Gr12 Genetics

• Genetics is the biological branch of study that focuses on the heredity and variation in organisms.

  This includes physical and behavioural traits that are transmitted from parent to offspring.
• Gregor Mendel, an Austrian monk (1822 - 1884), worked in the gardens of the monastery and through the cross-breeding of pea plants discovered the basics of heredity and genetics. He is the "Father of Genetics" 

   

  Mendel examined 7 traits during his pea plant experimentation. The seven traits are shown below. Strangely, he found that when he allowed the pea plants to reproduce, certain traits were hidden while others were expressed in the offspring plants.
• Law of segregation


  States that, for each characteristic, an organism possesses 1 gene consisting of 2 alleles which separate during meiosis so that each gamete only contains 1 allele
• Law of Dominance


  States that when two individuals with pure breeding (homozygous) contrasting characteristics are crossed, the individuals of the F1 generation all display the dominant characteristics
• Law of Independent Assortment

  States that the various "factors" controlling the different characteristics (seeds shape, seed colour, plant height, and flower colour) are separate entities, not influencing each other in any way, and sorting themselves out independently during gamete formation
• • Gene - particular length of DNA at a particular location (locus) on a chromosome that influences a particular characteristic.
• • Allele - alternative form of a gene found at the same location (locus).
• • Dominant allele - allele that is always expressed in the phenotype and represented by a capital letter.
• • Recessive allele - allele that is only expressed in the phenotype if the dominant allele is not present. Represented by the lower case letter, same letter as the dominant allele.
• • Locus - the location of a gene on the chromosome.
• • Phenotype - the external (physical) appearance of an organism.
• • Genotype - the genetic composition of an organism.
• • Heterozygous - organism has 2 different variations of the allele for that characteristic.
• • Homozygous - organism has 2 copies of the same allele for that characteristic.
• • Autosome - referring to a chromosome that is not a sex chromosome.
• • Gonosome - referring to a sex chromosome.
• • Genome - the entire genetic blueprint of an organism.
• • Karyotype - an individual's collection of chromosomes.
• • sex-linked allele an allele located on the sex chromosome.
• • sex-linked disorder - a disease or abnormality determined by the genes located on the sex chromosomes.
• • carrier - a heterozygous organism that carries an allele for a particular disorder without it being expressed.
• • mutation - a sudden unexpected change in the genetic structure of a cell.
• • mutagen - factor that cause mutations.
• Homologous chromosomes


  One chromosome from biological mother and other chromosome from biological father
• Alleles


  Variations of a gene located at the same locus (location) on homologous chromosomes
• Homozygous allele pair


  • Alleles carry the same variation of the gene
• Homozygous allele pair


  • Homozygous dominant (both alleles are for the dominant trait)
  • Homozygous recessive (both alleles are for the recessive trait)
• Heterozygous allele pair


  Alleles carry different variations of the gene
• Dominant alleles will be expressed physically in the individual, that trait will be the phenotype. Thus dominant alleles will "mask" a recessive allele if it present in the genotype, which is why the physical appearance will only show the dominant allele. If there are two dominant alleles present, the genotype is referred to as homozygous dominant; however if there is a dominant allele and a recessive allele the genotype is referred to as heterozygous - the phenotype for both of these genotypes will be the physical expression of the dominant trait.
• Recessive alleles will only be expressed in the phenotype, when there are NO dominant alleles present, thus both alleles are recessive in the genotype, and will be referred to as homozygous recessive.
• Alleles are represented by letters, usually with the first letter of the dominant characteristic.

  The dominant trait/characteristic is represented by a capital letter and the recessive trait/characteristic is represented by a lower case letter.

  For example for height, where tall is the dominant trait, tall = T, and short is the recessive trait, short = t.
• Monohybrid - crossing of one characteristic.

  In order to work out the possibilities offspring have of inheriting a specific trait from their genetic parents, we use one of two methods:

  • Monohybrid Cross
  • Punnett Square
• If we take Mendel's pea plants as a working example, and look at the height of the plant as our trait to cross - so our trait will be represented by the letter T in the dominant form for tall and t in the recessive form for short.
• If we take Mendel's pea plants as a working example, and look at the height of the plant as our trait to cross - so our trait will be represented by the letter T in the dominant form for tall and t in the recessive form for short.
• Before you draw your cross/Punnett Square

  • Characteristic being crossed; 
  • Identification of the letters representing the dominant allele, and the recessive allele;
  • Phenotype of parents (P1);
  • Genotype of parents (P1);
  • Meiosis as a heading (to indicate that meiosis has taken place to get the gametes)
  • Gametes of the parents.
  • Fertilisation as a heading before the genetic diagram (to indicate the possible outcomes that could be achieved with fertilisation)
• Draw your genetic diagram (Cross/Punnett Square)

  After you draw your cross/Punnett square

  • Phenotype of offspring as a percentage
  • Phenotype of offspring as a ratio
  • Genotype of offspring as a percentage
  • Genotype of offspring as a ratio.
• Let's have a look at another example with All the required components:

  Characteristic: height of stem

  Key: T = tall; t = short

  P1:            Phenotype: Tall x Tall       (P1 representing the Parent generation)

                   Genotype: Tt x Tt

  Meiosis: (must be included)

  Gametes:  T  t   x   T   t

  Fertilisation: (must be included)

  F1:           Genotype:  25% TT; 50% Tt: 25% tt   1:2:1  (F1 representing the offspring (filial) generation)

                  Phenotype:  75% Tall; 25% Short    3:1
• Co-dominance:

  Both alleles are expressed equally in the heterozygous individual. Both alleles are equally dominant. An example of this is coat colour in cattle and blood groups.
• Incomplete/partial dominance:

  Neither allele is dominant, so a blending of the phenotypes are expressed, a mixing of the phenotypes.

  An example of this are some flower colours or curly hair crossing with straight hair results in wavy hair. 
• The phenotypic ratio in a dihybrid cross between two heterozygous individuals is 9:3:3:1, this is based on the inference that there is complete dominance in the genes