is dominant, while in incomplete dominance both alleles are recessive.

• Codominance applies to polygenic traits, while incomplete dominance applies to monohybrid traits.

• Codominance results in an intermediate phenotype, while incomplete dominance results in both alleles being expressed fully.

Multiple Choice

A scientist crosses two tall tomato plants. Most of the offspring are tall, but some are short. What does this outcome tell you about the genotypes of the parent plants?

• Both parents must be heterozygous (Tt).

• Both parents must be homozygous dominant (TT).

• One parent is heterozygous (Tt) and the other is homozygous recessive (tt).

• One parent is homozygous dominant (TT) and the other is heterozygous (Tt).

Multiple Choice

Human skin color is a polygenic trait, influenced by multiple genes. How does this mode of inheritance explain the wide spectrum of human skin tones?

• The combination of many different alleles from multiple genes allows for a large number of possible phenotypes, creating continuous variation.

• A single dominant allele determines all skin tones.

• Environmental factors are the only cause for variation in skin tone.

• It results in only two possible skin tones, dark or light, with no intermediate shades.

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Summary

• Genotype is the genetic code, while phenotype is the observable physical trait.

• Alleles can be dominant or recessive, forming homozygous or heterozygous genotypes.

• Punnett squares are used to predict the probability of offspring inheriting traits.

• Inheritance patterns can be simple, complex, or influenced by the environment.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

• 1

• 2

• 3

• 4