Genetic Terminology

1. Genetic Terminology • Gene - Segment of DNA that codes for formation of a protein • Locus – Position of gene on a chromosome • Trait - any characteristic that can be passed from parent to offspring • Heredity - passing of traits from parent to offspring • Genetics - study of heredity

2. Alleles • A gene that controls one function can exist in different forms. These different forms are called alleles. • Each different allele is identified by its specific phenotypic action. • Alleles are commonly represented by letters of the alphabet. • Eg. The gene LDLR controls blood cholesterol levels. Located on chromosome 19, it has two allelic forms: • B = abnormally high cholesterol levels • b = normal range

3. Autosomal Genotype • Remembering that non-sex chromosomes occur in homologous pairs in the diploid cell – there are two copies of each gene. • The double set of genetic instructions present makes up the genotype. • The number of possible genotypes depends on the number of allelic forms of the gene.

4. Genotype terminology Homozygous (pure) genotype - gene combination involving 2 dominant or 2 recessive genes (e.g. RR or rr) Heterozygous (hybrid) genotype - gene combination of one dominant & one recessive allele    (e.g. Rr)

5. Phenotype • The visible expression of the genotype is called the phenotype. The expression may be a physical, biochemical or physiological trait. • Dominant trait: require only a single copy of the responsible allele for its phenotypic expression • Recessive trait: refers to a trait that is not expressed in a heterozygote • Co-dominant trait: both alleles in the heterozygote are expressed in the phenotype

6. e.g. Genotype & Phenotype in Flowers Genotype of alleles:R = red flowerr = yellow flower All genes occur in pairs, so 2 alleles affect a characteristic Possible combinations are: GenotypesRR Rr rr PhenotypesRED RED YELLOW

7. Genes and Environment Determine Characteristics

8. The relationship between genotype and phenotype is rarely simple! Phenotype = Genotype + Environmental Factors Hydrangeas: pink or blue? Both plants have the pigment for colour called anthocyanin. In acidic soils (low pH) the flowers are blue. In alkaline soils (high pH) the flowers are pink). Same genotype – different phenotype

9. Multiple Alleles AA Ai Allele Combination Phenotype For some genes, three or more alleles can be present in the population. You will only inherit two alleles (one on each chromosome) The combination of any two alleles determines the final phenotype. The ABO blood groupings in humans is an example. Three alleles are involved in controlling blood group IA, IB and i. or A or B AB O BB Bi AB ii

10. Monogenic Traits • Monogenic traits are due to the action of a single gene with two or more allelic forms. • These traits show discontinuous variation - the members of the population can be grouped into a few discrete and non-overlapping classes. • E.g. blood types

11. Polygenic Traits • Polygenic traits are due to the actions of many genes (and their allelic forms). These traits show continuous variation (e.g. height).

12. Human Sex Chromosomes • Traits (genes) located on the sex chromosomes • XX genotype for females • XY genotype for males • The X chromosome may carry up to 1500 genes. Genes located on the X chromosome are said to be X-linked. • Females have two alleles of a particular gene whereas males have only one (hemizygous genotype). This accounts for why many X-linked diseases show up more frequently in males than in females. • The Y chromosome has less than 300 genes. Genes located on the Y chromosome are said to be Y-linked. Males are also hemizygous for Y-linked genes.

13. X Inactivation in Female Mammals • Females who have two X chromosomes but one is subject to inactivation. • 75% of alleles on one X chromosome are switched off in early embryonic development. • 15% remain activated with another 10% altering their activation state in different females and in different cells within the same female. • See page 315 for case study

14. Mendel’s Pea Plant Experiments

15. Gregor Johann Mendel • Austrian monk • Studied the inheritance of traits in pea plants • Developed the laws of inheritance • Mendel's work was not recognized until the turn of the 20th century • Between 1856 and 1863, Mendel cultivated and tested some 28,000 pea plants • He found that the plants' offspring retained traits of the parents • Called the “Father of Genetics"

16. Site of Gregor Mendel’s experimental garden in the Czech Republic

17. Mendel stated that physical traits are inherited as “particles” Mendel did not know that the “particles” were actually chromosomes & DNA Particulate Inheritance

18. Reproduction in Flowering Plants • Pollen contains sperm • Produced by the stamen • Ovary contains eggs • Found inside the flower • Pollen carries sperm to the eggs for fertilization • Self-fertilization can occur in the same flower • Cross-fertilization can occur between flowers

19. How Mendel Began Mendel produced pure strains by allowing the plants to self-pollinate for several generations

22. Mendel’s Experimental Results

23. Did the observed ratio match the theoretical ratio? • The theoretical or expected ratio of plants producing round or wrinkled seeds is 3 round :1 wrinkled • Mendel’s observed ratio was 2.96:1 • The discrepancy is due to statistical error • The larger the sample the more the results approximate to the theoretical ratio

