Genetics: The Science of Heredity Chapter 3 Section 1: Gregor Mendel • A young priest from Europe, became a ____________ and cared for hundreds of pea plants • Became curious about why some of the plants had different ____________ characteristics, or ____________ • He noticed that some plants were ____________ and some were ____________ • He also noticed some had ____________ seeds while some had ____________ seeds • Mendel observed that some pea plants had different ____________ that their parents • The ____________ of traits from parents to offspring is called ____________ • The study of heredity is ____________ Mendel’s Peas • Mendel chose pea plants to study because many of their traits exist in only ____________ forms: for example, height is either tall or short, not medium • Mendel developed a way to cross-pollinate, or “____________” pea plants • He removed pollen from a flower on one plant and ____________ it on a flower of a second plant Mendel’s Experiments • To study ____________ of traits, Mendel decided to cross plants with ____________ forms of a trait… • For example, he crossed tall plants with short plants to see what the ____________ would be • He always started his experiment with ____________ plants • A purebred plant is one that always produces ____________ with the same form of a ____________ as the ____________ (purebred short peas always produce short offspring) First Experiment • In his first experiment, Mendel crossed ____________ tall plants with purebred short plants (“P generation”) • He called the offspring of the “____________” the first filial, or the “F1 generation” (Filial means ____________ in Latin) • To his surprise, all of the F1 generation plants were tall even though one of the parents were short Second Experiment • Mendel let the F1 generation plants ____________ • These offspring were known as the ____________ • Mendel was surprised to see the ____________ were both ____________ and____________! • This occurred even though all of the F1 parents were____________! • Mendel discovered about ____ of the plants were tall and ____ were short Other Traits • Mendel also did experiments to check for: Seed____________, seed____________, seed ____________ color, pod____________, pod____________, and flower ____________ Dominant and Recessive Alleles • Factors that control each trait exist in ____________ …one from the ____________ parent and one from the ____________ • One factor in a pair can ____________ the other factor • In the F1 generation, the ____________ factor hid the short factor • Today, factors that control traits are called ____________ • Different forms of genes are called ____________ • In Mendel’s work, each pea plant inherits a ____________ of two alleles from its parents – Either: • Two alleles for ____________ stems • Two alleles for ____________ stems • One of each • Individual alleles control the ____________ of traits • Some alleles are ____________ – A trait that always shows up in the organism when the allele is present • Some alleles are ____________ – These are____________, or covered up, when the dominant allele is present – A trait controlled by a ____________ allele will only show up if the organism does not have the dominant trait Understanding Mendel’s Crosses • Here is how Mendel’s results work: • The P generation tall plants (____________) had two alleles for tall stems • The purebred short plants had two ____________ for short stems • The ____________ received one tall and one short allele • The F1 plants are called ____________ (they have different alleles for the trait) • All the F1 plants are tall because the ____________ allele (tall) covers the recessive allele (short) • When the F1 plants (hybrids) were crossed with each other, some inherited two ____________ alleles (these became tall plants) • Some inherited one dominant and one ____________ allele (these were tall) • Some inherited two ____________ alleles (these were short!) Symbols in Genetics • ____________ are used to represent traits in today’s world • A dominant allele is represented by a ____________ letter (T) • A recessive allele is represented by a ____________ letter (t) • TT would mean two ____________ alleles • tt would mean two ____________ alleles • Tt would mean one ____________ and one ____________ allele Mendel’s Contribution • In 1900, three scientists rediscovered Mendel’s work • Many of Mendel’s discoveries are still used today • Because of his important work, Gregor Mendel is known as the “________________” Section 2: Probability • Probability is the ____________ that a particular event will occur • If you flip a coin… the probability of landing on ____________ is 50%, and the probability of landing on ____________ is 50% Principles of Probability • There are three ways to express probability • As a____________: 1 to 2 • As a____________: 50% • As a ____________: ½ • The laws of probability predict what ____________ occur. • If you flip a coin 10 times you ____________ get 5 heads and 5 tails, or you ____________ get 1 head and 9 tails… Mendel and Probability • Remember…when Mendel crossed two ____________ plants (Tt), ¾ were tall and ¼ were short • Mendel stated that the ____________ of producing a tall plant was 3 in 4, and the probability of producing a short plant was 1 in 4 • Mendel was the first scientist to recognize that the principle of ____________ can be used to predict the results of ____________ crosses Punnett Squares • A ________________________ is a chart that shows possible combinations of alleles that can result from a genetic cross • They are also used to determine the probability of a particular ____________ • TT = 25% • Tt = 50% • tt = 25% Phenotypes • An organisms phenotype is its ____________ appearance • Phenotypes are ____________ traits • An example: Pea plants can have one of two different phenotypes for stem height: ____________ or ____________ These are physical characteristics of something! Genotypes • A genotype is the genetic makeup or the ____________ combinations • TT, Tt, tt • An organism that has two ____________ alleles for a trait is ____________ (TT, tt) • An organism with ____________ alleles for a trait is ____________ (Tt) Section 3: Chromosomes and Inheritance • In humans, each body cell (skin, blood, liver, etc.) has ____________ (or 23 pairs) • Sex cells (sperm and egg) contain ____________ • Walter Sutton, an American geneticist, discovered this • He concluded that genes are carried from parents to their offspring on____________. • This is known as the “Chromosome Theory of ____________” Meiosis • Meiosis is the process by which the number of chromosomes is ____________ by half to form sex cells (sperm and egg) • During meiosis the chromosome pairs separate and are distributed to two ____________ cells. • The sex cells that are created have only ____________ as many chromosomes as the other cells in the organism. Chromosomes • In body cells of humans, there are 23 pairs (46 total) chromosomes • Chromosomes are made of many ____________ joined together like beads on a string • Each gene controls a particular ____________ Section 4: DNA Connection • The main function of genes is to control the production of ____________ in cells • Proteins help to determine the____________, ____________, and other traits • ____________ is a major component in chromosomes • A = Adenine • T = Thymine • G = Guanine • C = Cytosine • Remember, these four bases form the “____________” of the DNA ladder Mutations • Any ____________ that occurs in a gene or chromosome is called a ____________ • Mutations can cause a cell to produce an ____________ protein during protein synthesis • If this happens, an organism’s phenotype (____________) will be different from what it should have been • If a mutation happens in a ____________ cell, only that cell is affected • If a mutation happens in a ____________ cell, offspring can be affected Effects of Mutations • Some mutations can be harmful – These mutations can ____________ an organism’s chance of survival • Some mutations can be helpful – These mutations can ____________ an organism’s chance of survival • Some can be neither