genetics

• Gene


  Codes for means of communication, a sequence of bases on a DNA molecule that codes for a protein (polypeptide)
• Allele


  A variant version of a gene. There can be many different alleles of a single gene, but organisms including humans only carry two alleles of each gene, one from each parent. The order of bases in each allele is slightly different - they code for different versions of the same characteristic. They're represented using letters, e.g. the allele for brown eyes (B) and the allele for blue eyes (b)
• Genotype


  The genetic constitution of an organism - the alleles an organism has, e.g. BB, Bb or bb for eye colour
• Phenotype


  The expression of the genetic constitution and its interaction with the environment - an organism's characteristics e.g. brown eyes
• Dominant


  An allele whose characteristic appears in the phenotype even when there's only one copy. Dominant alleles are shown by a capital letter. E.g. the allele for brown eyes (B) is dominant - if a person's genotype is Bb or BB, they have brown eyes
• Recessive


  An allele whose characteristic only appears in the phenotype if two copies are present. Recessive alleles are shown by a lower case letter. E.g. the allele for blue eyes (b) is recessive - if a person's genotype is bb, they'll have blue eyes
• Codominant


  Alleles that are both expressed in the phenotype, neither one is recessive, e.g. the alleles for haemoglobin
• Locus


  The fixed position of a gene on a chromosome. Alleles of a gene are found at the same locus on each chromosome in a pair
• Homozygous


  An organism that carries two copies of the same allele, e.g. BB or bb
• Heterozygous


  An organism that carries two different alleles, e.g. Bb
• Carrier


  A person carrying an allele which is not expressed in the phenotype but that can be passed on to offspring
• Genetic Diagrams Show the Possible Genotypes of Offspring

  1. Humans are diploid organisms (we have two sets of chromosomes) so we have two alleles for each gene

  2. Gametes (sex cells) contain only one allele for each gene

  3. When gametes from two parents fuse together, the alleles they contain form the genotype of the offspring produced

  4. At each locus, the genotype can be homozygous or heterozygous
• Monohybrid Crosses


  1. Monohybrid inheritance is the inheritance of a characteristic controlled by a single gene

  2. Monohybrid crosses show the likelihood of the different alleles of that gene (and so different versions of the characteristic) being inherited by offspring of certain parents
• Monohybrid Cross Example: Fruit Fly Wing Length

  • N - normal wings allele
  • n - vestigial (little) wings allele
  • NN - normal wings
  • Nn - normal wings
  • nn - vestigial wings
• The normal winged parent only produces gametes with the allele for normal wings (N). The vestigial winged parent only produces gametes with the allele for vestigial wings (n).
• All F1 offspring are heterozygous (Nn), as one allele is inherited from each parent.
• The F2 offspring could have either normal or vestigial wings. But there's a 75% chance they'll have the normal wings phenotype (genotype of NN or Nn) and a 25% chance they have the vestigial wings phenotype (genotype nn). So you'd expect a 3:1 ratio of normal: vestigial wings in the offspring.
• Whenever you do a monohybrid cross with two heterozygous parents you get a 3:1 ratio of dominant: recessive characteristic.
• Punnett square


  A way of showing a genetic diagram to predict the type of offspring
• Punnett square


  1. Work out the alleles the gametes would have

  2. Cross the gamete alleles of the F1 generation to do the possible genotypes

  3. The genotypes show a 75% chance offspring will have all wings and a 25% chance they'll have vestigial wings
• Crossing parents' gametes


  Show the possible genotypes of the F1 generation
• Codominance


  Both alleles are expressed in the phenotype
• Codominance example


  • Sickle-cell anaemia in humans
• Sickle-cell anaemia


  • People homozygous for normal haemoglobin don't have the disease
  • People homozygous for sickle haemoglobin have sickle-cell anaemia
  • People heterozygous have sickle-cell trait - they have some normal haemoglobin and some sickle haemoglobin
• Crossing parents with sickle-cell trait

  Show the possible offspring genotypes
• Multiple alleles


  More than two alleles of the same gene
• Multiple alleles example


  • ABO blood group system in humans
• Phenotypic ratio


  The ratio of different phenotypes in offspring
• Genetic diagrams allow you to predict the phenotypic ratios in F1 and F2 offspring
• Phenotypic ratios for different crosses

  • Monohybrid - 3:1
  • Dihybrid - 9:3:3:1
  • Codominant - 1:2:1
  • Homozygous for one allele, heterozygous for the other - 1:2:1
• Genetic diagrams


  • Can show how more than one characteristic is inherited
  • Can be used for hybrid crosses to look at how two different genes are inherited
• Sometimes the expected (predicted) phenotypic ratio won't be obtained, due to sex linkage, autosomal linkage or epistasis
• Linkage


  Genes located on the same chromosome are said to be linked
• Epistasis


  One gene masks the expression of another gene
• Sex-linked characteristics are carried on sex chromosomes
• Sex chromosomes


  • The genetic information for biological sex is carried on them
  • Females have two X chromosomes, males have one X and one Y chromosome
• Sex-linked characteristic


  A characteristic where the allele that codes for it is located on a sex chromosome
• The Y chromosome is smaller than the X chromosome and carries fewer genes
• Most genes on the sex chromosomes are carried on the X chromosome
• Hemizygous


  Individuals with only one copy of a gene (e.g. males for sex-linked characteristics)