24. Generation “Gap” • Parental P1 Generation = the parental generation in a breeding experiment. • F1 generation = the first-generation offspring in a breeding experiment. (1st filial generation) • From breeding individuals from the P1 generation • F2 generation = the second-generation offspring in a breeding experiment. (2nd filial generation) • From breeding individuals from the F1 generation

25. Following the Generations Cross 2 Pure PlantsTT x tt Results in all HybridsTt Cross 2 Hybridsget3 Tall & 1 ShortTT, Tt, tt

26. Monohybrid Cross • A trait determined by one gene with two or more allelic forms.

27. Punnett Square Used to help solve genetic problems

29. P1 Monohybrid Cross • Trait: Seed Shape • Alleles: R – Round r – Wrinkled • Cross: Round seeds x Wrinkled seeds RR x rr Genotype: Rr Phenotype: Round GenotypicRatio: All alike PhenotypicRatio: All alike r r Rr Rr R R Rr Rr

30. P1 Monohybrid Cross Review • Homozygous dominant x Homozygous recessive • Offspring all Heterozygous (hybrids) • Offspring called F1 generation • Genotypic & Phenotypic ratio is ALL ALIKE

31. F1 Monohybrid Cross • Trait: Seed Shape • Alleles: R – Round r – Wrinkled • Cross: Round seeds x Round seeds Rr x Rr R r Genotype:RR, Rr, rr G.Ratio:1:2:1 Phenotype: 3 Round & 1 wrinkled P.Ratio: 3:1 RR Rr R r Rr rr

32. F1 Monohybrid Cross Review • Heterozygous x heterozygous • Offspring:25% Homozygous dominant RR 50% Heterozygous Rr 25% Homozygous Recessive rr • Offspring called F2 generation • Genotypic ratio is 1:2:1 • Phenotypic Ratio is 3:1

33. …And Now the Test Cross • Mendel then crossed a pure & a hybrid from his F2 generation • This is known as an F2 or test cross

34. F2 Monohybrid Cross (1st) • Trait: Seed Shape • Alleles: R – Round r – Wrinkled • Cross: Round seeds x Round seeds • RR x Rr Genotype: RR, Rr Phenotype: Round GenotypicRatio: 1:1 PhenotypicRatio: All alike R r RR Rr R R RR Rr

35. F2 Monohybrid Cross (2nd) • Trait: Seed Shape • Alleles: R – Round r – Wrinkled • Cross: Wrinkled seeds x Round seeds • rr x Rr R r Genotype:Rr, rr Phenotype: Round & Wrinkled G. Ratio:1:1 P.Ratio: 1:1 Rr rr r r Rr rr

36. F2 Monohybrid Cross Review • Homozygous recessive x heterozygous(hybrid) • Offspring: 50% Homozygous rr 50% Heterozygous Rr • Phenotypic Ratio is 1:1

37. Test crosses • Can be used to determine if an individual of dominant phenotype is homozygous or heterozygous • There are two possible testcrosses: Homozygous dominant x Homozygous recessive = All heterozygous dominant Hybrid x Homozygous recessive = Mix of dominant and recessive phenotypes

38. Monohybrid cross Practice Problems

39. t t T T 1. Breed the P1 generation • tall (TT) x dwarf (tt) pea plants

40. T t T t 2. Breed the F1 generation • tall (Tt) vs. tall (Tt) pea plants

41. t t produces the F1 generation Tt Tt T Tt Tt T All Tt = tall (heterozygous tall) 1. Solution: tall (TT) vs. dwarf (tt) pea plants

42. T t produces the F2 generation Tt TT T 1/4 (25%) = TT 1/2 (50%) = Tt 1/4 (25%) = tt Tt tt t 1:2:1 genotype 3:1 phenotype 2. Solution: tall (Tt) x tall (Tt) pea plants

43. Results of Monohybrid Crosses • Inheritable factors or genes are responsible for all heritable characteristics • Phenotype is based on genotype • Each trait is based on two genes, one from the mother and the other from the father • True-breeding individuals are homozygous (both alleles) are the same

44. Law of Dominance • In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. • All the offspring will be heterozygous and express only the dominant trait. • RR x rr yields all Rr (round seeds)

45. Law of Dominance

46. Law of Segregation • During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. • Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.

47. Applying the Law of Segregation

48. Law of Independent Assortment • Alleles for different traits are distributed to sex cells (& offspring) independently of one another. • This law can be illustrated using dihybrid crosses.

49. Dihybrid Cross • A breeding experiment that tracks the inheritance of two traits. • Mendel’s “Law of Independent Assortment” • a. Each pair of alleles segregates independently during gamete formation • b. Formula: 2n (n = # of heterozygotes)

50. Question:How many gametes will be produced for the following allele arrangements? • Remember: 2n (n = # of heterozygotes) • 1. RrYy • 2. AaBbCCDd • 3. MmNnOoPPQQRrssTtQq