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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 August 2008 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 August 2008 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. August 2008 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 August 2008 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 August 2008 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 August 2008 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 August 2008 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. August 2008 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. August 2008 Punnett Square • Developed by mathematician R.C. Punnett • Illustrates the possible combinations of a particular trait August 2008 Punnett Square R r 75% R RR Red Rr Red Homozygous Heterozygou s r August 2008 Rr Red rr Heterozygou s Homozygous White Red 25% White 3:1 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 August 2008 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) August 2008 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 August 2008 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 August 2008 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 August 2008 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 August 2008 Sex Determination • Female has XX Female XX X X XX August 2008 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 August 2008 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 August 2008 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 August 2008 Plant Breeding • All plants came from wild plants that were domesticated • The domesticated species are called cultivars August 2008 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 August 2008 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 August 2008 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 August 2008 Hybrid Corn • Crosses are made by collecting pollen from one plant and depositing on silk of recipient plant. A B AB August 2008 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. August 2008 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) August 2008 Santa Gertrudis Developed All on the King Ranch in Texas Santa Gertrudis are descendants of the bull “Monkey” August 2008 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 August 2008 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 August 2008