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Transcript
The Science of Genetics Original Power Point Created by Andy Harrison Modified by the GA Agriculture Education Curriculum Office July 2002 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 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. 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 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 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 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 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. 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. Punnett Square Developed by mathematician R.C. Punnett Illustrates the possible combinations of a particular trait Punnett Square R r 75% R RR Red Rr Homozygous Heterozygous Red 25% White 3:1 r Rr Red rr Heterozygous Homozygous White Red Red to White 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 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) 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 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 Gene Transfer At fertilization the egg and sperm unite Newly formed cell contain two chromosomes Haploid For example, sex cells have only one chromosome Diploid There is a pair of alleles for each trait Chromosomes are in pairs Some cells have more than two pair Also known as polyploid 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 Sex Determination Female has XX Female XX X X XX Male has XY Male XY X Y XY 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 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 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 Plant Breeding All plants came from wild plants that were domesticated The domesticated species are called cultivars 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 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 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 Hybrid Corn Crosses are made by collecting pollen from one plant and depositing on silk of recipient plant. A B AB 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. 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) Santa Gertrudis Developed All on the King Ranch in Texas Santa Gertrudis are descendants of the bull “Monkey” 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 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