2. Chapter 10 Discovery of genes, alleles and Deoxyribonucleic Acid

3. There have been many scientists and biologists who have contributed to our understanding of genetic material. We will take a look at some of the important individuals and how their contributions have helped our understanding of genetics.

4. Gregor Mendel Modern genetics had its beginnings in a monastery garden, where an Austrian monk named Gregor Mendel documented a particular mechanism of inheritance. He discovered the basic principles of heredity by breeding garden peas in carefully planned experiments.

5. Mendel’s work In order to study inheritance, Mendel chose to use peas, probably as they are available in many varieties. The use of plants also allowed strict control over the breeding.

6. Mendel’s work Mendel’s work supported the “Particulate Inheritance” model as opposed to the “Blending Inheritance” model. Mendel’s work was ignored until the 1900s and was rediscovered by three biologists (Bateson, Punnett and Sutton).

7. Mendelian Genetics Mendel studied a number of characteristics in pea plants including, but not limited to: Height - short or TALL Seed color - green or YELLOW Seed shape - wrinkled or ROUND

8. Genetic crosses To breed two varieties of pea plants, Mendel used a paint brush. He transferred pollen from a true breeding (homozygous) white flower to the carpel of a true breeding (homozygous) purple flower.

9. Tracking heritable characteristics Mendel tracked heritable characters for 3 generations. When F 1 hybrids were allowed to self-pollinate a 3:1 ratio of the 2 varieties occurred in the F 2 generation.

10. First filial generation (F 1 ) This shows a heterozygote with one gene, two alleles: one allele for purple flowers (P) and white flowers (p).

11. Mendel’s Law of Segregation By carrying out these monohybrid crosses, Mendel determined that the 2 alleles for each character segregate during gamete production. i.e. each allele is inherited independently of the other alleles. This is called Independent Assortment of Alleles. Mendel discovered this in 1860.

12. Mendel’s terminology True breeding: When the plants self-pollinate, all their offspring are of the same variety. Hybridisation: Mating, or crossing of two varieties. Monohybrid cross: A cross that tracks the inheritance of a single character. P generation: True breeding parents. F 1 generation: (first filial) Hybrid offspring of the P generation. F 2 generation: (second filial) Offspring from the self- fertilisation of the F 1 hybrids. Factors: what we now call alleles

13. Walter Sutton In 1902, Walter Sutton recognised that there were thread-like structures in cells which he called chromosomes. He then came to the realisation that Mendel’s factors were located on these chromosomes.

14. William Bateson and Reginald Crundall Punnett Also in the early 1900s W. Bateson and R.C. Punnett found that certain characteristics in peas were inherited together. It was later found that these characteristics must be carried on the same chromosome in order to be inherited together.

15. Thomas Hunt Morgan In 1910 T.H Morgan confirmed the results of Sutton, Bateson and Punnett. Morgan’s experiments with Drosophila showed that genes were located on chromosomes. Morgan also confirmed that when genes were located close together on homologous chromosomes, their alleles tended to be inherited together, i.e. linked.

16. Frederick Griffith Frederick Griffith in 1928 discovered a substance that could change a harmless organism into a disease causing killer organism. Bacteria (like pneumococci) also showed variation. The harmless avirulent form (rough) has been ‘transformed’ by something from the virulent form (smooth).

17. Oswald Avery Oswald Avery discovered that the chemical compound containing genes was deoxyribonucleic acid (DNA).

18. Discovering DNA After it was found that genetic information is inherited through DNA, the race was on to find the structure of DNA. During the 1940s and 1950s a number of different scientists studied DNA to find the proportions of the 4 different nucleotides (adenine, guanine, thymine and cytosine). It was found that Adenine and Thymine were found in equal proportions and Guanine and Cytosine occurred in equal proportions. (Chargaff’s Rules)

19. James Watson and Francis Crick In 1953 James Watson and Francis Crick, discovered that the structure of DNA was as two nucleotide chains arranged to form a double helix. Watson and Crick used results obtained by Rosalind Franklin. Rosalind Franklin used X-ray crystallography to obtain the image on the right.

20. Deoxyribonucleic Acid Watson and Crick’s 1954 model of DNA