2. GENES AND GENETIC STUDIES
A gene is the unit that transmits genetic information
Genotype is the set of genes that an individual has
Phenotype is the set of observable traits that an individual has
A gene can have different variants alleles
Homozygous: when the alleles are the same
Heterozygous: when the alleles are different
The alleles can be: dominant or recessive
The first person who studied the transmission of biological traits was Mendel
3. MENDEL’S LAWS
1.- PRINCIPLE OF UNIFORMITY:
When two purebred individuals are crossed, all of their offspring will have identical phenotypes and genotypes
2.- PRINCIPLE OF SEGREGATION:
When two heterozygous individuals from the first generation of offspring are crossed, a second generation
Is produced that is composed of two different phenotypes.
3.- PRINCIPLE OF INDEPENDENT ASSORTMENT:
When two individuals that have two or more different traits are crossed, the transmission of each trait is
Independent from the other traits
4. SPECIAL CASES
1.- MULTIPLE ALLELES
More than two alleles exist for a trait
2.- GENE INTERACTION
A pai of alleles influence the expression of another pair of alleles
3.-LETHAL GENES
They cause death and modify the normal proportions of phenotypes and genotypes in the offspring
4.- QUANTITATIVE INHERITANCE
Some particular traits can have a number of alternative expressions
5. LOCATION OF GENES
THEORY OF THOMAS HUNT MORGAN
Genes are located on chromosomes. Each gene is made up of a segment of DNA.
Each gene has a specific place (locus) on a chromosome.
The two alleles which determine a specific trait are located on two chromosomes which are the same size and
shape: homologous chromosomes
7. DETERMINING SEX
SEX CHROMOSOMES
They are the different chromosomes in males and females.
If the chromosomes are common to both sexes are called: autosomes
There are different models of sex chromosome determination:
XX/XY system:
The female has two identical sex chromosomes (XX) and the male has an X and a Y.
It is typical of mammals, some fish and amphibians.
ZZ/ZW system:
Males have two identical chromosomes (ZZ) and females different ones (ZW).
It is typical of birds and reptiles.
XX/XO system:
There are only X chromosomes: females have two and males have just one.
It determines the sex in some insects.
8. MUTATION
Occasionally, gees can be altered due to unexpected and random changes called
mutations.
Mutations can sometimes be harmful to individuals.
TYPES OF MUTATIONS
1. Gene mutation: it affects the structure of DNA within the genes.
2. Chromosome mutation: the alteration affects the structure of chromosomes.
3. Numerical mutations: it affects the number of chromosomes.
CAUSES OF MUTATIONS
1. Physical mutagens: sometimes because of radiation.
2. Chemical mutagens: changes caused by certain pollutants (pesticides,
dyes...).
3. Biological mutagens: caused by some viruses that interfered with the cell
genetic material
