MENDELIAN GENETICS Monohybrid Crosses Vs Dihybrid Crosses Dihybrid

MENDELIAN GENETICS Monohybrid Crosses Vs. Dihybrid Crosses

Dihybrid Cross • Mendel’s Law of Independent Assortment inheritance pattern of one trait will not affect the inheritance pattern of another • Independent assortment occurs during meiosis • Use Punnett Square to determine probability of how the alleles of 2 traits are being inherited at a time

Dihybrid Cross Step 1: Identify Alleles of Traits In snapdragons, tallness (T) is dominant to dwarfness (t). Flowers can be red (R), white (r), or pink (Rr) in color. A dwarf, pink snapdragon is crossed with a plant homozygous for tallness & for red flowers. Give the possible genotypes and corresponding phenotypes for all of the possible F 1 generation.

Step #2: Determine Genotypes of Parents • dwarf, pink snapdragon is crossed with a plant homozygous for tallness & red flowers • dwarf, pink = tt. Rr • homozygous tall, red = TTRR • tt. Rr x TTRR

Step #3: Determine Genotypes of Gametes • dwarf, pink snapdragon is crossed with a plant homozygous for tallness & red flowers • Use FOIL method dwarf, pink = t t R r t. R tr

Step #3: Determine Genotypes of Gametes • dwarf, pink snapdragon is crossed with a plant homozygous for tallness & red flowers • dwarf, pink = tt. Rr t. R tr • homozygous tall, red = TTRR TR TR

Step #4: Punnett Square gametes from TALL red parent Punnett square determines the genotypes of the offspring gametes from dwarf pink parent TR TR TR t. R Tt. RR Tt. RR tr Tt. Rr Tt. Rr TR

Step #5: Identify Phenotype Ratios in Offspring Tt. RR Tall, red = 8/16 Tall, pink = 8/16 Tt. Rr Tt. RR Tall, white = 0/16 Short, red = 0/16 Short, pink = 0/16 Tt. Rr Short, white = 0/16

Dihybrid Punnett Square Practice: Example 1 • cross a heterozygous round, green pea with a wrinkled, heterozygous yellow pea. • 1. identify the alleles of traits Y = yellow R = round y = green r = wrinkled • 2. show cross of the two parents Rryy x rr. Yy • 3. show possible genotypes of gametes use FOIL method from parent 1: Ry, ry, Ry, ry from parent 2: r. Y, ry, r. Y, ry

Dihybrid Punnett Square Practice: Example 1 • cross a heterozygous round, green pea with a wrinkled, heterozygous yellow pea. • 4. create & complete Punnett Square for Rryy x rr. Yy Ry r. Y ry ry Ry ry

Dihybrid Punnett Square Practice: Example 1 • cross a heterozygous round, green pea with a wrinkled, heterozygous yellow pea. • 4. create & complete Punnett Square for Rryy x rr. Yy Ry ry r. Y Rr. Yy rr. Yy ry Rryy rryy

Dihybrid Punnett Square Practice: Example 1 • cross a heterozygous round, green pea with a wrinkled, heterozygous yellow pea. • 5. determine phenotypic ratios Ry ry r. Y Rr. Yy rr. Yy ry Rryy rryy Genotype Phenotype ¼ Rr. Yy ¼ Round, Yellow ¼ Rryy ¼ Round, green ¼ rr. Yy ¼ wrinkled, Yellow ¼ rryy ¼ wrinkled, green

Dihybrid Punnett Square Practice: Example 2 • In guinea pigs, allele for short hair (S) is dominant to long hair (s), & allele for black hair (B) is dominant over allele for brown hair (b). Cross 2 parents that are heterozygous for both traits?

Dihybrid Punnett Square Practice: Example 2 • In guinea pigs, allele for short hair (S) is dominant to long hair (s), & allele for black hair (B) is dominant over allele for brown hair (b). Cross 2 parents that are heterozygous for both traits? • 1. identify the alleles of traits S = short hair B = black hair s = long hair b = brown hair • 2. show cross of the two parents Ss. Bb x Ss. Bb • 3. show possible genotypes of gametes from parent 1: SB, Sb, s. B, sb from parent 2: SB, Sb, s. B, sb

Dihybrid Punnett Square Practice: Example 2 SB Sb s. B sb • What is the probable offspring phenotype SB SSBb Ss. BB Ss. Bb ratio for a cross involving 2 parents that are heterozygous for Sb SSBb SSbb Ss. Bb Ssbb both traits? • 9/16 Black, Short hair s. B Ss. Bb ss. BB ss. Bb • 3/16 Black, long hair • 3/16 brown, Short hair sb Ss. Bb Ssbb ss. Bb ssbb • 1/16 brown, long hair