Biology Chapter 9 9.1 - Principles of Genetics: An OverviewGenes determine traits:- A genetic trait is any inherited characteristic of an organism that can be observed or detected in some manner. - At the molecular level, a gene consists of a stretch of DNA. - Some genetic traits are invariant or nearly invariant, meaning that they are about the same in all individuals in the population. - The display of a particular version of a genetic trait in an individual is the phenotype of that individual. Diploid cells have two copies of every gene: - The two copies make up a homologous pair for each chromosome type. - One member of the homologous pair is inherited from the male parent and is called the paternal homologue. - The other member of the pair is inherited from the female parent and is called the maternal homologue. - A diploid cell has two copies of every gene located on those chromosomes. - Gametes- sperm and egg cells – are haploid because these cells have only one set of chromosomes. - Because a gamete has only one copy of each pair of homologous chromosomes, it possesses only one copy of every gene. Genotype directs phenotype:- Different versions of a particular gene are known as alleles of that gene. - The genotype of an individual is the allelic make up of that individual for a specific genetic trait. - In other words, a genotype is the pair of alleles that code for a phenotype.- An individual that carries two copies of the same allele is said to be homozygous for that gene. - For example, dogs with the WW or ww genotype are homozygotes. - An individual with a genotype consisting of two different alleles is said to be heterozygous. Some phenotypes are controlled by dominant alleles: - The allele that exerts a controlling influence on the phenotype is said to be dominant. - An allele that has no effect on the phenotype when paired with a dominant allele is said to be recessive. Gene mutations are the source of new cells:- Different alleles of a gene arise by mutation which we can define briefly here as any change in the DNA that makes up a gene. - When a mutation occurs, the new allele instructions may result in a protein whose form differs from the original version. - Harmful or nonfunctional alleles tend to be recessive: their effects do not show up unless two copies are present. Controlled crosses help us understand patterns of inheritance: - A genetic cross so a controlled mating experiment performed to examine how a particular trait may be inherited. - The parent generation in a genetic cross is called P generation. - The first generation of offspring is called the F1 generation. - When the individuals of the F1 generation are crossed with each other, the resulting offspring are said to be the F2 generation. 9.2 – Basic Patterns of InheritanceMendel’s genetic experiments began with true-breeding pea plants: - Mendel proposed that the inherited characteristics of organisms are controlled by heredity factors known as genes and that one factor for each trait is inherited from each parent. - Mendel crossed true-breeding lines with contrasting phenotypes. - He tracked the phenotypes through two generations of hybrid offspring – that is, offspring resulting from a cross between two purebreds. Mendel inferred that inherited traits are determined by genes: - These results led him to propose a new theory of inheritance exist as discrete factors that do not lose their unique characteristics when crossed.Mendel summarized the results of his experiments in two laws: the law of segregation and the law of indepentdent