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Basic summary if genetics
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TOPIC 5: BASIC AGRICULTURAL GENETICSGenetic concepts
Genetic terminology:
- Genetics: the study of heredity or how characteristics are passed on from parents to
offspring.
- Heredity: the transfer of characteristics, such as height and eye colour, from parents
to their offspring
- Gene: A section of DNA molecule that contains the information for a particular
genetic characteristic.
- Chromosomes: Threadlike structures in the nucleus of the cell, made up of DNA
- Alleles: Different forms of the same gene.
- Homozygous: individual has the same alleles for a particular gene on both
chromosomes of a homologous pair.
- Homozygous: homo means the same
- Homozygous individuals are called homozygotes. For example, a pea plant that is
homozygous for a flower colour has two purple alleles for flower colour or two white
alleles for flower colour.
- Heterozygous: individual has different alleles for a particular gene in each of the
chromosomes of a homologous pair.
- Heterozygous: hetero means different
- Heterozygous individuals are called heterozygotes. For example, a pea plant that is
heterozygous for flower colour has one purple allele and one white allele for flower
colour.
- Mutagenic agent (mutagen): any factor that causes a mutation to occur
Distinction between genotype and phenotype
- Genotype: The genetic composition of an individual
- Phenotype: The visible characteristics of an individual
Distinction between dominant and recessive genes
- Dominant genes: allele that overrides or masks the characteristics of a recessive
allele.
- Recessive genes: allele that is masked by the dominant allele
,The monohybrid inheritance/crosses: Mendel’s First Law: Law of Segregation
- Mendel’s First Law: Law of Segregation: The separation of a pair of alleles on
homologous chromosomes.
Mendel’s First Law: states that there are two genes or alleles controlling a given
characteristic, and these alleles separate and go to different gametes when a parent
reproduces.
- Monohybrid cross: A genetic cross between parents that have different alleles for
one particular gene.
- Example of a monohybrid cross: the cross between a pea plant with purple flowers
and a pea plant with white flowers.
- Both parents must be pure breeding individuals.
- Pure breeding individuals always produce the same characteristics as the parent
plants when they are self pollinated.
- Pure breeding individuals are homozygous.
Two steps of monohybrid cross
1. Parental cross: The parent plants are cross pollinated to produce the first
generation of offspring. First generation called F1 generation (filial generation).
- Filial= sons or daugheters of a parent
2. F1 cross: The F1 individuals are then allowed to self pollinate to produce a second
generation of offspring. Second generation called F2 generation (second filial
generation).
A genetic diagram to show a monohybrid cross
, The punnet square
The Dihybrid inheritance/dihybrid crosses: Mendel’s Second Law: Law of Independent
Assortment
- Mendel’s Second Law: Law of Independent Assortment: The alleles of different
genes are arranged independent of one another
- Dihybrid crosses: a study of inheritance patterns for organisms differing in two traits
The use of various methods such as Punnet square, genetic diagrams and schematic
representations to illustrate the crosses
Mendel’s second Law: The law of Independent Asortment (independent recombination
of characteristics)
- States that pairs of alleles separate independently during the formation of gametes.
This means that traits are passed to offspring independently of one another.
Qualitative and quantitative characteristics
Qualitative characteristics
- Characteristics such as eye colour or gender (i.e. whether the organism is male or
female).
- In other words, qualitative characteristics involve a particular quality of the organism.
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