BIO MIDTERM: PUNNET SQUARES

Alleles and Monohybrid Crosses

• Alleles: Different versions of a gene that arise by mutation and are found at the same place on a chromosome.

  • Example: Dominant allele (capital letter) and recessive allele (lowercase letter).

• Monohybrid: Refers to an individual that is heterozygous for a single trait (e.g., one dominant and one recessive allele).

  • Example: If the trait is freckles: F (dominant) for freckles and f (recessive) for no freckles.

• Monohybrid Cross: A genetic cross between two heterozygous individuals (e.g., Ff x Ff).

Phenotypic Ratios and Gametes

• Phenotypic Ratio: The ratio of different phenotypes in the offspring from a genetic cross.

  • In a monohybrid cross, the expected phenotypic ratio is 3:1.

  • Breakdown:

    • 3 offspring with the dominant trait (at least one capital allele).

    • 1 offspring with the recessive trait (two lowercase alleles).

• Gamete Formation: To determine potential offspring, separate alleles into gamete boxes.

  • Example with alleles: Ff results in gametes F and f.

Cross Example: Straight Thumb Trait

• Straight Thumb (dominant): Represented as H (dominant) and h (recessive).

  • Perform the cross:

    • Parent 1: Hh

    • Parent 2: Hh

• Possible combinations: HH, Hh, hH, hh.

• The resulting offspring's genotype frequency:

  • 3 dominant (HH, Hh, hH) to 1 recessive (hh).

  • Gives a 75% chance of having a straight thumb.

Punnett Squares

• Punnett Squares: Used to predict the ratio of genotypes and phenotypes in offspring.

  • Arrange gametes from both parents in a grid.

  • Count the resulting combinations for genotypes and phenotypes.

Blood Types and Codominance

• Blood Types: Involves alleles A (I^A), B (I^B), and O (i).

  • I^A I^A or I^A i results in Blood Type A.

  • I^B I^B or I^B i results in Blood Type B.

  • I^A I^B results in Blood Type AB (codominance).

• In AB type, both traits are expressed equally.

X-Linked Recessive Disorders

• Color Blindness: An example of an X-linked recessive disorder caused by a mutation.

  • Genotypes involved:

    • Normal vision: X^N (dominant).

    • Color blindness: X^n (recessive).

• Females must have two copies of the recessive allele to express color blindness (X^nX^n) because they have two X chromosomes.

  • Males require only one copy of the mutant allele (X^nY) to express the disorder due to one X and one Y chromosome.

• Carriers: Females with one normal and one mutant allele (X^N X^n) are carriers, meaning they do not express the trait, but can pass it on to their offspring.