The Science of Genetics

1. The Science of Genetics Original Power Point Created by Andy Harrison Modified by the GA Agriculture Education Curriculum Office July 2002

2. Introduction • Genetics began as people noticed the similarity of offspring to parents • Animals or plants were selected based on certain desirable traits • Later became known as selective breeding

3. Gregor Mendel • Austrian monk: 1822 –1884 • Noticed the texture of seeds differed • For example, some were smooth and round, others were not smooth and not round • Also noticed the difference in colors of flowers, length of stems, etc.

4. Mendel’s Experiments • Used purebred plants • Chose plants with traits that would be carried to next generation each time seeds produced. • Crossed purebred plants with other purebred plants. • Removed petals • Removed pollen from stamens and placed on stigma of pistil of different type of plant

5. Mendel’s Experiment • Some produced red flowers, and some only white • He then crossed all red with all white • Yielded some plants with all red flowers • Reasoned that some factor must cause the red to be dominant over white

6. Mendel’s Experiment • Law of Dominance – overriding or dominant factors make certain recessive traits disappear. • Allele – matched pair of genes that control a trait • Law of Segregation – alleles responsible for trait separate and then combine with other parent at fertilization – each parent provides one of two genes for the trait

7. Inheritance • Mendel let some plants self-pollinate • Offspring called an F1 generation • Some all red and some all white • Let F1 reproduce • Plants from all red produced some white flowers • Called the F2 generation

8. Inheritance • Mendel concluded that traits or factors associated with red or white flowers separated before pollen merged with egg • Law of segregation - alleles responsible for traits from each parent are separated and then combined with factors from other parents at fertilization.

9. Inheritance • Law of Independent Assortment – factors or genes for certain characteristics are passed on to the next generation separate from the factors or genes that transmit other traits.

10. Punnett Square • Developed by mathematician R.C. Punnett • Illustrates the possible combinations of a particular trait

11. Punnett Square • 75% Red • 25% White • 3:1 Red to White

12. Genetics • The study of how traits are passed on to future generations • Genotype – the genetic composition of an individual • Phenotype – how the allele expresses itself

13. Genetics • Homozygous – Genes that contain 2 dominant alleles (RR) or 2 recessive alleles (rr) • Heterozygous – Genes containing 1 dominant allele and 1 recessive allele (Rr)

14. Gene Transfer • Traits are located on chromosomes • Chromosomes are made of strands of DNA • Contains proteins and amino acids • Each segment of the chromosome that controls a trait is a GENE • Some traits are controlled by one gene, others under multi-genetic control • In Mendel's law of dominance, one allele is expressed and one is hidden

15. Meiosis • Similar to mitosis • Has an additional step • Chromosomes are divided so each sperm or egg contains a chromosome • Results in one egg or 4 sperm

16. Gene Transfer • At fertilization the egg and sperm unite • Newly formed cell contain two chromosomes • There is a pair of alleles for each trait • Haploid • For example, sex cells have only one chromosome • Diploid • Chromosomes are in pairs • Some cells have more than two pair • Also known as polyploid

17. Sex Determination • Each parent contributes one chromosome which pairs with a chromosome from the other parent • Sex Chromosomes • X: comes from the female • Y: comes from the male

18. Female XX Male XY X Y X X XX XY Sex Determination • Female has XX • Male has XY

19. Gene Transfer • DNA shaped in spiral a coil • Also known as a double-helix • If flattened out it would resemble a ladder • The two “sides” of the ladder are connected by nitrogen containing bases • Adenine - A • Thymine - T • Cytosine - C • Guanine - G • Adenine can only pair with Thymine • Cytosine can only pair with Guanine

20. Gene Transfer • During mitosis and meiosis, the chromosome separates • Prior to cell division, DNA copies itself by replication • Messages in DNA are transferred by RNA (a messenger substance) • This “message” determines how the molecules of new DNA are to be arranged

21. Plant Breeding • Systematic process of matching genetic factors from parent plants to produce offspring that are superior to parents • Early man used a primitive form of plant breeding

22. Plant Breeding • All plants came from wild plants that were domesticated • The domesticated species are called cultivars

23. What Plant Breeders Look For • Increased yield • Disease resistance • More efficient water use • Temperature tolerance • Ease of harvest • Uniform maturity • Quality of fruit or usable portion

24. Plant Breeding • Breeders have increased adaptation of plants by a systematic system of plant breeding • Hybrid – crossing two purebred lines to result in a superior plant • Corn most common hybrid • Many vegetables and flowers now hybrids • Heterosis – the result of the crossing of unrelated parents • Results in increased performance

25. Hybrid Corn • First developed in 1909 • Information on use taught to growers by Extension Service and Vocational Ag Classes in schools • Breeder develops parent lines by inbreeding selections from populations • Then crosses parent lines to create hybrids

26. A B AB Hybrid Corn • Crosses are made by collecting pollen from one plant and depositing on silk of recipient plant.

27. Animal Breeding • Less animals than in 1950, but production has increased • Most slaughter animals are crossbred due to hybrid vigor (heterosis benefit) • Selected for weight gain, size, conformation, litter size, etc.

28. Animal Breeding • Some new breeds have been developed by crossbreeding • Santa Gertrudis - cross of two different species: Bos taurus X Bos indicus • Most cattle in America or Europe – Bos taurus • Cattle from India - Bos indicus (Brahman)

29. Santa Gertrudis • Developed on the King Ranch in Texas • All Santa Gertrudis are descendants of the bull “Monkey”

30. Animal Breeding • Texas cattlemen wanted to combine gentle nature and carcass quality of the Shorthorn breed with vigor, heat resistance, and insect resistance of the Brahman cattle • Other breeds developed by crossing Bos indicus – Brangus, Simbrah, Braford, and Charbray

31. Animal Breeding • Computers have aided in sorting data • Results in expected progeny differences (EPD’s) • Use data of offspring to make selections of which animals to breed to others or to keep • Animals selected with leaner carcasses and better quality muscle result in increased flavor