Module 1 - Genetics

• Genetics is the study of how genes and traits are passed down from one generation to the next.
• Genetics was derived from the Greek word gen, meaning to become or to grow into something. The term was coined by William Bateson in 1906.
• Inheritance in genetic terms means an offspring, or child receiving certain traits, behaviors, or characteristics from your mother or father.
• The set of all the genes found in a cell responsible for the characteristics of an individual is called a genome.
• • In sexually reproducing organisms, the genome is composed of two sets of chromosomes, paternal and maternal sets. Thereby, in a diploid (2n) individual, every chromosome is in a pair.
• • In humans, there are pairs of homologous chromosomes, which refer to a pair of paternal and maternal chromosomes that are of the same size, have similar gene locations, common banding patterns, and have the same centromere location. 
• Alleles are the number of alternative forms of the same gene occupying a given position on a chromosome.
• Genotypes

  • It is the totality of the genetic makeup of an individual.
  • Also refers to the combination of alleles found in a homologous chromosome.
  • It is represented by symbols (letters). Examples: AA, Aa, or aa
• Phenotypes

  • The observable expression of a particular genotype
  • Examples are eye color (blue or black), seed shape (round or wrinkled), and flower color (purple or white).
• When two similar alleles are present in a pair (such as BB or bb), the genotype is called homozygous.
• When two different alleles are paired (such as Bb), the genotype is called heterozygous
• The homozygous genotype contains the same allele while the heterozygous genotype contains different alleles.
• The allele that is expressed in a heterozygous condition is called the dominant allele whereas the one that is not expressed is called the recessive allele.
• Genetic variation is the variation of genomes between individuals in the same species due to the genetic mutations that occur during sexual reproduction.
• Chromosomal crossing over is responsible for introducing new genotypes to the species.
• Genetic variation is an important mechanism, which forces evolution through natural selection. It is important in maintaining biodiversity among species as well.
• Genetic variation can be caused by mutations of genes, gene flow, random mating, random fertilization, and crossing over between homologous chromosomes.
• Random mating, random fertilization, and chromosomal crossing over lead to the reshuffling of alleles within a particular species.
• Mutations introduce new genes or alleles to the population.
• Environmental variation is the alteration of the phenotype of a particular genotype as a response to the environment.
• Environmental variation is also known as phenotypic plasticity.
• Genetic variation and environmental variation are two factors that bring alterations in the phenotype of a population.
• Genetic variations are caused by the changes in the genome of individuals. It mainly occurs during sexual reproduction.
•  The effect of the external environmental factors on the genotype makes changes in the phenotype in environmental variation.
•  Only genetic variation affects evolution.
• Meiosis is a process where a single cell divides twice to produce four cells containing half the original amount of genetic information.
• Segregation occurs because each gamete inherits only one copy of each chromosome.  Each chromosome has only one copy of each gene; therefore each gamete only gets one allele.

  
• Segregation occurs when the homologous chromosomes separate during meiotic anaphase I.
• Independent assortment occurs because homologous chromosomes are randomly segregated into different gametes, that is, one gamete does not only get all maternal chromosomes while the other gets all paternal chromosomes.

  
• Independent assortment occurs when homologous chromosomes align randomly at the metaphase plate during meiotic metaphase I.
• Incomplete dominance is exhibited when the heterozygote has an intermediate phenotype between that of either homozygote superficially resembles the idea of blending inheritance, but can still be explained using Mendel’s laws with modification. In this case, alleles do not exert full dominance and the offspring resemble a mixture of the two phenotypes.

  
• co-dominance occurs when pairs of alleles are both expressed equally in the phenotype of a heterozygous individual. Heterozygotes, therefore, have an altered phenotype as the alleles are having a joint effect.

  
• When more than two alleles exist in a certain character, the pattern of inheritance is called multiple alleles. An example of this is the human blood type. Human red blood cells can be categorized into different blood groups based on the structure of a surface glycoprotein (antigen).
• Height and other similar features are controlled not just by one gene, but rather, by multiple (often many) genes that each make a small contribution to the overall outcome. This inheritance pattern is sometimes called polygenic inheritance. It occurs when a trait is governed by two or more sets of alleles.
• Pleiotropy is a condition in which one gene causes several effects (multiple traits) in various organs of the body. Genes with pleiotropy effects tend to be more severe and may affect multiple systems. Pleiotropy will often result in a syndrome, which is a group of symptoms that may appear simultaneously, an indication of a particular genetic mutation.
• The sex chromosomes in females are XX, and those in males are XY.
• The X chromosome is quite large and contains nearly 2,000 genes, most of which have nothing to do with the gender of the individual.
• The Y chromosome is shorter and contains only about 80 genes, and most of these genes are concerned with sex differences between men and women.
• Sex-linked/ X-linked disorders are disorders that are caused by abnormalities in the genes carried on the X chromosome.
• A pedigree is a diagram that depicts the biological relationships between an organism and its ancestors. Often, it is used to look at the transmission of genetic disorders. These are also used to analyze the pattern of inheritance of a particular trait throughout a family.