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Mendel and the Gene Idea Chapter 14 • Gregor Mendel - monk - studied pea plants, looked at traits. • Pea plants many varieties with distinct heritable features (characters) with different variants (traits). • Pea plants self-fertilize; Mendel cross-fertilized to study traits. • Mendel cross-pollinated (hybridize) 2 contrasting, true-breeding pea varieties. • True-breeding parents - P generation; hybrid offspring - F1 generation. • F1 hybrids then self-pollinate to produce F2 generation. http://nitro.biosci.arizona.edu/courses/EEB320-2005/Lecture02/pics/pea.jpeg • Thought genes blended - purple flower crossed with white flower result would be light purple flowers. • All the flowers purple. • When flowers self-fertilized, white flower reappeared in next generation. • Ratio of purple to white in F2 generation was 3:1. • Developed hypothesis to explain process. • 1Alternative versions of genes (alleles) account for variations in inherited characters. • 2For each character organism inherits 2 alleles, 1 from each parent. • Alleles can be same or different. • 32 alleles differ - 1 (dominant allele) fully expressed in organism. • Other (recessive allele) no noticeable effect on organism’s appearance. • 42 alleles for each character segregate (separate) during gamete production. http://discover.edventures.com/images/termlib/d/dominant_allele/support.gif • Found that alleles not linked due to inheritance patterns. • Independent assortment of each pair of alleles during gamete formation - law of independent assortment. • Therefore, Mendel had three different laws. • The law of dominance and recessiveness states that one gene is dominant over the more recessive gene. • The law of segregation states that alleles separate during meiosis. • The law of independent assortment states that alleles organize in the gametes regardless of other alleles. • An organism with two identical alleles for a character is homozygous for that character. • Organisms with two different alleles for a character is heterozygous for that character. • A description of an organism’s traits is its phenotype. • A description of its genetic makeup is its genotype. • A Punnett square predicts the results of a genetic cross between individuals of known genotype. • A testcross, breeding a homozygous recessive with dominant phenotype, but unknown geneotype, can determine the identity of the unknown allele. • Mendel’s experiments focused on monohybrid crosses meaning that he looked at only one trait at a time. • Later on he started looking at dihybrid crosses involving probabilities of two different traits. • Mendel’s ideas are based on probability. • If you were to toss a coin 4 times, the coin has a ½ chance of coming up heads every time. • Each toss is independent of the one done before. • The probability of it coming up heads all four times is: ½ * ½ * ½ * ½ = 1/8. • This is known as the rule of multiplication. 12.2 • There is also a law of addition that determines the chances of an event happening in different ways. • For example, there are two ways that F1 gametes can combine to form a heterozygote. • The dominant allele could come from the sperm and the recessive from the ovum (probability = 1/4). • Or, the dominant allele could come from the ovum and the recessive from the sperm (probability = 1/4). • The probability of a heterozygote is 1/4 + 1/4 = 1/2. • Incomplete dominance can also occur in offspring. • In incomplete dominance, heterozygotes have a completely different phenotype than homozygotes. • This happens in snapdragons. • Homozygous recessive flowers are white; homozygous dominant flowers are red; heterozygotes are pink. • Another inheritance pattern is codominance in which two alleles affect the phenotype in separate, distinguishable ways. • An example of this is blood type. • If you inherit an A allele and a B allele, your blood type will be AB; if it is AA or AO, your blood type will be A. • This means that A is dominant to O, B is dominant to O, but A is codominant to B. • Blood type is important because type A has anti-B antibodies. • If exposed to B blood, it will clump together causing a transfusion reaction. • People with blood type O have both antibodies and therefore can donate to any other blood type. • On the other hand, AB has neither antibodies and therefore can receive from any blood type. • Dominant genes do not mean that they are more popular in a given population. • Also, most genes do not control only one trait but are pleiotropic, affecting more than one phenotypic character. • In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus. • In mice, one gene determines whether or not there will be a coat color. • If that gene is turned off, the mouse will be white; if it is turned on, another locus will determine what the color is (brown or black). • Quantitative characters vary in a population along a continuum. • This is because of polygenic inheritance which is when more than one gene controls a single trait. • An example of this is skin color which is controlled by at least three different genes and is responsible for the variety of skin colors. • Phenotype also depends on environment. • For humans, nutrition influences height, exercise alters build, sun-tanning darkens the skin, and experience improves performance on intelligence tests. • Genetic experiments cannot ethically be performed on humans, so geneticists use pedigrees to look at traits found in families. • A family tree is then created showing the absence or presence of a specific trait to determine how it is passed. • Ethnicity plays a role in genetic disease patterns. • For example, sickle-cell anemia is found predominately in African-Americans. • This disease causes the red blood cells to be sickle shaped instead of the normal disk shape causing the cells to get stuck in the vessels. • Cystic fibrosis affects mostly Caucasians. • Cystic fibrosis is a multi-system disease that causes mucous to build up in various organs, especially the lungs. • Tay-Sachs affects people of Jewish descent. • Tay-Sachs affects the brains of small children, ultimately causing the death of the child prior to 5 years old. • Some genetic diseases, such as dwarfism, are dominant diseases. • This means that a child has a 50% chance of inheriting the disease because one of the parents has the disease. • Huntington’s disease is also a dominant disease that affects the nervous system. • Most dominant diseases are not lethal (Huntington’s disease is). • Genetic counseling is a field of study that works with people that have a history of genetic disease in the family. • A child with a recessive disease can be born to phenotypically normal parents. • There are several tests that can be performed to determine a couple’s risk. • One technique is amniocentesis. • Cells are extracted from the amniotic fluid surrounding the fetus and then analyzed to search for potential problems in a technique called karyotyping. • Karyotyping is essentially mapping out the chromosomes of an individual. • A second technique, chorionic villus sampling (CVS) can allow faster karyotyping and extracts a sample of fetal tissue from the chorionic villi of the placenta. • A more routine test is an ultrasound which detects only physical abnormalities that are present.