Introduction to Genetics

2. Introduction to Genetics Dr. Abid Ali Department of Biochemistry

3. Learning Objectives • At the end of lecture students will be able to • Describe Mendel’s Law • Discuss Punnett Squares • Know about Pedigree Chart • State DNA & RNA structure

4. What is genetics? • Genetics is the branch of Biology that deals with heredity. • Heredity is… the passing on of characteristics from parents to offspring. Mendelian Genetics

5. Chromosome • All living organisms consist of cells. • In each cell there is chromosomes. • Chromosomes are strings of DNA • Chromosome consists of genes, blocks of DNA. Each gene encodes a particular protein.

6. Chromosome • Each gene has its own position in the chromosome. • This position is called locus. • Complete set of genetic material (all chromosomes) is called genome. • Particular set of genes in genome is called genotype. • The genotype is with later development after birth base for the organism's phenotype, its physical and mental characteristics, such as eye color, intelligence etc.

7. DNA • DNA is a double helix composed of two intertwined nucleotide chains oriented in opposite directions. • The double helix composed of building block called nucleotides

8. DNA • Each nucleotides consist : • Phosphate group • Deoxyribose sugar molecule • One of four different nitrogenous bases either Purines - Adenine and Guanine, or Pyrimidines -Cytosine and Thymine)

9. DNA • The functional units of DNA are genes. • A gene is a segment of DNA that can be copied to make RNA. • The nucleotide sequence in RNA is translated into the amino acid sequence of a protein. • Proteins are the main determinants of the basic structural and physiological properties of an organism.

10. Protein synthesis • DNA is duplicated before a cell divides • A process called replication

11. Protein synthesis • Genes are transcribed into RNA (transcription). • mRNA Transported out of the nucleus • proteins are built Outside the nucleus, the (translation).

12. TERMS USED IN GENETICS

13. What is a Trait? • A trait is a specific characteristic that varies from one individual to another. • Examples: Brown hair, blue eyes, tall, curly

14. What is an Allele? • Alleles are the different possibilities for a given trait. • Every trait has at least two alleles (one from the mother and one from the father) • Example: Eye color – Brown, blue, green, hazel Examples of Alleles: A = Brown Eyes a = Blue Eyes B = Green Eyes b = Hazel Eyes

15. What are Genes? • Genes are the sequence of DNA that codes for a protein and thus determines a trait.

16. Gregor Mendel Mendelian Genetics • Gregor Mendel was the first person to study genetics. 1. He did his genetics experiments using pea plants. 2. He crossed (mated) two pea plants that had certain traits. 3. He then planted their offspring and observed what traits they expressed. 4. He carefully selected each plant that he used and carefully reviewed the results.

17. What did Mendel observe and conclude? • Mendel observed that there were different forms of a trait (controlled by a gene) called alleles. • Each parent donated one allele for each trait, which meant that each offspring had two alleles for each trait. Mendelian Genetics

18. P1 and F1 generations • In a cross (mating) between organisms, the parent generation = P1 generation. • The first generation = F1 generation. • The second generation = F2 generation. Mendelian Genetics

19. Mendel’s Conclusions • Law of Segregation – Two alleles for each trait separate when gametes form; Parents pass only one allele for each trait to each offspring • Law of Independent Assortment – Genes for different traits are inherited independently of each other

20. Genotype and Phenotype • The phenotype an organisms appearance and behavior (how the trait expresses itself). • The genotype for an offspring or parent is the gene combinations or the set of two alleles that they have for each trait. Mendelian Genetics

21. Genotype vs. Phenotype • Genotype – The genetic makeup of an organism; The gene (or allele) combination an organism has. • Example: Tt, ss, GG, Ww • Phenotype – The physical characteristics of an organism; The way an organism looks • Example: Curly hair, straight hair, blue eyes, tall, green

22. Homozygous and Heterozygous • If the alleles are the same for a trait then the organism’s genotype is homozygous for a certain trait. • If the organism has two differentalleles for each trait then its genotype is heterozygous (these are often called hybrids). Mendelian Genetics

