1 Mendelian Genetics

2 Gregor Mendel ( ) Discovered the laws of heredity by working with garden peas.

Why use pea plants???  Mendel created true breeding plants through self-pollination to produce the PARENTAL generation.  Then Mendel used these parent plants to produce the F 1 generation. 3

Parental Generation (P)  First Filial Generation (F 1 ) What happened to the short trait?

The same thing happened for several traits! 5 Seed Shape Flower Position Seed Coat Color Seed Color Pod Color Plant Height Pod Shape Round Wrinkled Round Yellow Green Gray White Smooth Constricted Green Yellow Axial Terminal Tall Short YellowGraySmoothGreenAxialTall

6 Principle of Dominance Some alleles are dominant, some are recessive.  dominant-allele that is visible and observable  represented by a capital letter in the genotype  Covers up the recessive allele for a given trait  recessive-allele of an organism that is masked by a dominant trait IF the dominant allele is present.  Represented by a lower case letter

7 Genetic Terminology  Genotype - the allele combination an organism contains  Each genotype contains 2 alleles – one on each homologous chromosome  Phenotype - the physical expression of a trait Ex: tall, short

8 Types of Genotypes  homozygous-organism with two alleles the same  Ex: homozygous dominant (TT) and homozygous recessive (tt)  Also called “pure”  heterozygous-organism with both a dominant allele and a recessive allele  Individuals heterozygous for a given trait are carriers for the recessive allele.  Ex: Tt  Also called a “hybrid”

The alleles for height in pea plants are tall (T) and short (t). Which allele(s) is(are) dominant? A.Tall B.Short C.Tall and short D.Neither tall nor short 9

The genotype tt will have the phenotype A.Homozygous short B.Heterozygous short C.Heterozygous tall D.Homozygous tall 10

Which of the following does NOT describe the genotype Tt? A.Tall B.Homozygous C.Heterozygous D.Dominant 11

12 Law of Dominance

13 Following the Generations Cross 2 Pure Plants Results in all Hybrids Cross 2 Hybrids Results in 3Tall & 1Short

14 Principle of Segregation (figure 11-5 page 266) Mendel observed that in future generations the recessive traits reappear. Therefore, the alleles must separate at some time.Mendel observed that in future generations the recessive traits reappear. Therefore, the alleles must separate at some time. During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. Parents randomly pass on one of their alleles for each gene in the egg or sperm.During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. Parents randomly pass on one of their alleles for each gene in the egg or sperm.

15 Probability-likelihood that an event will occur.  used to determine possible allele combos for a given trait Equation: Probability= # of one kind of possible outcome total # of all possible outcomes total # of all possible outcomes  Examples: tossing a coin = rolling a die = rolling a die = sex of child = sex of child =

Monohybrid Crosses – shows allele combinations that might result from two parents Does not represent actual outcome. The more individuals in a sample, the closer you get to the expected ratios shown in a Punnett square. Does not represent actual outcome. The more individuals in a sample, the closer you get to the expected ratios shown in a Punnett square.

17 In pea plants, tall height is dominant over short height. A heterozygous plant is pollinated by another heterozygous plant. What are the probable phenotypic and genotypic ratios of the plant’s offspring? 1.Choose a letter to represent the genes in the cross 2.Write the genotypes of the parents.

18 3. Determine the possible gametes that the parent will produce. Remember, alleles will segregate when gametes are formed. Enter the gametes at the top and side of the Punnett square.

19 4. Complete the Punnett Square by writing the alleles from the gametes in the appropriate boxes. This step represents fertilization. Letters inside the boxes represent the probable genotypes of the offspring.

20 5. Determine genotypic and phenotypic ratios of the offspring

21 Let’s Solve A Problem! Yellow seeds are dominant to green seeds. Determine the genotypic and phenotypic ratios of the offspring created from a cross of a plant heterozygous for yellow seeds and a plant with green seeds.

22 Yellow seeds are dominant to green seeds. Determine the genotypic and phenotypic ratios of the offspring created from a cross of a plant heterozygous for yellow seeds and a plant with green seeds.

23 Principle of Independent Assortment Alleles for different traits are distributed to sex cells (& offspring) independently of one another.  can be illustrated using dihybrid crosses. Dihybrid Cross - used to determine all possible combinations of alleles for two traits

24 Dihybrid Cross Traits: Seed shape & Seed color Alleles: Alleles: R round r wrinkled Y yellow y green RrYy x RrYy All possible gamete combinations

25 Dihybrid Cross

26 Dihybrid Cross Round/Yellow: 9 Round/green: 3 wrinkled/Yellow: 3 wrinkled/green: 1 9:3:3:1 Figure 11-9 and pages