Dihybrid Crosses General Biology 2006-2007

Monohybrid cross Some of Mendel’s experiments followed the inheritance of single traits monohybrid crosses flower color seed color height General Biology 2006-2007

Monohybrid Review Tongue rolling gene is dominant to non-rolling gene. Cross a heterozygous roller with a non-roller. 1st – Identify the dominant and recessive genes 2nd – Identify parent genotypes 3rd – Now use the Punnett Square General Biology 2006-2007

Monohybrid Examples Rr x rr R r Rr rr 50% chance of being Rollers Kinds of sperm Kinds of eggs R r Rr rr 50% chance of being Rollers 50% chance of being non-rollers **This does not mean that if you have four children, 2 will automatically be able to roll their tongues and 2 will not. Each Punnett square probability is for a single event. General Biology 2006-2007

Dihybrid cross Other experiments by Mendel followed the inheritance of 2 different characters “Are genes inherited as a package?” Dihybrid crosses seed color (green vs. yellow) seed shape (round vs. wrinkled) General Biology 2006-2007

Mendel’s Dihybrid Experiment Round is dominant to wrinkled. Yellow is dominant to green. Cross a homozygous round, homozygous yellow plant with a homozygous wrinkled, homozygous green plant. Parents? RRYY x rryy General Biology 2006-2007

Dihybrid cross P YYRR yyrr 100% F1 YyRr 9:3:3:1 F2 x true-breeding yellow, round peas true-breeding green, wrinkled peas x YYRR yyrr Y = yellow R = round y = green r = wrinkled 100% F1 generation (hybrids) yellow, round peas YyRr Wrinkled seeds in pea plants with two copies of the recessive allele are due to the accumulation of monosaccharides and excess water in seeds because of the lack of a key enzyme. The seeds wrinkle when they dry. Both homozygous dominants and heterozygotes produce enough enzyme to convert all the monosaccharides into starch and form smooth seeds when they dry. self-pollinate 9:3:3:1 F2 generation 9/16 yellow round peas 3/16 green round peas 3/16 yellow wrinkled peas 1/16 green wrinkled peas General Biology 2006-2007

Which system explains the data? What’s going on here? If genes are on different chromosomes… how do they assort in the gametes? together or independently? YyRr Is it this? Or this? YyRr YR yr YR Yr yR yr Which system explains the data? General Biology 2006-2007

 Is this the way it works? YyRr x YyRr YR yr YR YYRR YyRr yr YyRr 9/16 yellow round  YR yr 3/16 green round Well, that’s NOT right! YR YYRR YyRr 3/16 yellow wrinkled yr YyRr yyrr 1/16 green wrinkled General Biology 2006-2007

 Dihybrid cross YyRr x YyRr YR Yr yR yr YR Yr yR yr YYRR YYRr YyRR or Dihybrid cross YyRr x YyRr 9/16 yellow round YR Yr yR yr YR Yr yR yr  3/16 green round YYRR YYRr YyRR YyRr BINGO! YYRr YYrr YyRr Yyrr 3/16 yellow wrinkled YyRR YyRr yyRR yyRr 1/16 green wrinkled YyRr Yyrr yyRr yyrr General Biology 2006-2007

Mendel’s 2nd law of heredity Can you think of an exception to this? Mendel’s 2nd law of heredity Law of independent assortment different genes separate into gametes independently non-homologous chromosomes align independently classes of gametes produced in equal amounts YR = Yr = yR = yr yellow green round wrinkled YyRr Yr Yr yR yR YR YR yr yr 1 : 1 : 1 : 1 General Biology 2006-2007

Law of Independent Assortment Which stage of meiosis creates the law of independent assortment? Metaphase 1 Remember Mendel didn’t even know DNA —or genes— existed! EXCEPTION If genes are on same chromosome & close together will usually be inherited together rarely crossover separately “linked” General Biology 2006-2007

Review: Mendel’s laws of heredity Law of segregation monohybrid cross single trait each allele for a gene segregates into separate gametes established at Metaphase 1 Law of independent assortment dihybrid (or more) cross 2 or more traits genes on separate chromosomes assort into gametes independently established by Metaphase 1 EXCEPTION linked genes metaphase1 General Biology 2006-2007

Rules of a Dihybrid Cross Parents now have 4 letters! (2 traits) Gametes have 2 letters (1/2 of parent!) Gametes must have 1 gene from EACH trait. (1 of each letter on the outside) Offspring (inside boxes) have 4 letters Always write the dominant letter first, if present. Don’t split your genes! General Biology 2006-2007

Example: YyRr x YyRr First! Find the possible gamete combinations (FOIL). R r Y y YR yR yr Yr General Biology 2006-2007

Example: YyRr x YyRr YR yR Yr yr YYRR YyRR YYRr YyRr yyRR yyRr YYrr Phenotypes: 9/16 yellow, round 3/16 yellow, wrinkled 3/16 green, round 1/16 green, wrinkled General Biology 2006-2007

Dihybrid Problem 1 In humans, red hair is recessive to brown hair and brown eyes are dominant to blue eyes. Cross a red-haired, blue-eyed man with a woman heterozygous for both brown hair and eyes. What are the genotypes of the offspring? General Biology 2006-2007

Dihybrid Problem 1 Cont’d. 1st – Assign letters. 2nd – Figure out the parents. 3rd – Figure out the possible gametes (FOIL) 4th – Work out using a large Punnett Square 2 of each letter on the outside 4 letters on the inside Capitol letter first Don’t split your genes! B,b and E,e bbee x BbEe be x BE, Be, bE, be General Biology 2006-2007

Dihybrid Problem 1 Cont’d BE Be bE be BbEe Bbee bbEe bbee be be be be Genotypes: BbEe, 4/16 Bbee, 4/16 bbEe, 4/16 bbee, 4/16 General Biology 2006-2007

Remember Probability? x x x ?% Use rule of multiplication to predict crosses YyRr YyRr x Yy x Rr x yyrr 1/16 ?% yy rr 1/4 1/4 x General Biology 2006-2007

Dihybrid Problem 2 In summer squash, white fruit color is dominant over yellow fruit color. Disk-shaped fruit is dominant over sphere-shaped fruit. Cross a heterozygous white, homozygous disk-shaped plant with a yellow, sphere-shaped fruit plant. What are the genotype and phenotype ratios of the offspring? General Biology 2006-2007