GENETICS

1. GENETICS

2. Vocabulary Genes & Alleles Genotype vs. Phenotype Dominant vs. Recessive Homozygous vs. Heterozygous Pure-breeding vs. Hybrid Monohybrid vs. Dihybrid Punnett Square Generations: P, F1, F2, etc.

3. Gregor Mendel’s Peas

4. Dihybrid Cross Mating of parents that are both heterozygous for two traits Supports Mendel’s Laws Exhibit a phenotypic ratio of 9:3:3:1

5. Di- and Trihybrid Crosses • The hard way… BIG PUNNETT SQUARES

6. Di- and Trihybrid Crosses • The easy way… BRANCHING OR TREE METHOD 

7. The Testcross Crossing an unknown dominant (homo/hetero?) with a recessive

8. The Law of Segregation Separation of alleles to gametes During meiosis, the two members of each gene pair must separate into different gamete cells so that each gamete receives only one copy

9. The Law of Independent Assortment • Independent assortment of alleles • The different genes of different traits are transmitted independently of each other • Not true of linked genes

10. Exceptions to Mendelian Genetics • Incomplete Dominance • Codominance • Multiple Alleles & Polymorphism • Gene-Environment Interaction • Polygenic Inheritance

11. Incomplete Dominance Heterozygotes have an intermediate phenotype Often in flowers (white as recessive) Dominance is not an “all-or-none” rule

12. Codominance The heterozygotes display the phenotype of both alleles of a single gene Neither allele is dominant or recessive to the other Roan cattle: intermixed white and pigmented hairs

13. Multiple Alleles & Polymorphism • Many genes have more than two alleles • Polymorphism: when more than two distinct phenotypes are present in a population due to multiple allelism • Blood types: 3 alleles, 4 phenotypes • IA, IB, i alleles(A&B=codominant, O=recessive) • A/B glycoproteins on RBC • Immune response to carbs you don’t have (antibody agglutination)

14. Gene-Environment Interaction Most phenotypes are strongly influenced by the physical environment as well as their genotypes PKU (phenylketonuria) Causes dietaryphenylalanine to accumulate in the body, results in mental retardation Individuals placed on a low-phenylalanine diet, however, develop normally

15. Polygenic Inheritance Interaction of many genes to shape a single phenotype Often results in continuous variation (traits expressed as a range of varieties), “quantitative” trait Skin color, height (most human traits)

16. Model Organisms in Genetics A good model organism: Easy to maintain and breed Reproduce quickly, high fecundity Controllable sexual reproduction Small genome (easy to manipulate DNA) Easily recognizable traits First: garden peas (Mendel) Common: E.coli, mice, fruit flies (Morgan), mustard plant

17. Morgan’s Fruit Flies • Drisophila melanogaster • He first identified different phenotypes • Wild-type: The most common phenotype for each trait • Mutant: A change in the phenotype

18. Morgan’s Experiments Red eyes are wild-type and white eyes are mutations Reciprocal cross Gender affected inheritance patterns The gene for eye color (Ww) is only on the X chromosome (X-linked) Females (XX) – inherit 2 copies Males (XY) – inherit 1 copy

19. Sex Chromosomes Determine the sex of the offspring Humans and most mammals: XY system Females XX Produce all X gametes Males XY Produce ½ X and ½ Y gametes Other species have other systems: ZW in birds, others XO in some insects Haplodiploidy in bees, ants Temperature-determined in some reptiles

20. SRY Gene • Sex-determining Region Y gene • Codes for a transcription factor • In mammals • Initiates male sex determination (development of testes) • Significance • The presence of a Y chromosome leads to maleness, (even XXY, XXXY) • If the SRY gene is missing or defective, appear female but infertile • XX with translocated SRY gene – appear male

21. X Inactivation • One of the two copies of a female’s X chromosomes is inactivated (becomes a Barr body) • Random, irreversible • All cells in female mammals are not functionally identical! • Calico Cats: • Fur color is X-linked; in development its random as to which X was inactivated for that area of the body

22. Linked Genes The physical association of two or more genes found on the same chromosome Predicted to be transmitted together during gamete formation violating the law of independent assortment

23. Morgan’s Experiment on Linked Genes

24. Recombinant Chromosomes Linked genes are inherited together unless crossing over occurs. When crossing over takes place, genetic recombination occurs.

25. Linkage Mapping Data on the frequency of crossing over is used to create a geneticmap showing the relative positions of genes along a particular chromosome Map unit = % of time allele is known to cross over Add distances for frequency

26. Point Mutations • Single base substitution (A, T, C, G) • Significance: May alter codon, amino acid, protein structure • May be advantageous, neutral, or deleterious

27. Chromosomal Mutations Deletion Insertion or Duplication Inversion Translocation Basis for evolution: - Duplications can lead to specialization of similar genes - Inversions may alter mRNA splicing or the reading frame

28. Nondisjunction • Failure of chromosome pairs or chromatids to separate properly during mitosis or meiosis

29. Aneuploidy • Change in part of the chromosome set (abnormal chromosome number) • Human genetic disorders: • Monosomy - One missing chromosome • Example: Turner Syndrome (XO female) • Trisomy - One extra chromosome • Example: Klinefelter Syndrome (XXY male)

30. Polyploidy • More than two complete sets of chromosomes • Fatal in human embryos • Common in many flowering plants • Commercial advantages in plants • Larger flowers, seeds, fruits • Seedless fruits (bananas, watermelon) • Disease resistance

31. Pedigrees Family trees, showing: Relationships Sex of individual Presence of trait Determine whether a trait is due to a dominant or recessive allele and whether the gene responsible is located on a sex chromosome or an autosome

32. Autosomal Recessive Traits Individuals with the trait must be homozygous Parents of an affected individual are heterozygous carriers for the trait

33. Autosomal Dominant Traits Expressed in any individual with at least one dominant allele (either homozygous or heterozygous for the trait) Ex: Huntington’s disease

34. Human Genetic Disorders • Autosomal Recessive • Cystic Fibrosis • Thick mucous secretions in the lungs and digestive tract • Trinucleotide deletion (loss of 1 amino acid) • Tay-Sachs • Neurological deterioration • Various point mutations • Sickle Cell Anemia • Distorted hemoglobin • A-T point mutation • Autosomal Dominant • Achondroplasia • Dwarfism • G-A Point Mutation • Homozygous Lethal • Huntington’s Disease • Neurodegenerative • Trinucleotide repeat (CAGCAGCAG)

35. Sex-Linked Traits Trait is not equally expressed between males and females X linked recessive Usually skips generations, males more likely affected Example: Hemophilia X-linked dominant Rarely skip generations An affected male has all affected daughters but no affected sons

36. Pleiotropy & Genetic Disorders Pleiotropy = a single genotype that results in multiple phenotypes Marfan syndrome: A single gene results in a connective tissue disorder, leading to increased height and limb length, heart problems, etc.