Genetics: Key Concepts & Problems (Monohybrid & Dihybrid)

Feature (Character): Something you can see in a living thing, like eye color or height.
Version of a Feature (Trait): How a feature actually looks, like "blue eyes" or "tall."
Looks (Phenotype): What you observe outside (e.g., a tall plant).
Genetic Code (Genotype): The hidden genetic information that determines how something looks.
Gene Varieties (Alleles): Different versions of a gene.
Strong (Dominant): Shown with a capital letter (e.g., T). This version usually appears.
Weak (Recessive): Shown with a small letter (e.g., t). This version is often hidden if a strong one is present.
How Genes Show Up:
- A tall plant has either two strong genes (T T) or one strong and one weak gene (T t).
- A short plant must have two weak genes (t t) because if a strong T were present, it would be tall.

Rule of Separation (Law of Segregation): You get one gene copy from each parent for a trait. These two copies separate when you make sex cells, so each sex cell gets only one copy.
Rule of Independent Mixing (Law of Independent Assortment): Genes for different traits usually pass on independently, especially if they are far apart on different chromosomes. (For example, hair color genes don't affect eye color genes.)
Sometimes, gene parts can swap (crossing over), which can mix how traits are passed on if they are close together.

What it is: Mating two individuals that are different for only one trait (e.g., a tall plant and a short plant).
When both parents have mixed genes (like Aa x Aa):
- What they look like (Phenotype): Expect roughly 3 with the strong trait and 1 with the weak trait (a 3:1 ratio).
- Their genetic code (Genotype): Expect 1 pure strong (AA), 2 mixed (Aa), and 1 pure weak (aa) (a 1:2:1 ratio).
Punnett Square: A simple chart used to predict all possible gene combinations in offspring.

Why use it?: To find out if an organism showing a strong trait (e.g., a tall plant) has pure strong genes (AA) or mixed genes (Aa).
How?: Mate the unknown organism with one that has two weak genes (aa).
- If the unknown organism was pure strong (AA), all offspring will show the strong trait.
- If the unknown organism was mixed (Aa), about half the offspring will show the strong trait and half will show the weak trait (a 1:1 ratio).

Polydactyly: Having extra fingers or toes, usually from a strong gene (P). Having five fingers is from a weak gene (p).
Cross Example: A female with pure extra fingers (PP) mates with a male with pure five fingers (pp).
- First Kids (F1 generation): All will have mixed genes (Pp) and show extra fingers.
- Second Kids (F2 generation, from mating two F1s):
  - Genes: 1/4 pure strong (PP), 1/2 mixed (Pp), 1/4 pure weak (pp).
  - Looks: About 3 out of 4 will have extra fingers, and 1 out of 4 will have five fingers (a 3:1 ratio).
Remember, these are averages; actual numbers in a small family might be a bit different.

What it is: Looking at two traits at the same time (e.g., skin color and navigation skill) where their genes don't affect each other.
Cross Example: Mating two individuals both carrying mixed genes for two traits (AaBb x AaBb).
- What they look like (Phenotype) in F2: Expect about 9 to show both strong traits, 3 strong for the first and weak for the second, 3 weak for the first and strong for the second, and 1 weak for both (a 9:3:3:1 ratio).