Download Basic Principles and Genetic Crosses

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.
traits are said to be X linked or Sex Linked
 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.