Study Notes on Punnett Squares and Mendelian Genetics

Punnett Squares

Introduction to Punnett Squares

  • Punnett Squares were developed by Reginald Punnett in the early 1900s to extend Mendel's work on inheritance.

  • They provide a schematic approach to predicting the outcomes of breeding.

  • The probabilities of resulting genotypes and phenotypes of offspring resulting from parent genotypes can be calculated easily for single traits.

  • The Punnett Square tool determines possible outcomes based on the alleles of each parent.

  • It is based on Mendel's law of segregation, which states that a parent can contribute only one of two possible alleles to an offspring.

Setting Up a Punnett Square

  • Determining Parental Alleles: Each parent can only contribute one allele of each gene to each offspring, as gametes are haploid.

    • During meiosis, parents distribute one of their two alleles to each haploid gamete.

    • Each allele is equally likely to be passed on, meaning:

    • AA parent can contribute A or A to each gamete.

    • Aa parent can contribute A or a.

    • aA parent can contribute a or A.

    • aa parent can contribute a or a.

  • Typically, the top of the Punnett Square contains the alleles from one parent represented as capital letters and those of the other parent are represented similarly on the side.

  • Example Setup: For a true-breeding plant with purple flowers (PP) and a true-breeding plant with white flowers (pp), set up the squares as follows:

    • Top: PP

    • Side: pp

Filling In the Punnett Square

  • The Punnett Square is divided into four equal smaller squares.

  • Fill each square with one letter from the top and one letter from the side; usually, the capital letter (dominant) is written first.

    • For example:

    P

    P

    p

    Pp

    p

    Pp

  • Each square represents a 25% chance of the genotype being the offspring's genotype following random fertilization of gametes.

  • F1 Generation Prediction: Since P is dominant, all offspring from this example will have purple flowers. Thus, the possible genotypes are:

    • P, P, p, p leads to Pp, Pp, Pp, Pp.

Crossbreeding F1 Generation for F2 Generation

  • When crossing two offspring (Pp x Pp) of the F1 generation to produce F2, use the Punnett Square again:

    • Setup for F2 Generation:

    • Top: P, p

    • Side: P, p

Probability of F2 Generation Genotypes

  • Results from F2 Punnett Square:

    • Genotypes: PP, Pp, Pp, pp.

    • Probability for each genotype:

    • PP: 25%

    • Pp: 50%

    • pp: 25%

  • Thus, the genotypic ratio is 1:2:1.

Phenotypic Ratios in F2 Generation

  • Considering phenotypes:

    • PP and Pp will have purple flowers.

    • pp will have white flowers.

    • Phenotypic Ratio: Out of the offspring:

    • 75% purple flowers

    • 25% white flowers

  • Thus, the expected phenotypic ratio for this example is 3:1.

Definitions

  • Genotype: The organism's set of genes, e.g., PP, Pp, pp.

  • Phenotype: The observable characteristics of an organism, e.g., purple or white flowers.

  • All genotypes PP and Pp result in purple flowers because P is dominant.

  • Only the pp genotype results in white flowers since p is recessive.

Example Applications of Punnett Squares

Example 1: Tall and Short Pea Plants

  • If a homozygous tall pea plant (TT) is crossed with a heterozygous tall pea plant (Tt), the offspring percentages are:

    • Genotypes: 50% homozygous dominant (TT), 50% heterozygous (Tt)

    • Phenotypes: 100% tall plants.

    • Punnett Square calculation:

    T

    T

    T

    TT

    t

    Tt

Example 2: Yellow and Green Seed Pea Plants

  • If two heterozygous yellow seed pea plants (Yy) are crossed:

    • Yellow seed (Y) is dominant to green seed (y).

    • Expected offspring:

    • Genotype distribution: 25% homozygous dominant (YY), 50% heterozygous (Yy), 25% homozygous recessive (yy)

    • Phenotype distribution: 75% yellow, 25% green.

    • Punnett Square representation:

    Y

    y

    Y

    YY

    y

    Yy

    y

    Yy

    y

    yy

Test Crosses

  • A test cross is used to determine the genotype of an individual by breeding it with a homozygous recessive individual.

  • If any offspring display the recessive phenotype, the parent must be heterozygous.

Dominance and Commonality in Populations

  • Importantly, just because an allele is dominant does not imply it is more common in the population compared to its recessive counterpart.

  • Example of Dominant Allele: Polydactylism (extra fingers or toes) occurs in 1 out of 400 births.

  • The allele for polydactylism is dominant to the more common five digits per appendage trait, indicating that recessive alleles can be more prevalent in a population despite the presence of dominant traits.