Punnett Squares (don’t need to copy) In 1905 Reginald Punnett, and English biologist devised a shorthand way of finding the expected proportions of possible genotypes in the offspring of a cross. This method is called a punnett square. If you know the genotypes of the parents, you can predict the possibility of the genotypes of the offspring.

Mendel’s Dihybrid Crosses Mendel then performed another set of crosses where he took two different traits (round, yellow vs wrinkled green) of pea seeds to see if the traits would stay together in the F1 and F2 generations, or if they would be separated. This is called a di (meaning two), dihybrid cross.

In the F1 generation all the seeds were yellow and round meaning that both of these traits are dominant. In the second (F2) generation, Mendel let the plants pollinate themselves. Not only did Mendel find round yellow and wrinkled green seeds, but he found round green and wrinkled yellow seeds as well. The traits separated themselves.

Mendel formed his second law: The Law of Independent Assortment This law states that genes for different traits are inherited independently of each other.

If you know the genotypes of the parents, you can predict the possibility of the genotypes of the offspring.

Show punnett square using Tt vs Tt. After you determine the genotypes, you can determine the phenotypes. T t TtTt

Show punnett square using Tt vs Tt. After you determine the genotypes, you can determine the phenotypes. T T T t t t T t TtTt

Genotype = ¼ homozygous dominant = ½ heterozygous = ¼ homozygous recessive Phenotype= ¾ tall = ¼ short

Example 1: HOMOZYGOUS X HOMOZYGOUS P = Dominant Purple p = Recessive White Genotype PP X Genotype pp –The combination of Alleles in the Punnett Square indicate all the possible genotypes that can result from the cross. The predicted genotype is Pp in every case. –There is a 100% probability that the offspring will have the Genotype Pp (Heterozygous) and the phenotype purple flower color.

Example 1: Homozygous dominant x Homozygous recessive

Example 2: HOMOZYGOUS X HETEROZYGOUS B = Dominant Black b = Recessive Brown Genotype BB X Genotype Bb –The combination of Alleles in the Punnett Square indicates all the possible genotypes that can result from the cross. The predicted genotype BB is 2/4 or 50 % and the genotype Bb is 2/4 or 50 %. –There is a 50% probability that the offspring will have the Genotype BB (Homozygous Dominant) & the Phenotype Black. –There is a 50% probability that the offspring will have the Genotype Bb (Heterozygous) and the Phenotype Black. –The probability of the Phenotype of Black coat in every case is 4/4 or 100%.

Example 2: HOMOZYGOUS X HETEROZYGOUS

Example 3: HETEROZYGOUS X HETEROZYGOUS B = Dominant Black b = Recessive Brown Genotype Bb X Genotype Bb –The combination of Alleles in the Punnett Square indicates all the possible genotypes that can result from the cross. The predicted genotype BB is 1/4 or 25 % and the genotype Bb is 2/4 (1/2) or 50 % and genotype bb is 1/4 or 25%. –3/4 or 75% of the offspring from this cross are predicted to have a Black Coat and 1/4 or 25% of the offspring are predicted to have a Brown Coat. –The Ratio of the Genotypes that appear in offspring is called the GENOTYPIC RATIO. FIG. 9-7 = 2:1:1 –The Ratio of the Phenotypes that appear in offspring is called the PHENOTYPIC RATIO. FIG. 9-7 = 3:1

Example 3: HETEROZYGOUS X HETEROZYGOUS

Example 5: INCOMPLETE DOMINANCE –Sometimes, the F1 offspring will have a Phenotype in between that of the Parents, a relationship called INCOMPLETE DOMINANCE. –Incomplete dominance occurs when Two or More Alleles Influence the Phenotype, resulting in a Phenotype Intermediate between the two dominant Traits. –In four o’clocks (these flowers) both the Allele for Red Flowers (R) and the Allele for White Flowers (R’) influence the Phenotype.

–Neither Allele is completely dominant over the other Allele. –When four o’clocks self-pollinate, red flowering plants produce only red flowering offspring, and white flowering plants only produce white flowering offspring. –However, when red four o’clocks are crossed with white four o’clocks the F1 offspring all have pink flowers. 100% of the offspring of this cross have the RR’ Genotype, which results in the Pink Phenotype.

RR’ R’ RR’ R’R’

Snapdragons have incomplete dominance; red R, white R’, and pink RR’.

Example 6: CODOMINANCE –CODOMINANCE occurs when Both Alleles for a gene are Expressed in a Heterozygous offspring. In Codominance neither Allele is Dominant or Recessive, nor do alleles blend in the phenotype. A capital letter is used for both traits. In chickens, both black and white feathered birds are dominant. So what happens when the bird is heterozygous? The bird has both black feathers and white feathers! See figure 12.8 page 323.

PREDICTING RESULTS OF DIHYBRID CROSSES –A DIHYBRID CROSS is a cross between individuals that involves TWO Pairs of Contrasting Traits. –Predicting the results of a Dihybrid Cross is more complicated that predicting the results of a Monohybrid cross because there are more possible combinations.

R = Dominant Round r = Recessive Wrinkled Y = Dominant Yellow y = Recessive Green –Suppose you wanted to predict the results of a cross between a pea plant that is homozygous for round, yellow seeds (RRYY), and one the is homozygous for wrinkled, green seeds (rryy). –The Punnett square is used to predict the results of the cross, and contains 16 boxes. –The independently assorted alleles from one parent – RY, RY, RY, RY, listed along the left side of the Punnett Square. –The independently assorted alleles from one parent – ry, ry, ry, ry, listed along the top of the Punnett Square. –The Genotype for all the offspring of this cross will be Heterozygous for both Traits, RrYy, and the Genotype of all the offspring will have Round and Yellow Seeds.

Example 1: HOMOZYGOUS X HOMOZYGOUS

Example 2: HETEROZYGOUS X HETEROZYGOUS

Probability Punnett squares are good for showing the possible combinations of gametes and the likelihood that each will occur. In real life, the results are like tossing a coin; it follows the rule of chance. The larger the sample, the more likely that the probability will occur. i.e. ¾ of the pea plants are round.

PROBABILITY –Probability is the likelihood that a specific event will occur or is the likely outcome a given event will occur from random chance. –A Probability may be a Decimal (0.75), a Percentage (75%), or a Fraction (3/4). Probability is determined by the following Equation: PROBABILITY = Number of times an event is expected to happen Number of opportunities for an event to happen