Biology 250 Mendelian Genetics

1. Biology 250Mendelian Genetics It’s all about Gregor Mendel

2. The History of Genetics • Genetics - The field of biology devoted to understanding how characteristics are transmitted from parents to offspring. • Believe it or Not! Genetics began with the work of an Austrian monk named Gregor Mendel (now known as the “father” of genetics!)

3. The History of Genetics (continued) • Some important facts about Gregor Mendel: • His task at the monastery was to tend the gardens. (This is where he was first introduced to his favorite test subject: peas) • He had taken courses in both science and mathematics including statistics. (This enabled him to see patterns in his experimental results which would otherwise have gone unnoticed.) • Genetics gave rise to heredity – the study of the transmission of characteristics from parents to offspring

4. Mendel’s Experiment • Mendel Experimented with pea plants. Qualities of the pea plant which made it useful for this purpose: • Has 7 characteristics which occur in 2 contrasting traits. (purple OR white flowers for example) • Pea plants can either self-pollinate (fertilize themselves) or cross-pollinate (fertilize each other)

5. Mendel’s Experiment (continued) • Plant Terminology: • Pure – Plants which are pure for a specific trait will always produce offspring with that trait. (Example: A pure purple flowering plant will always produce seeds which produce purple flowering plants.) • Strain – A group of plants which are pure for a specific trait

6. Mendel’s Experiment (continued) • Terms dealing with generations: • P1 (Parental Generation) – the starting generation of the experiment. • F1 (First Filial Generation) – the generation produced by crossing the parental generation. (Example: If your parents are the P1 generation, you are the F1 generation) • F2 (Second Filial Generation ) – the generation produced by the F1 generation (Example: if your parents are the P1 and you are the F1 then your children will be the F2)

7. Vocabulary Review! Genetics How are these two terms similar/related? Heredity Pure Strain F1 Generation F2 Generation P1 Generation

8. Mendel’s Experiment (continued) • Mendel allowed pea plants to self-pollinated until they were pure for a specific trait. These plants were his P1 Generation. • He crossed the members of his P1 Generation (white flower plant with purple flower plant) to produce the F1 generation • He allowed the F1 generation to self-pollinated to produce the F2 generation.

9. Mendel’s Results & Conclusions • Mendel concluded that each trait was controlled by a pair of alleles – alternative form of a gene • Dominant Allele – Must be expressed when present. Will mask the appearance of other alleles. • Recessive Allele – Will only show if no dominant allele is present.

10. Alleles • Dominant alleles are abbreviated with capital letters (A, B, D) • Recessive alleles are abbreviated with lower case letters (a, b, d) • Each trait will have TWO alleles. (AA, Aa, aa) • If an individual has two dominant alleles for a gene (AA) they are said to be HOMOZYGOUS DOMINANT • If an individual has two recessive alleles for a gene (aa) they are said to be HOMOZYGOUS RECESSIVE • If an individual has one dominant and one recessive allele for a gene they are said to be HETEROZYGOUS

11. Practice Practice Practice RR = ? rr = ? Rr = ? Genotype - The genes an organism has. (AA, Aa, aa) Phenotype – The physical appearance (purple or white flowers) In peas, purple is dominant to white so: R = purple r = white

12. Probability • The likelihood that a specific event will occur. • Can be expressed as a decimal, fraction, or percentage. • 0.75 • 75% • ¾

13. Predicting the results of crosses Punnett Square – Diagram used to predict the probability of a specific genotype or phenotype from a cross between two individuals. Let’s try some Punnett Squares! Does NOT show you the actual offspring. It shows the likelihood of having a specific type of offspring.

14. Reporting results of Punnett Squares • If you are asked for the genotypic ratio you need to give the ratio of the genotypes. • 1AA: 2Aa: 1aa • If you are asked for the phenotypic ratio you need to give the ratio of the phenotypes • 3 purple : 1 white

15. Performing a Testcross • We will always know the genotype of an individual with the recessive phenotype. • Why? • To determine the genotype of an organism with the dominant phenotype, we perform a testcross. • In a testcross you cross the organism of unknown genotype with a homozygous recessive individual. If A= purple and a = white AA = purple Aa = purple aa = white The only way to get the recessive phenotype is to have the aa genotype!

16. Testcross continued… • How will this help us determine the genotype? • If the unknown individual is homozygous dominant then…. • If the unknown individual is heterozygous then…. Let’s practice a testcross! All offspring show the dominant phenotype! Half the offspring will have the dominant phenotype and half will have the recessive phenotype!

17. Dihybrid Crosses • Sometimes you want to cross two individuals while looking at TWO different traits (flower color AND seed texture). • Flower color: A = purple a = white • Seed texture: R = smooth r = wrinkled • Write the genotype of an individual who is heterozygous for flower color and homozygous dominant for seed texture. • Write the genotype of an individual who is homozygous recessive for flower color and heterozygous for seed texture. Let’s see how it’s done! AaRR aaRr

18. Special Types of Inheritance • We’ve been working with complete dominance – when one allele is completely dominant over the other. • Sometimes alleles show incomplete dominance – when both alleles influence the phenotype of a heterozygote and the phenotype is a blend.

19. Example of Incomplete Dominance • Four o’clock flowers show incomplete dominance. • R = red • r = white • Rr = pink

20. Special Types of Inheritance cont. • Codominance – When neither allele is dominant or recessive to the other and BOTH are expressed in a heterozygote. • R = red • R’ = white • RR’ = red and white • Example: The roan horse.