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Heredity Mrs. Nell 7th Life Science What is heredity? Heredity is the passing of traits from parents to offspring. These traits are controlled by genes. Genes Genes are found on chromosomes and are made up of DNA. When pairs of chromosomes separate into sex cells during meiosis, pairs of genes also separate from one another. Each sex cell ends up with one form of a gene for each trait. The different forms a gene may have for a trait are its alleles. The study of how traits are inherited through the actions of alleles is the science of genetics. Widow’s Peak Straight hairline The Father of Genetics • The first recorded scientific study of how traits pass from one generation to the next was done by Gregor Medel, a Monk. In 1856, he began experimenting with garden peas. He found that it was possible to predict the kinds of flowers and fruit a plant would produce. He realized that information about the parents of the plants was needed before such a prediction could be made. Scientific Method Mendel made extremely careful use of scientific methods in his research. He studied pea plants for eight years! He eventually became known as the Father of Genetics. Purebred? An organism that always produces the same traits in its offspring is called a purebred. Can you recognize these purebreds? Mendel’s Work In nature, bees and other insects pollinate plants as they go from flower to flower. Mendel cross- pollinated the pea plants he studied (he took pollen from the male reproductive structure of flowers of purebred tall plants and placed it on the female reproductive structure of flowers of pure short plants). Dominant or recessive? Mendel discovered that tall plants crossed with short plants produced all tall plants. Mendel called the tall height form that appeared the dominant factor, because it dominated or covered up the short height form. He called the form that seemed to disappear the recessive factor. Recessive Reappears! Mendel allowed the new tall plants to self-pollinate. When he planted the new seeds he found that the recessive form had reappeared… both tall and short plants grew. He found that for every three tall plants there was one short plant, a 3:1 ratio. Using a Punnett Square A handy tool used to predict results like Mendel did for genetics is called the Punnett square. In a Punnett square dominant and recessive alleles are represented by letters. A capital letter represents the dominant allele and a small letter represents the recessive allele for a trait. The letters show the genetic makeup, or genotype, of an organism. Homozygous or Heterozygous? Most cells in your body have two alleles for a trait. An organism with two alleles for a trait that are exactly the same is called homozygous. (example: TT or tt) An organism with two different alleles for a trait is called heterozygous. (example: Tt) The physical trait that shows a a result of a particular genotype is its phenotype (example: tall or short) Summary: How Traits are Inherited 1) Traits are controlled by alleles on chromosomes. 2) An allele may be dominant or recessive in form. 3) When a pair of chromosomes separates during meiosis, the different alleles for a trait move into separate sex cells. Genetics Since Mendel While Mendel’s results were true for pea plants, different plants sometimes varied from Mendel’s predictions. When scientists crossed pure red four o’clock plants with pure white four o’clocks they expected to get all red flowers… but all the flowers were pink! Neither allele for the flower color seemed dominant. Incomplete dominance is the production of a phenotype that is intermediate to those of the two homozygous parents. Multiple Alleles Phenotype Genotype Mendel studied traits in peas A AA or AO that were controlled by just B BB or BO two alleles. However, many traits are controlled by more AB AB than two alleles- they are O OO controlled by multiple alleles. Human blood typing is controlled by multiple alleles. Multiple Genes Some traits are produced by a combination of many genes. Polygenic inheritance occurs when a group of gene pairs act together to produce a single trait. Eye color and fingerprints are examples. Height, weight, body build, hair, and skin color, are as well. Human Genetics Sometimes a gene undergoes mutation that results in an unwanted trait. Some mutations aren’t harmful, while others result in genetic disorders in humans, like sickle-cell anemia and cystic fibrosis. Sickle-Cell Anemia Sickle-cell anemia is a homozygous recessive disorder in which red blood cells are sickle-shaped. These cells can’t deliver oxygen to the cells in the body and don’t move through blood vessels easily. Often the patient will die as children. Sickle cell patients can be treated with drugs to increase oxygen in the blood or by transfusions of blood containing normal cells. Sickle cell Cystic Fibrosis Cystic fibrosis is another homozygous recessive disorder. Patients have a thick mucus in their lungs and intestinal tract instead of a thin fluid. Mucus makes it hard for cystic fibrosis patients to breathe and causes lung damage. Mucus in the digestive tract damages the pancreas. Patients are helped with antibiotics, special diets, and physical therapy. Cystic fibrosis is the most commonly inherited genetic disorder among Caucasians. Sex Determination Females produce eggs that have only an X chromosome. Males produce sperm that contain either an X or a Y chromosome. If an egg is fertilized by an X sperm the offspring is XX- a female. If the egg is fertilized by a Y sperm the offspring is XY- a male. Sex-Linked Disorders Some inherited conditions are closely linked with the X and Y chromosomes that determine the sex of an individual. Individuals who are red-green color blind have inherited an allele on the X chromosome that prevents them from seeing these colors. What number do you see? Sex-Linked Gene An allele inherited on a sex chromosome is a sex- linked gene. Another sex- linked gene disease is hemophilia, a disorder in which blood does not clot properly. Females are usually carriers of sex- linked diseases… can you explain why? Normal Mother + Father with Hemophilia Each pregnancy has a 50% chance of resulting in a female carrier and a 50% chance of resulting in a normal male. Sons of hemophiliac fathers and normal mothers will not have hemophilia. A Closer Look at Sex-Linked Traits Carrier Mother + Normal Father Carrier Mother + Father with Hemophilia Each pregnancy has a 25% chance of resulting in a Each pregnancy has a 25% chance of resulting in a female normal female, a 25% chance of resulting in a female carrier, a 25% chance of resulting in a female with carrier, a 25% chance of resulting in a normal male, and hemophilia, a 25% chance of resulting in a normal male, a 25% chance of resulting in a male with hemophilia. and a 25% chance of resulting in a male with hemophilia. A Closer Look at Sex-Linked Traits Mother with Hemophilia + Father with Hemophilia Mother with Hemophilia + Normal Father Each pregnancy has a 50% chance of resulting in a Each pregnancy has a 50% chance of resulting in a female with hemophilia and a 50% chance of resulting female carrier and a 50% chance of resulting in a male in a male with hemophilia (actual occurrence is with hemophilia (actual occurrence is extremely rare). extremely rare). Pedigrees A pedigree is a tool for tracing the occurrence of a trait in a family. Males are represented by squares and females by circles. Why is Genetics Important? Knowing how genes are inherited is important when a couple decides to have a child. Couples who discover genetic traits that could lead to disorders in their children may elect to adopt healthy children instead. Genetic Engineering Through genetic engineering, scientists are experimenting with biological and chemical methods to change the DNA sequence that makes up a gene. It is presently being used to make medicines (like insulin for diabetics) and to improve crop production.
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