23. What did Mendel observe and conclude? • Mendel noticed that some traits masked the expression of other traits. • For example, offspring from a cross between tall and short pea plants were all tall. Mendelian Genetics

24. Dominant and Recessive Alleles • Mendel came up with his rule of dominance : an allele that masks the expression of another allele is dominant. • The dominant allele is usually capitalized. • The other allele is recessive and it is written lower case. Mendelian Genetics

25. How it works…. • For example, the allele for a tall pea plant is dominant over the allele for short pea plants. • The dominant allele is expressed when the organism is: • homozygous =TT or • heterozygous =Tt • The recessive allele for a trait is expressed only when the organism is • Homozygous for recessive= tt Mendelian Genetics

26. Punnet Squares Punnet Squares! Punnett squares (devised by Reginald Punnett) give us a shorthand way to determine the expected genotypes and phenotypes of the offspring that result from a cross.

27. What can we do with a Punnet Square? • From this, you can calculate: • the numbers of each genotype and phenotype. • the probability of an offspring having a certain phenotype. • And how many offspring should exhibit a certain form of each trait by multiplying the number of offspring by the probability that they will have a certain trait. Punnet Squares

28. Punnett Square Parent Offspring Parent

29. Tt x tt Step One: Set Up Punnett Square (put one parent on the top and the other along the side) T t t t

30. Tt x tt Step Two: Complete the Punnett Square T t t t

31. Tt x tt Step Two: Complete the Punnett Square T t t t Genotype: Tt - 2 (50%) tt - 2 (50%) Phenotype: 50% Tall 50% Short Remember: Each box is 25%

32. Pedigree Charts • The family tree of genetics

33. A pedigree is a chart of the genetic history of family over several generations. • Scientists or a genetic counselor would find out about your family history and make this chart to analyze.

34. Symbols commonly used in Pedigree

35. Interpreting a Pedigree Chart • Determine whether the disorder is dominant or recessive. • If the disorder is dominant, one of the parents must have the disorder. • If the disorder is recessive, neither parent has to have the disorder because they can be heterozygous.

36. Dominant

37. Recessive

38. Autosome = all chromosomes are called autosomes except sex chromosomes • Humans normally have 22 pairs of autosomes (44 autosomes) in each cell, together with 2 sex chromosomes • A single abnormal gene on one of the first 22 nonsex (autosomal) chromosomes from either parent can cause an autosomal disorder.

39. Autosomal dominant

40. Structure of DNA and RNA

41. Nucleic Acids • There are two kinds of nucleic acids in cells: • 1) ribonucleic acids (RNA) • 2) deoxyribonucleic acids (DNA) • Both RNA and DNA are polymers built from monomers called nucleotides. • A nucleotide is composed of: • a base, a monosaccharide, and a phosphate.

42. Nucleic Acids • made up of nucleotides • found in all living cells except RBC • deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) • DNA is in the nucleus • RNA is in the cytoplasm • function in the storage and transmission of genetic material • And control and direct all protein synthesis

43. each nucleotide contains • 1) a sugar • 2) a base • 3) phosphoric acid unit 1) Ribose and Deoxyribose DNA RNA

44. Bases in Nucleic Acids Bases for DNA: A, G, C, T Bases for RNA: A, G, C, U

45. Formation of a Nucleotide • Alternating phosphate, sugar molecules form the backbone • The reaction between phosphate and sugar forms an ester bond with the elimination of water • The sugar bonds with a base, forming tertiary amine, with the elimination of water

46. DNA and RNA Strand • The sequence of the bases in DNA or RNA form the primary structure DNA

47. Structure of DNA and RNA

49. DNA - 2° Structure • Secondary structure: the ordered arrangement of nucleic acid strands. • the double helix model of DNA 2° structure was proposed by James Watson and Francis Crick in 1953. • Double helix: a type of 2° structure of DNA in which two polynucleotide strands are coiled around each other in a screw-like fashion.

50. The DNA Double Helix • Figure 25.4 Three-dimensional structure of a DNA double helix.