Mendelian Genetics

• Define key terms: gene, allele, genotype, and phenotype.

• Explain the difference between dominant and recessive alleles.

• Use a Punnett square to predict the traits of offspring.

• Distinguish between Mendelian and non-Mendelian inheritance patterns.

Key Vocabulary

Gene

A gene is a section of DNA that holds the instructions for building specific proteins.

Allele

An allele is one of the different forms or variations of a particular gene.

Genotype

The genotype is the specific combination of alleles that an organism has for a trait.

Phenotype

The phenotype is the observable physical trait that results from an organism's genotype, like eye color.

Dominant Allele

This allele's trait is always expressed in the phenotype if at least one copy is present.

Recessive Allele

This allele’s trait is only expressed when two copies are present in the organism's genotype.

The Basics of Genetics

• Gregor Mendel discovered genes control traits passed from parents to offspring.

• An organism’s genotype is its genetic makeup; its phenotype is its appearance.

• A dominant allele (B) is expressed over a recessive allele (b).

• Organisms are homozygous (BB, bb) or heterozygous (Bb) for each trait.

Monohybrid Cross

• A Punnett square predicts the genotypes and phenotypes of offspring from a genetic cross.

• A monohybrid cross tracks the inheritance of a single trait, such as flower color.

• The resulting phenotypic ratio between two heterozygous parents is typically 3 dominant to 1 recessive.

Dihybrid Cross

• A dihybrid cross tracks the inheritance of two different traits at the same time.

• This cross requires a larger 4x4 Punnett square to show all possible offspring allele combinations.

• The typical phenotypic ratio for this cross between two heterozygous parents is 9:3:3:1.

Non-Mendelian Inheritance

Codominance

• A phenotype where both alleles for a gene are expressed equally and distinctly.

• Neither allele is dominant or recessive over the other allele in the pair.

• For example, a flower shows both red and white petals at once.

Incomplete Dominance

• This occurs when one allele is not completely dominant over the other allele.

• This results in a third phenotype that is a blend of the two.

• A red and a white flower can produce a new pink flower.

Polygenic Traits

• These are characteristics that are controlled by more than one single gene.

• Polygenic traits often show a wide range of variations in a population.

• Examples in humans include skin color, eye color, and overall height.

• Gregor Mendel established that genes, in different forms called alleles, determine traits.

• An organism's specific combination of alleles is its genotype, which determines the phenotype.

• Dominant alleles mask recessive alleles, and Punnett squares help predict offspring traits.

• Codominance and incomplete dominance are more complex non-Mendelian inheritance patterns.