Genetics

• Genetics


  Study of inheritance in all its manifestations, from distribution of traits from parent to offsprings to the biochemistry of genetic material DNA
• Homologous Chromosomes


  Pairs of chromosomes which carry the same sets of genes, with one of each air being derived from each biological parent
• Gene


  Basic unit of heredity for a particular characteristic
• Allele


  2 or more alternative forms of the same gene, which occupy corresponding loci on homologous chromosomes and give rise to contrasting characteristics
• Locus


  Position of an allele/gene on a DNA molecule
• Karyotype


  Complete chromosome complement of a somatic cell
• Heterozygote


  Diploid conditions where one possesses 2 different alleles for the same gene
• Homozygote


  Diploid condition where one possesses 2 of the same allele for the same gene
• Dominant Allele


  Allele which is expressed in the phenotype even in the presence of an alternative allele, i.e. in both homozygous and heterozygous state
• Recessive Allele


  Allele which is expressed in the phenotype only in the homozygous state
• Phenotype


  Visible appearance/characteristics of an individual; affected both by alleles possessed AND environment
• Genotype


  Genetic make-up for a particular trait, in terms of pair of alleles
• F1 Generation


  Offspring resulting from cross between parental individuals
• F2 Generation


  Offspring obtained from crossing 2 F1 individuals
• Pure bred line


  Group of individuals with similar genetic background
• Monohybrid Inheritance


  Study of the inheritance pattern of a single characteristic controlled by a single gene
• Law of Segregation


  A diploid organism possesses alleles which occur in pairs, each of which coming from either parent. During anaphase I of meiosis, alleles are randomly separated such that each gamete produced only contains 1 allele
• Test Cross


  Test used to distinguish between homozygous dominant and heterozygous state by crossing with a homozygous recessive individual and observing the resultant offspring
• Back Cross


  Same as test cross but specifically using a homozygous recessive parent
• Dihybrid Cross


  Study of the inheritance pattern of alleles of 2 genes for different characteristics which are carried on different chromosomes, i.e. unlinked genes through 2 generations
• Recombinant Phenotypes


  Phenotypes in dihybrid inheritance where individuals have 1 characteristic from each parent rather than resembling either of the parents directly
• Law of Independent Assortment

  During meiosis, allele pairs of 2 unlinked genes for different traits are segregated into gametes independently of each other
• Pedigree


  Record in diagrammatic form to represent the pattern of inheritance of a particular trait through several generations of a family
• Pedigree Analysis


  Study of illustration of the inheritance pattern of a trait in several generations of a family to understand the inheritance pattern and predict possible genotypes based on phenotypes
• Co-Dominance


  When 2 contrasting alleles for a particular trait are equally dominant over each other and are both expressed phenotypically in heterozygotes
• Incomplete Dominance


  When heterozygotes for a particular characteristic show an intermediate phenotype between the homozygous phenotypes
• Multiple Alleles


  When, amongst a population, more than 2 alternative forms of the gene exist; i.e. when there are 3 or more alleles for the same gene amongst individuals of the same population – dominance hierarchy
• Gene Complex


  Multiple genes at different loci interact to form 1 functional unit and contribute to the phenotype
• Epistasis


  Interaction between 2 independent genes, such that one gene (epistatic) masks/suppresses the expression of the other gene (hypostatic), therefore modifying the overall phenotype
• Polygenic Inheritance


  Interaction when the phenotype is a result of the combined effect of multiple genes, giving rise to continuous variation (gradation of small differences), which give bell-shaped normal distribution frequency curve
• Environment Interactions


  Physical environment can interact with the genetic constitution of an individual, therefore affecting the ultimate phenotype observed; i.e. degree to which certain alleles are expressed depends on environment over and above alleles themselves
• Discontinuous Variation


  When phenotypes are distinct classes/categories, unaffected by environment and usually controlled by few genes – illustrated as bar graph
• Autosomal Linkage


  When 2 genes occur on the same autosomal (non-sex) chromosome
• Linkage Group


  Group of genes with loci on the same chromosome, which therefore are inherited as a unit and are not subject to independent assortment in meiosis
• Recombinant Frequency


  Total no. of recombinants / total no. of offspring ; Reflection of the distance between 2 linked genes on the same chromosome
• Chromosome Map


  Representation of a chromosome showing the loci of linked genes on the DNA molecule and the distance between them based on the cross-over frequencies
• Map Unit


  1 m.u. = 1% cross-over frequency = 1 cM
• Homogametic


  Gender which produces only 1 type of gamete/ gamete with only 1 type of chromosome e.g. all human female gametes possess X chromosome
• Heterogametic


  Gender produces 2 types of gametes, some possessing the X chromosome and some possessing the Y e.g. human sperm [NOT THE CASE FOR BIRDS AND SOME INSECTS]
• Sex Linkage


  Occurs when genes other than those for sexual characteristics are carried on the sex chromosomes