Introduction to Genetics

Learning Objectives

Define genetics, heredity, trait, allele, dominant, and recessive.

Distinguish between genotype and phenotype.

Use a Punnett square to predict the probable outcomes of a monohybrid cross.

Explain the work of Gregor Mendel as the 'Father of Genetics'.

The Science of Heredity

Why do you have your mother's eyes or your father's smile? Heredity is the passing of physical or mental characteristics, called traits, from parents to offspring. The science that studies heredity is called genetics.

Gregor Mendel: The Father of Genetics

Our modern understanding of genetics began with the work of an Austrian monk named Gregor Mendel in the 1860s. Mendel studied heredity by experimenting with pea plants. He carefully tracked traits like flower color, seed shape, and plant height.

Through his experiments, Mendel discovered that traits are determined by discrete 'factors' that are passed down from parents. We now call these factors genes.

Key Genetic Terms

Gene: A segment of DNA that codes for a specific trait.

Allele: Different forms or variations of a single gene. For example, for the 'flower color' gene in peas, there is a purple allele and a white allele.

Dominant Allele: An allele that will always be expressed if it is present. It 'masks' the recessive allele. It is represented by a capital letter (e.g., P for purple).

Recessive Allele: An allele that will only be expressed if two copies are present. It is masked by the dominant allele. It is represented by a lowercase letter (e.g., p for white).

Genotype: The actual combination of alleles an organism has for a trait. It's the 'genetic code'. (e.g., PP, Pp, or pp).

Phenotype: The physical appearance or observable trait that results from the genotype. It's what you actually see. (e.g., purple flowers or white flowers).

Homozygous: Having two of the same alleles for a trait (e.g., PP or pp). Also called purebred.

Heterozygous: Having two different alleles for a trait (e.g., Pp). Also called a hybrid.

A plant with the genotype PP will have purple flowers. A plant with the genotype Pp will also have purple flowers, because the purple allele (P) is dominant over the white allele (p). The only way for a plant to have white flowers is to have the genotype pp.

The Punnett Square

A Punnett square is a simple diagram used to predict the possible genotypes and phenotypes of the offspring from a genetic cross.

Example: Let's cross two heterozygous pea plants (Pp x Pp).

1.Draw a 2x2 grid.

2.Write the alleles from one parent across the top (P, p).

3.Write the alleles from the other parent down the side (P, p).

4.Fill in the boxes by combining the letters from the top and side.

| | P | p |

| :--- | :--: | :--: |

| P | PP | Pp |

| p | Pp | pp |

Results:

Genotypic Ratio: 1 PP : 2 Pp : 1 pp

Phenotypic Ratio: 3 Purple : 1 White (Because PP and Pp both result in purple flowers)

This means there is a 3 in 4 (75%) chance the offspring will have purple flowers, and a 1 in 4 (25%) chance it will have white flowers.

Key Terms

**Genetics: The scientific study of heredity and the variation of inherited characteristics.
**Heredity: The passing on of physical or mental characteristics genetically from one generation to another.
**Trait: A specific characteristic of an organism, such as eye color or height.
**Allele: One of the different forms of a gene. For example, the gene for eye color has alleles for blue, brown, etc.
**Dominant Allele: An allele whose trait always shows up in the organism when the allele is present.
**Recessive Allele: An allele that is masked when a dominant allele is present. Its trait only appears if two copies are present.
**Genotype: An organism's genetic makeup, or allele combinations.
**Phenotype: An organism's physical appearance, or visible traits.
**Punnett Square: A chart that shows all the possible combinations of alleles that can result from a genetic cross.