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

Genotype

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

Phenotype

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

Allele

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

Dominant Allele

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

Recessive Allele

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

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).

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.

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.

Non-Mendelian Inheritance

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.

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.

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.

Common Misconceptions

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.