Gregor Mendel - Pioneer of Genetics

1. Chapter 18 Section 18.1 Gregor Mendel – Pioneer of Genetics

2. Gregor Mendel • Gregor Mendel (1822-1884) was an Austrian monk whose experiments with garden peas laid the foundation for the science of genetics. • He tracked and recorded the transmission of seven visible genetic traits through seven generations of garden peas. • The reason that Mendel chose to work with garden peas was because most characteristics of the garden pea have only two alternative forms. (Ex. Round or Wrinkled seeds)

3. Mendel used cross-pollination to breed his garden peas. • He would remove pollen (sperm cells) from the male part of one plant and place it on the female part of a second plant. • The pollen grains then fertilize the female eggs, resulting in a new organism called the progeny or the offspring.

4. To refer to his seven generations of peas he used the following system: • First cross = Parental Generation (P) • Offspring of first cross = First Filial Generation (F1) • Offspring of second cross = Second Filial Generation (F2) • F3, F4, F5, F6, and F7

5. The 7 Traits • Flower colour • Flower position • Seed colour • Seed shape • Pod colour • Pod shape • Stem length

6. Some Genetics Terms • Progeny – new individuals that result from reproduction; offspring. • Dominant trait – a characteristic that is expressed when one or both alleles in an individual are present. • Recessive trait – a characteristic that is expressed only when both alleles in an individual are present.

7. Alleles – the alternative forms of genes. • Ex. Purple flowers or White flowers • Each of these is an allele for the flower colour gene. • Homozygous – having identical alleles for the same gene. • Ex. P = purple flowers, p = white flowers. • Homozygous would be PP, or pp. • Heterozygous – having different alleles for the same gene. • Heterozygous would be Pp.

8. Genotype – the genetic code of an organism. • Ex. PP, pp, Pp • This is the genetic code for flower colour. • Phenotype – the observable characteristics of an organism. • The phenotype is the colour itself, purple or white.

9. Mendel’s Experiments • Mendel discovered 2 important concepts about genetics through his experiments: • The Principle of Dominance • Mendel’s Principle of Segregation

10. The Principle of Dominance • When Mendel used pollen from a pea plant with round seeds to fertilize a pea plant with wrinkled seeds, he found that all the offspring in the F1 generation had round seeds. • He tested this many times and always got the same results. • This trend suggested to him that the allele for round seeds, masks the allele for wrinkled seeds.

11. He referred to this idea as the principle of dominance because the round seeds were always dominant over the wrinkled seeds. • From then on the round seeds were known as the dominant trait and the wrinkled seeds were known as recessive trait. • The allele for dominant traits is always indicated by an uppercase letter (R for round seeds). • The allele for recessive traits is designated by a lowercase letter (r for wrinkled seeds).

12. Mendel’s Traits Mendel eventually discovered the dominant and recessive traits for each of his 7 characteristics.

13. Mendel’s Principle of Segregation • Mendel next let the F1 plants self-fertilize to see what would happen in the F2 generation. • Mendel was expecting the same result, that 100% of the plants would have round seeds. • He was astonished to find that 25% of the F2 generation had wrinkled seeds instead of round. • He again repeated this several times and found that the F2 generation always consisted of 75% round and 25 % wrinkled, a 3:1 ratio.

14. To explain these ratios, Mendel reasoned that each plant must carry two copies (alleles) of each gene, that can be the same or different. • An individual with round seeds can have two RR alleles or one of each because it is dominant, Rr. • An individual with wrinkled seeds can only have two rr alleles. • RR and rr = homozygous • Rr = heterozygous

15. He also reasoned that the alleles must separate during meiosis, one allele going to one sex cell and the other allele going to the other sex cell. • He called this segregation – the separation of alleles during meiosis. • Only one of these alleles from a parent can get passed to one individual offspring. • The offspring will receive two alleles, one from ‘mom’ and one from ‘dad’.

16. Segregation of P Generation Round Seed Wrinkled Seed RR rr R R r r Crossing a pure round seeded plant with a pure wrinkled seeded plant will produce a round seeded plant 100% of the time. Cross Pollination Occurs Only one round allele is needed because round is dominant over wrinkled. Rr Round Seed

17. Segregation of F1 Generation Rr Rr Round seeds can appear when homozygous or heterozygous. Wrinkled seeds only appear when homozygous. R r R r rr RR Rr Rr Round Round Round Wrinkled

18. 18.1 textbook questions • Pg. 600 #1-5