Leaving Certificate Agricultural Science

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Leaving Certificate Agricultural Science
Genetics - Introduction
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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.

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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.

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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 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).

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Genetics' Problems Monohybrid
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Genetics' Problems Monohybrid 2
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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.

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Mendels 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.

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Mendels 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.
  AaBb, each of the As can join with either of the
  Bs at gamete formation.
• Thus we can have four gametes AB, Ab, aB and ab.

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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.

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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 traits.

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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.