Monohybrid & Dihybrid Crosses Probability and Genetics!

Why did Mendel use Pea Plants?

- easy to control pollination Pea Flower Stamens (male) produce pollen Pistil (female) produces ovules

- seven distinct, inherited traits Plant size Flower location Flower color Pod color Pod shape Seed shape Seed color

P1P1 F1F1

F1F1 F2F2

Cross Fertilization of Parents Purebred Purple-flowered Parent Purebred White-flowered Parent Cross-Fertilize All Purple-flowered Offspring Pollen P F1F1 P

F1F1 Self-Fertilize F2F2 F2F2 F2F2 F2F2 75% Purple 25% White Self-fertilization of F 1

Punnett squares The probability the next generation will exhibit a certain trait can be shown with a Punnett square. The genotype of the mother is shown on the top of the square (TT) The genotype of the father is shown on the side of the square (tt) The various combinations of genes (genotypes) are shown in the squares. All of these offspring will be (T) tall and carry the recessive (t) short allele. T T tttt Tt

Monohybrid Cross (crosses that examine the inheritance of only ONE specific trait) What happens if you cross a tall (heterozygous) plant (Tt) with a short plant (tt)? Genotypic Ratio: TT :Tt :tt 0 :2 :2 Phenotypic Ratio: Tall : Short 2 : 2 T t tttt tt Tt tt

Punnett squares If deafness in dogs are recessive (dd), show what offspring could result from two dogs that are heterozygous (Dd) 3 possible genotypes: DD, Dd, dd Genotypic Ratio: DD : Dd : dd 1 : 2 : 1 2 possible phenotypes: Not deaf or Deaf Phenotypic Ratio: Not Deaf : Deaf 3 : 1 D d DdDd DD dd Dd

Test Cross Farmer Dan breeds guinea pigs. There is a gene in guinea pigs for teeth length. Teeth can either be normal (T) or bucked (t). Farmer Dan buys what he thinks is a normal TT guinea pig. How can he be sure it is “pure” (TT) and not a heterozygote (Tt)?

Test Cross: Breed the unknown genotype with a homozygous recessive. A homozygous dominant guinea pig will result in offspring that all have normal teeth. T T t Tt Tt t T tt tt tt t Tt Tt tt tt A heterozygous guinea pig will result in a 2:2 or 1:1 ratio of normal to buck-teethed offspring.

Dihybrid Cross (crosses that examine the inheritance of only TWO different traits) Mendel also studied dihybrid crosses. – He crossed a plant with yellow round peas with a plant with green wrinkled peas.

Mendel and Dihybrid Crosses Mendel began his experiments with purebred (homozygous) parent plants. RRYY x rryy – Remember each trait has 2 genes due to getting half your genes from mom and half from dad. How many gamete options do you have for RRYY?? RY How many gamete options do you have for rryy? ry These are the gametes from the “P” (parent) generation.

Mendel and Dihybrid Crosses “P” generation: RRYY x rryy RRYY rryy RY ryRrYy ryRrYy ryRrYy ryRrYy

Mendel and Dihybrid Crosses What is the genotypic ratio? 16 RrYy:0 anything else Phenotypic ratio? 16 Round Yellow: 0 anything else RY ryRrYy ryRrYy ryRrYy ryRrYy

Mendel and Dihybrid Crosses He then crossed two “F 1 ” organisms: RrYy x RrYy RYRyrYry RY RRYYRRYyRrYYRrYy Ry RRYyRRyyRrYyRryy rY RrYYRrYyrrYYrrYy ry RrYyRryyrrYyrryy RrYy

Mendel and Dihybrid Crosses What is the genotypic ratio for the F2 generation? 1 RRYY:2 RRYy: 1 RRyy: 2 RrYY: 4 RrYy: 1 rrYY: 2 Rryy: 2 rrYy: 1rryy RYRyrYry RY RRYYRRYyRrYYRrYy Ry RRYyRRyyRrYyRryy rY RrYYRrYyrrYYrrYy ry RrYyRryyrrYyrryy

Mendel and Dihybrid Crosses Phenotypic ratio for the F2 generation? RYRyrYry RY RRYYRRYyRrYYRrYy Ry RRYyRRyyRrYyRryy rY RrYYRrYyrrYYrrYy ry RrYyRryyrrYyrryy 9:16 Round Yellow, 3:16 Round Green, 3:16 Wrinkled Yellow, 1:16 Wrinkled Green

Mendel and Dihybrid Crosses 9:16 yellow and round, 3:16 green and round, 3:16 wrinkled and yellow, 1:16 wrinkled and green.

Mendel and Dihybrid Crosses Mendel continued to find this approximately 9:3:3:1 phenotypic ratio in the F 2 generation, regardless of what 2 different traits he chose.

Mendel and Dihybrid Crosses – This is due to his Law of Independent Assortment. Each allele pair separate independently from other allele pairs during meiosis (gamete formation)… – different traits are inherited separately.

Dihybrid Cross Practice Suppose you are studying the color and texture of kernels on a corn cob. – Kernels can be either purple (R) or yellow (r). – They can also be smooth (T) or wrinkled (t). – Predict the phenotypic and genotypic outcome of crossing a dihybrid (heterozygous) purple smooth with a yellow wrinkled. Don’t forget to answer the questions on the bottom of your note template.

Answer: phenotypic and genotypic outcome of crossing a dihybrid (heterozygous for both) purple smooth with a yellow wrinkled Cross: RrYy x rryy RrYy rryy RYrYRyry RrYyrrYyRryyrryy ryRrYyrrYyRryyrryy ryRrYyrrYyRryyrryy ryRrYyrrYyRryyrryy 4:16 Purple Smooth, 4:16 Purple Wrinkled, 4:16 Yellow Smooth, 4:16 Yellow Wrinkled

Parent generation (AABB x aabb) are purebred for both traits because they are homozygous for both traits. Possible gametes AaBb-> AB, Ab, aB, ab Genotype: AAbb -> Phenotype: Purple, short