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HEREDITY What is Genetics? Eye color, nose shape and other physical types of traits may be inherited from your parents. These traits are controlled by genes. Heredity is the passing of traits from parent to offspring. Genes control all traits, and are made of DNA. When pairs of chromosomes separate during meiosis, the pairs of genes also separate. If a trait is for hairlines, sex cells may control this form of trait. Alleles are different forms of a gene that you may have for a trait. Genetics is the study of how these traits are inherited through the actions of alleles. Gregor Mendel, born 1822, in Austria was an important figure in the study of genetics. He studied science and math, and later became a priest and teacher. In 1856, he experimented with garden peas. His assumption was that it was possible to predict the kinds of flowers and fruit a plant would produce. In order to do this he used the scientific method in his research. After eight years of work on inheritance of plants, Mendel presented his research paper to Natural Science Society of Brǜnn, Austria. This was published in 1866 and titled “Experiments with Plant Hybrids.” It wasn’t until 1900, that three other scientists rediscovered Mendel’s work. Mendel’s first worked with purebred plants. A purebred is an organism that always produces the same traits in its offspring. With this is mind, tall plants will always produce tall plants, and short plants will always produce short plants. In addition to the height of plants, Mendel studied six other traits of garden peas. Mendel crossed pea plants taking pollen from the male reproductive structures of purebred tall plants placing them on the female reproductive structures of pure short plants. The dominant Factor was tall height, and the recessive Factor was short height, which seemed to disappear. The result was all tall plants. Mendel then allowed the new tall plants to self-pollinate. The recessive factor reappeared, at a ratio of 3:1, tall to short. Mendel discovered that he could predict what plants would grow. He used probability which is a branch of mathematics that helps you predict the chance that something will happen. Mendel’s predictions were accurate because he worked with large numbers of plants. He counted every plant and thousands of seeds. Mendel studied almost 30 000 pea plants over eight years. This increased the chance of repeated patterns. Suppose Mendel had 100 second generation pea plants. How many would he expect to be tall? How many would he expect to be short? Mendel used a tool to predict results in Mendelian genetics. This is called the Punnett square. When using the Punnett square dominant alleles are represented by capital letter (T for tall), and recessive alleles are represented by small letter (t for short). The Punnett square will show the genotype of an organism, its genetic makeup. Most cells in your body have two alleles for every trait. Homozygous cells have two alleles that are exactly the same. Heterozygous cells have two different alleles. The phenotype is the physical trait that shows as a result of a particular genotype. For example red is the phenotype for red flowering plants, and short is the phenotype for short plants. Another phenotype is the color of your hair. Brown hair is the phenotype for the hair color of someone having brown hair. Finally Mendelian inheritance states the following: 1. Traits are controlled by alleles on chromosomes, 2. an allele may be dominant or recessive in form, 3. and when a pair of chromosomes separate during meiosis, the different alleles for a trait move into separate sex cells. HEREDITY Genetics Since Mendel The production of a phenotype that is intermediate to those of two homozygous parents is called incomplete dominance. When you cross pure red four o’clock plants with pure white four o’clocks the expected result is all red flowers. This is not the case. When crossing these all flowers were pink. Multiple alleles are traits that are controlled by more than two alleles. An example is blood type. There are three blood types: A (dominant), B (dominant), and O (recessive). AB blood type shows both alleles in his or her phenotype. A blood type is inherited from genotype AA or AO, B blood type is inherited from genotype BB or BO, and O blood type is inherited from genotype OO. Polygenic inheritance is a third type of way traits are affected. Polygenic inheritance occurs when a group of gene pairs act together to produce a single trait. The effect may be small, but the combination produces a wide variety. Blue or brown eye color is an example. Look at your classmate’s eyes to see if they have the same shade of color as you do. The chances are they won’t. This is because eye color is affected by polygenic inheritance. Finger prints are another example of polygenic inheritance. They come in a whorl, arch, loop pattern, or a combination of these patterns. For human’s height, weight, body build, shape of eyes, lips, and ears are all affected by polygenic inheritance. It’s not just humans that are affected by this. Other examples are grain color in wheat, milk production in cows, and egg production in chickens. HEREDITY Human Genetics Genes sometimes undergo mutations, resulting in an unwanted trait. Not all mutations are harmful, but some have resulted in genetic disorders in humans. There are about 1000 simple recessive disorders known to man. These disorders are recessive genetic disorders. One type of recessive genetic disorder is Sickle-Cell Anemia. Sickle-cell is a homozygous recessive disorder in which red blood cells are sickle-shaped instead of discshaped. They can’t deliver enough oxygen to cells in body, and the body cells are damaged due to a lack of oxygen. The cells don’t move through blood vessels very easily. Many die as children, and a few make it to adulthood. This disease can be treated. It is commonly found in tropical areas, and affect a small percentage of African Americans. Cystic Fibrosis is another homozygous recessive disorder in which a thin fluid is produced by the body to lubricate the lungs and intestinal tract. Cystic fibrosis has thick mucus in these areas instead of thin fluid. The mucus builds up in the lungs and digestive system. It makes it hard to breathe and causes lung damage. Cystic fibrosis also prevents the flow of digestive enzymes. It is commonly inherited by Caucasians. 1:20 carry this recessive allele. Cystic fibrosis is helped with antibiotics, special diets, and physical therapy. Scientists have discovered that fruit flies have eight chromosomes and among these chromosomes are either two X chromosomes, or an X chromosome and a Y chromosome. Scientists have concluded that X and Y chromosomes contain genes that determine the sex of individuals. Females carry XX chromosomes, and males carry XY chromosomes. When an egg from a female is fertilized from the X sperm, an XX female is born. When and egg from a female is fertilized from the Y sperm an XY male is born Some inherited conditions are closely linked to the X and Y chromosomes. This brings about sex-linked disorders. A sex-linked gene is an allele that is inherited on a sex chromosome. Color blindness is an example. More males than females are color blind. Why? Hemophilia is another sex-linked disorder in which blood does not clot properly. A scrape can be life threatening. Males who inherit the X cell will have this disorder. The ration is 1:7 000 males. Females need both the XX cells to be infected. They are usually carriers. Scientists use a pedigree, a tool for tracing the occurrence of a trait to a family. Normally squares are males and circles are females. Colored and filled circles and squares show a person who has the condition. Half colored circles and squares are carriers. The same laws that govern inheritance of human traits also govern the traits of watermelon, wheat, and even mice. Knowing how genes are inherited causes some people to seek advice of genetic counselors. Genetic engineering has resulted. Genetic engineering is when biological and chemical methods are used to change the DNA sequence that makes up a gene. They are used to help produce large quantities of medicines such as insulin for diabetes. They are also used to find new ways to improve crop production. Identifying genes that are desirable traits in one plant, then inserting in another plant is an example. Barley has the following strengths: maturity, strength, height, and resistant to drought. Genetic engineers are planning to insert these genes in other plants to make them more resistant and easily grown.