Genetics Fundamentals: Vocabulary, Inheritance, and Crosses

Genetics Vocabulary

• Gene: The basic physical and functional unit of heredity, composed of DNA.
• Chromosomes: Thread-like structures of DNA and proteins found in the nucleus of most living cells.
• DNA: The molecule that carries genetic instructions for development and function.
• Karyotype: A visual representation of all chromosomes within a cell, used to identify chromosomal counts.
• Genotype: The specific genetic makeup of an organism that determines its traits.
• Phenotype: Observable physical characteristics, such as eye color, hair color, or height.
• Dominant: A gene that expresses its trait even if only one copy is present.
• Recessive: A gene that only expresses its trait when two copies are present.
• Homozygous: Having two identical versions of a gene (e.g., AA or aa).
• Heterozygous: Having two different versions of a gene (e.g., Aa).

Inheritance and DNA

1. What are the types of inheritance?

Inheritance describes how traits are passed from parents to offspring.

• Dominant: Only one dominant gene is required to express the trait (e.g., brown eyes).
• Recessive: Two recessive genes are required to express the trait (e.g., blue eyes).
• Codominant: Both traits are expressed simultaneously (e.g., AB blood type).
• Incomplete Dominance: Traits blend together (e.g., a red flower and a white flower producing a pink flower).

Explanation: Genes act as biological instructions inherited from both parents.

2. What is DNA and its structure?

DNA is the genetic code that dictates how living organisms grow and function. It features a double helix structure, resembling a twisted ladder.

• Sides: Composed of sugar and phosphate.
• Steps (Bases): Adenine (A), Thymine (T), Cytosine (C), and Guanine (G).
• Pairing: A always pairs with T; C always pairs with G.

Punnett Squares and Pedigrees

How to use a Punnett Square

A Punnett Square predicts potential offspring traits. For parents with genotypes Bb and Bb:

Results: 75% Brown eyes (BB, Bb, Bb), 25% Blue eyes (bb).

Pedigree Symbols

A pedigree is a family tree tracking traits across generations.

• □: Male
• ○: Female
• ● or ■: Expresses the trait
• ○ or □ (empty): Does not express the trait
• ─: Marriage or parents
• │: Children

Dihybrid Crosses

A dihybrid cross examines two traits simultaneously. For parents with genotype BbTt (B=Brown fur, b=White fur, T=Long tail, t=Short tail):

• Step 1: Possible combinations are BT, Bt, bT, and bt.
• Step 2: Use a 4x4 Punnett Square to map combinations.
• Step 3: The phenotypic ratio typically results in 9:3:3:1.

Memory Trick: "Di" means two; a dihybrid cross tracks two traits at once, whereas a standard Punnett Square tracks one.