Learning Objectives

• Define key genetic terms like gene, allele, genotype, and phenotype.

• Explain how dominant and recessive alleles determine an organism's traits.

• Use a Punnett square to predict genotypes and phenotypes from a monohybrid cross.

• Differentiate between Mendelian and non-Mendelian inheritance patterns like codominance and incomplete dominance.

Key Vocabulary

Gene

A segment of DNA that codes for a protein, controlling a specific expressed characteristic.

Allele

Variants of a specific gene that are responsible for determining the expression of a trait.

Genotype

The inherited genetic makeup of an organism, composed of a unique combination of its alleles.

Phenotype

The observable physical appearance or expression of an organism that is determined by its genotype.

Dominant Allele

An allele that is always expressed in the phenotype as long as it is present.

Recessive Allele

An allele that is only expressed in the phenotype when no dominant allele is present.

Genotype vs. Phenotype

• A genotype is the genetic makeup, with two alleles for each gene.

• Homozygous means identical alleles (BB, bb); heterozygous means different alleles (Bb).

• The phenotype is the physical expression or observable trait of the genotype.

• Dominant alleles are expressed in BB and Bb; recessive alleles only in bb.

Predicting Traits with Punnett Squares

• A Punnett square is a chart that predicts an offspring's potential traits.

• ​Parent genotypes are on the top and side; boxes show possible offspring.

• For two heterozygous (Bb) parents, offspring can be BB, Bb, or bb.

• This results in a 3:1 ratio of dominant to recessive phenotypes.

Beyond Mendel: Complex Inheritance

Codominance

• ​Both alleles for a gene are expressed equally in the organism’s phenotype.

• ​​Neither of the alleles is recessive, so both phenotypes are expressed together.

• ​A flower with both red petals and white petals is an example.

Incomplete Dominance

• ​The dominant allele is not fully expressed over the recessive allele.

• ​​This results in a third phenotype which is a blend of both alleles.

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

Polygenic Traits

• ​These characteristics are controlled by more than one gene.

• ​​This allows for a wide range of possible phenotypes to be expressed.

• ​Human height and skin color are examples of such polygenic characteristics.

Common Misconceptions

Summary

• Heredity is the passing of traits from parents to offspring through genes.

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

• Dominant alleles are expressed if present; recessive alleles need two copies to be expressed.

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