Genetics

• allele

  an alternative form of a gene that occupies the same position on homologous chromosomes
• artificial selection

  the process of breeding plants and animals with desirable characteristics in the hope that their offspring will inherit them
• asexual reproduction

  reproduction involving only one parent producing identical offspring
• bases

  adenine, thymine, cytosine, guanine
• carrier

  has inherited a genetic trait or mutation, but who does not display that trait, but can pass it on to their offspring
• characteristic

  an attribute of an individual eg tall plant, brown hair, blue eyes
• chromosome

  strand of DNA which contains many genes; a normal human cell has 46 (23 pairs)
• complementary base pairing

  used to describe how the bases A always bonds with T, and C with G
• continuous variation

  variation that shows a range eg skin colour / gradual changes, e.g. height and weight
• diploid

  cell with the full number of chromosomes (46 in humans)
• discontinuous variation

  variation that is "either/or" eg blood groups, ability to roll tongue or not
• DNA

  this is the chemical which carries genetic information in the nuclei of cells / made up of phosphate units, sugar units, and 4 types of bases
• dominant

  this is the allele which will be expressed in the phenotype if it is present as either a single gene or as two genes
• double helix

  name given to the twisted ladder shape of the DNA
• fertilisation

  male sex cell (gamete) combines with a female sex cell (gamete) to form a zygote
• gamete

  sex cell; an egg or a sperm cell
• gene

  a segment of DNA on a chromosome that codes for a specific trait
• genotype

  what genes you carry e.g. BB, Bb, bb
• haploid

  cell with half the full number of chromosomes
• heterozygous

  having two different alleles for a trait; a dominant and recessive allele e.g. Bb
• homozygous

  pair of alleles that are identical e.g. BB or bb
• hydrogen bonds

  attractive forces that hold the two strands of DNA together
• incomplete dominance

  situation where neither of the two genes (alleles) present mask the other
• karyotype

  a photograph or diagram of the chromosomes of a cell arranged in an orderly fashion
• meiosis

  this is a cell division that occurs in the gametes; reduces the number of chromosomes to half the normal number, producing 4 genetically unique daughter cells,
• mitosis

  this is a genetically exact division of a cell - produces 2 identical daughter cells
• monohybrid cross

  a straight forward cross involving only one pair of genes or alleles
• mutation

  this is a sudden, permanent change in a gene or whole chromosome
• natural selection

  organisms with characteristics best suited to their environment become more successful, and so expand in numbers
• nucleotide

  a unit built up of a base, a sugar and a phosphate
• pedigree chart

  a family tree drawn with standard genetic symbols, showing inheritance patterns for specific characteristics e.g. straight/curly hair
• phenotype ratio

  ratio of phenotypes e.g. 3 brown hair : 1 blond hair (a prediction of the phenotypes and their occurrence as a result of a cross)
• phenotype

  the expression of the genotype; this is what you look like and is a visible trait
• phosphate

  group that alternates with the ribose sugar in the DNA backbone
• punnet square

  name given to the grid of squares that may be drawn to show the range of combinations of genes that may occur
• pure breeding

  group of identical individuals that will always produce offspring of the same phenotype when interbred / are homozygous individuals e.g. BB or bb
• recessive

  this is the allele which is only expressed if there are two recessive alleles
• reduction division

  another name for meiosis
• ribose

  a sugar group that alternates with the phosphate group in the DNA backbone
• semi conservative

  each double-stranded DNA molecule is composed of one parental strand and one newly made strand