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Mendelian Genetics and Heredity CHAPTER 6 SECTIONS 3 - 6 Gregor Mendel laid the groundwork for genetics. Genetics is the study of biological inheritance patterns and variation. Gregor Mendel showed that traits are inherited as discrete units. Many in Mendel’s day thought traits were blended. Studied pea plants • Mendel used pollen to fertilize selected pea plants. – P generation (purebred parents) crossed to produce F1 generation (offspring) – interrupted the self-pollination process by removing male flower parts Mendel controlled the fertilization of his pea plants by removing the male parts, or stamens. He then fertilized the female part, or pistil, with pollen from a different pea plant. • Mendel allowed the resulting plants to self-pollinate. – Among the F1 generation, all plants had purple flowers – F1 plants are all heterozygous – Among the F2 generation, some plants had purple flowers and some had white Law of Segregation – Organisms inherit two copies of each gene, one from each parent. – The two copies segregate during gamete formation. purple white The same gene can have many versions. A gene is a piece of DNA that directs a cell to make a certain protein. Each gene has a locus, a specific position on a pair of homologous chromosomes. An allele is any alternative form of a gene occurring at a specific locus on a chromosome. – Each parent donates one allele for every gene. – Homozygous describes two alleles that are the same at a specific locus. – Heterozygous describes two alleles that are different at a specific locus. Genes influence the development of traits. All of an organism’s genetic material is called the genome. • A genotype refers to the makeup of a specific set of genes. Example of genotype: Aa and Bb • A phenotype is the physical expression of a trait. Example of phenotype: Blue eyes and Brown eyes • Alleles can be represented using letters. – A dominant allele is expressed as a phenotype when at least one allele is dominant. – A recessive allele is expressed as a phenotype only when two copies are present. – Dominant alleles are represented by uppercase letters; recessive alleles by lowercase letters. • Both homozygous dominant and heterozygous genotypes yield a dominant phenotype. • Homozygous dominant = AA • Heterozygous = Aa • Both contain a capital letter “A” and express the same phenotype. Punnett squares illustrate genetic crosses. The Punnett square is a grid system for predicting all possible genotypes resulting from a cross. The axes represent the possible gametes of each parent. The boxes show the possible genotypes of the offspring. • The Punnett square yields the ratio of possible genotypes and phenotypes. A monohybrid cross involves one trait. Monohybrid crosses examine the inheritance of only one specific trait. homozygous dominant parent crossed with homozygous recessive parent yield 100% heterozygous offspring (F1 generation), which all express dominant phenotype. – Heterozygous parent crossed with heterozygous parent will yield a genotypic ratio of 1:2:1 (which means 1 homozygous dominant to 2 heterozygous to 1 homozygous recessive; or a phenotypic ratio of 3:1 (which means 3 dominant to 1 recessive). • Heterozygous parent crossed with a homozygous recessive parent will yield a genotypic ratio of 1:1 (which means 1 heterozygous to 1 homozygous recessive); or a phenotypic ratio of 1:1 (which means 1 dominant to 1 recessive). •A testcross (backcross) is a cross between an organism with an unknown genotype and an organism with the recessive phenotype. •You must look at phenotype of offspring to determine genotype of unknown parent. A dihybrid cross involves two traits. Mendel wanted to see if pea shape and color were inherited together. In other words, are all round peas colored yellow? Mendel’s dihybrid cross of 2 heterozygous plants yielded a 9:3:3:1 phenotypic ratio. • Mendel found that pea shape and color are not inherited together. Factor gametes using FOIL method. First Outer Inner Last Law of Independent Assortment •The law of independent assortment states that allele pairs separate independently of each other during meiosis. Not all corn kernels are yellow! Heredity patterns can be calculated with probability. Probability is the likelihood that something will happen. Probability predicts an average number of occurrences, not an exact number of occurrences. number of ways a specific event can occur • Probability = number of total possible outcomes • Probability applies to random events such as meiosis and fertilization. Example: What are the chances of a quarter landing on heads if you flip it 3 times in a row? ½ x ½ x ½ = 1/8 Law of Segregation, Law of Independent assortment and crossing over during meiosis result in genetic diversity. Sexual reproduction creates unique combination of genes by independent assortment of chromosomes in meiosis random fertilization of gametes Unique phenotypes may give a reproductive advantage to some organisms. Example: dark skin is better adapted for tropical climates because the sunlight is most direct in these areas. Skin color is controlled by a pigment called melanin. Dark skin produces more melanin than light skin, which acts as a natural “sunscreen” to protect DNA from UV damage. Light skin is more at risk for skin cancer due to the lack of increased melanin production to protect the DNA from genetic mutation. Crossing over during meiosis increases genetic diversity. Crossing over is the exchange of chromosome segments between homologous chromosomes. occurs during prophase I of meiosis I results in new combinations of genes • Chromosomes contain many genes. – The farther apart two genes are located on a chromosome, the more likely they are to be separated by crossing over. – Genes located close together on a chromosome tend to be inherited together, which is called genetic linkage.