Chapter 14- Mendel and the Gene

14.1 Mendel’s Experimental System 

• Heredity is the inheritance or the transmission of traits from parents to offspring
• A trait is any observable characteristic of an individual, ranging from outward appearance such as height to molecular characteristics such as the prima structure of a particular membrane protein.
• A model organism is a species that is used for research because it is easy to work with and conclusions drawn from studying it may apply to many other species.
• Whenever a trait appears commonly in two or more different forms, for example purple or white flowers, it is called a polymorphic trait.
• Self-fertilization occurs when a flower's pollen falls on the female reproductive organ of that same flower.
•  Phenotype can be any observable characteristic, from molecular-level aits such as protein shape to the familiar aits of entire organisms at Mendel studied.
• A pure line consists of individuals that produce offspring identical to the parents when they are crossed to another member of the same pure-line population or are self-fertilized.
• Hybrids are offspring from matings between true breeding parents that differ in one or more traits. 

14.2 Mendel’s Experiments with a Single Trait

•  The individuals used in the initial cross are the parental generation. 
• Their progeny ( offspring) are the F1 generation.
• A mating between parents that each carry two different genetic determinants for the same trait-is called a monohybrid cross. 
• ==In genetics, the terms “dominant” and “recessive” identify only which phenotype is observed and which is masked in individuals carrying two different genetic determinants for a given trait.==
• Reciprocal cross is a set of matings where the mother’ s phenotype in the initial cross is the father’ s phenotype in a subsequent cross, and the father's phenotype in the initial cross is the mother’ s phenotype in a subsequent cross.
• Particulate inheritance is that the hereditary determinants for traits do not blend together or become modified through use.
• Different versions of the same gene are called alleles.
• The combination of alleles found in an individual is the genotype. 
• Two copies of the same allele are said to be homozygous.
• Two different alleles for the same gene are said to be heterozygous. 

14.3 Mendel’s Experiments with Two Traits 

• These F1 individuals are called dihybrids, and a mating between dihybrids is a dihybrid cross.
• The principle of independent assortment is that alleles of different genes are transmitted independently of one another. 
• In a traditional testcross, a parent with a dominant phenotype but unknown genotype is crossed with a parent that contributes only recessive alleles.

14.4 The Chromosome Theory of Inheritance 

• Common phenotypes are referred to as wild type
• Mutation is a heritable change in a gene.
• An individual with an unusual phenotype due to a mutation is referred to as a mutant. 
• A gene on the X chromosome is now described as an X-linked gene. 
• A gene on the Y chromosome is a Y-linked gene
• The general term for genes being located on either sex chromosome is sex-linked genes. 
• The patterns of inheritance of these genes are said to show sex-linked inheritance. 
• If the gene is on the X chromosome, it is X-linked inheritance, if on the Y chromosome, it’ s Y-linked inheritance.
• Genes on non-sex chromosomes are said to be autosomal, and their patterns of inheritance are called autosomal inheritance.

14.5 Extending Mendel’s Rules

• Linkage is the tendency of alleles of particular genes to be inherited together.
• Morgan referred to these individuals as recombinant because the alleles on their X chromosome were different (recombined) from the combinations present in their mother.
• ==Morgan concluded that alleles on the same chromosome then stay together, but not always.== 
• A genetic map is a diagram showing the relative positions (loci) of genes along a particular chromosome.
• The existence of more than two common alleles of the same gene is multiple allelism. 
•  In the traits Mendel studied, only the phenotype associated with the dominant allele appeared in heterozygotes. This form of dominance is called complete dominance. 
• A gene that influences many traits is said to be pleiotropic. 
• When two or more genes influence a single trait, it is called gene interaction.
• Epistasis occurs when the expression of a phenotype associated with a particular genotype of one gene can be completely masked by a particular genotype of a different gene.
• Environmental effects are anything that influences phenotypes other than the genotype, including temperature, sunlight, nutrient availability, competition, and even a mother’s hormone levels during development of an embryo.
• Discrete traits are traits that are clearly different from each other
• Continuously vaη ing traits that don’t fall into distinct categories are called quantitative traits.
• Polygenic is where each of many different genes adds a small amount to the value of the trait.

14.6 Applying Mendel’s Rules to Human Inheritance 

• A mode of transmission describes a trait as autosomal or sex-linked and the type of dominance of the allele. 
• To learn the mode of transmission, scientists construct a pedigree, or family tree, of affected and unaffected individuals. 
• Heterozygous individuals who do not have an inherited disease but carry a recessive allele for it are called carriers of the disease.

\