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Definition of Phenotype and Genotype
- Phenotype: The expressed traits of an organism; for example, in this context, the finger length is either short or long. - Genotype: The genetic makeup of an organism, represented by alleles.

Short Fingers vs Long Fingers
  - Genotype Notation:
    - Short fingers:
      - Homozygous dominant: $SS$ (capital S, capital S)
      - Heterozygous: $Ss$ (capital S, lowercase s)
    - Long fingers:
      - Homozygous recessive: $ss$ (lowercase s, lowercase s)
  - Phenotype Examples:
    - Genotype $SS$ or $Ss$ results in short fingers (dominant trait).     - Genotype $ss$ results in long fingers (recessive trait).   - Explanation:
    - The phenotype (observable trait) is expressed when at least one dominant allele ( $S$ ) is present. Only when both alleles are recessive ( $ss$ ), the recessive trait (long fingers) is seen.

Tall vs Short Pea Plants
  - Genotype Notation:
    - Tall plants:
      - Homozygous dominant: $TT$
      - Heterozygous: $Tt$
    - Short plants:
      - Homozygous recessive: $tt$
  - Phenotype Examples:
    - A pea plant with phenotype of shortness must have genotype $tt$ (homozygous recessive).   - Expression of Recessive Traits:
    - The recessive trait (short pea plants) only expresses itself when both alleles are lowercase.     - If a dominant trait allele is present (capital T), it expresses over the recessive trait.

Purpose of a Punnett Square:
  - A tool used to predict the probability of offspring genotypes and phenotypes from parental genetic makeup.   - Assign allele symbols and determine each parent's genotype.
  - Example: Purple flowers being dominant over white flowers
    - Use capital $P$ for purple (dominant) and lowercase $p$ for white (recessive).
Types of Genotypes:
  - Homozygous:
    - Two identical alleles (e.g., $PP$ or $pp$ )
  - Heterozygous:
    - One dominant and one recessive allele (e.g., $Pp$ ).
Example Phenotype Ratios:
- In a cross of $PP$ (homozygous dominant) x $pp$ (homozygous recessive), all offspring will be $Pp$ (purple). - In a heterozygous cross $Pp$ x $Pp$ , results will show: - 75% purple flowers ( $PP$ or $Pp$ )
- 25% white flowers ( $pp$ ).

Concept of Dihybrid Crosses:
  - A genetic cross involving two traits, typically resulting in a 9:3:3:1 phenotypic ratio.   - Example with yellow and green seed traits:     - Traits: Yellow seeds ( $Y$ ) vs Green seeds ( $y$ ), Round shape ( $R$ ) vs Wrinkled shape ( $r$ ).   - Offsprings generated from dihybrid crosses display various combinations of these traits based on the alleles provided by the parents.   - Calculating phenotypes:
    - For example, a dihybrid cross can be made, resulting in specific ratios such as:       - 9 offspring with yellow, round seeds
      - 3 offspring with yellow, wrinkled seeds
      - 3 offspring with green, round seeds
      - 1 offspring with green, wrinkled seeds.

Tips for Studying:
- Understand the differences between homozygous and heterozygous genotypes. - Practice with Punnett squares and their interpretations to solidify understanding of genetic probabilities. - Familiarize with common ratios arising from monohybrid and dihybrid crosses (e.g., 3:1 for monohybrid and 9:3:3:1 for dihybrid).
Important Reminders:
- Make sure to clarify any confusion before exams, focusing on genetic principles and trait expressions.