Genetics - Introduction

 Genetics is the study of inheritance.  In terms of agriculture, genetics is everything.  An understanding of genetics is critical in breeding strategies in any animal enterprise and in producing new strains of barley, wheat, potatoes etc.  Most of what we know about genetics was obtained from the work of an Austrian monk called Gregor Mendel.  He used pea plants to study how characteristics were passed from one generation to the next.  Pea plants grow quickly and are self pollinating.

 A gene is a section of a chromosome that controls a specific trait.  An allele is a different form of the same gene (e.g. A gene for flower colour could have different alleles, for example for purple or white)  A locus is the location of a gene on the chromosome.  The genotype is the set of genes an individual possesses.  Dominant genes are genes that are always expressed when present.  Recessive genes are genes that are only expressed in the absence of a dominant genes.  The phenotype is the physical effect produced by the gene.

 A mutation is any change in the structure of the gene, which may be inherited.  Homologous chromosomes are pairs of chromosomes with identical genes and loci on them.  An F1 Cross is a cross between two pure breeding parents.  Polyploidy occurs in cells containing multiple copies of a chromosome.  Heterozygous means having one dominant and one recessive gene for a characteristic (e.g.Pp)  Homozygous means having either two dominant or two recessive genes for a characteristic (e.g.PP or pp).

 Mendel studied traits that were clearly dominant or recessive  In incomplete dominance, no allele is dominant and the offspring produce traits that are “in-between”.  For example, in Snapdragons, when red flowers (PP) and white flowers (pp) cross pollinate (cross fertilise), the offspring (Pp) will produce pink flowers.  Heterozygous plants, when cross-fertilised with another heterozygous, will produce red, pink and white flowers.

 The Law of Segregation states:  Alleles of a gene exist in pairs but when gametes (sex cells) are formed the members of each pair pass into different gametes.  This means that a gamete has only one allele of each gene.  At fertilisation the offspring will have two alleles again, one from each parent.

 The Law of Independent Assortment state:  At Gamete formation, the separation of one pair of alleles is completely independent to the separation of all other alleles.  Example:  If we look at an organism with two genes e.g. AaBb, each of the A’s can join with either of the B’s at gamete formation.  Thus we can have four gametes: AB, Ab, aB and ab.

 In human, all cells (except gametes) contain 23 pairs (46) chromosomes.  44 of these are called autosomes, which means are found in both males and females.  The other 2 are called X and Y, and their presence determine male or female.  An individual with XX would be female, while XY male.

 It must be noted that the Y chromosome is missing some of the genes of the X chromosome.  Examples include colour vision, blood clotting and muscle development.  These traits are said to be X linked or Sex Linked traits.

 Fruit flies or Drosophila are used to study genetics today.  Fruit flies are suitable because:  They are easy to grow.  They reproduce a new set of offspring in just two weeks.  They have large chromosomes.  They have only four chromosomes.  They produce in large numbers.