Leaving Certificate Agricultural Science Genetics Introduction Introduction Genetics

Leaving Certificate Agricultural Science Genetics - Introduction

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.

Genetic Definitions �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.

Genetic Definitions �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 F 1 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).

Genetics' Problems – Monohybrid

Genetics' Problems – Monohybrid 2

Incomplete Dominance � 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.

Mendel’s First Law � 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.

Mendel’s Second Law � 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. Aa. Bb, each of the A’s can join with either of the B’s at gamete formation. � Thus we can have four gametes: AB, Ab, a. B and ab.

Sex Determination � 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.

Sex Determination - 2 � 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

Genetic Study Using Fruit Flies � 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.