Dihybrid Crosses General Biology Monohybrid cross Some of

Dihybrid Crosses General Biology

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

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

Monohybrid Examples Rr x rr rm f spe ds o Kin r R r General Biology Rr rr g fe r gs so ind K 50% chance of being Rollers Rr 50% chance of being non-rollers rr **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.

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

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

Dihybrid cross P true-breeding yellow, round peas Y = yellow R = round x YYRR yyrr true-breeding green, wrinkled peas y = green r = wrinkled yellow, round peas F 1 generation (hybrids) 100% Yy. Rr self-pollinate F 2 generation General Biology 9: 3: 3: 1 9/16 yellow round peas 3/16 green round peas 3/16 yellow wrinkled peas 1/16 green wrinkled peas

What’s going on here? § If genes are on different chromosomes… u u how do they assort in the gametes? together or independently? Yy. Rr YR General Biology yr Is it this? Or this? YR Yy. Rr Yr Which system explains the data? y. R yr

Yy. Rr Is this the way it works? YR yr or Yy. Rr x Yy. Rr YR YR yr General Biology yr YYRR Yy. Rr yyrr Well, that’s NOT right! Yy. Rr YR Yr y. R yr 9/16 yellow round 3/16 green round 3/16 yellow wrinkled 1/16 green wrinkled

Yy. Rr Dihybrid cross YR Yy. Rr x Yy. Rr YR Yr y. R yr or YR Yr yr Yy. Rr Yyrr y. R Yy. Rr yy. RR yy. Rr yr YYRr Yy. Rr General Biology YYrr Yyrr yy. Rr yyrr y. R yr 9/16 yellow round 3/16 green round YR YYRr Yy. RR Yy. Rr Yr Yy. Rr BINGO! 3/16 yellow wrinkled 1/16 green wrinkled

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

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

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

Rules of a Dihybrid Cross 1. Parents now have 4 letters! (2 traits) 2. Gametes have 2 letters (1/2 of parent!) 3. Gametes must have 1 gene from EACH trait. (1 of each letter on the outside) 4. Offspring (inside boxes) have 4 letters 5. Always write the dominant letter first, if present. 6. Don’t split your genes! General Biology

Example: Yy. Rr x Yy. Rr § First! Find the possible gamete combinations (FOIL). Y General Biology Yr R y YR y. R r yr

Example: Yy. Rr x Yy. Rr YR y. R Yr yr YYRR Yy. RR YYRr Yy. RR yy. RR Yy. Rr yy. Rr YYRr Yy. Rr YYrr Yy. Rr yy. Rr Yyrr yyrr YR y. R Yr yr Phenotypes: 9/16 yellow, round General Biology 3/16 green, round 3/16 yellow, wrinkled 1/16 green, wrinkled

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

Dihybrid Problem 1 Cont’d. 1 st – Assign letters. B, b and E, e 2 nd – Figure out the parents. bbee x Bb. Ee 3 rd – Figure out the possible gametes (FOIL) be x BE, Be, b. E, be 4 th – Work out using a large Punnett Square § § General Biology§ 2 of each letter on the outside 4 letters on the inside Capitol letter first

Dihybrid Problem 1 Cont’d BE Bb. Ee Be Bbee b. E be bb. Ee bbee be be Genotypes: Bb. Ee, 4/16 Bbee, 4/16 bb. Ee, 4/16 bbee, 4/16 General Biology

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

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