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 allelesHomozygous: 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

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 generationIs 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 isIndependent from the other traits

SPECIAL CASES

1.- MULTIPLE ALLELESMore than two alleles exist for a trait

2.- GENE INTERACTIONA pai of alleles influence the expression of another pair of alleles

3.-LETHAL GENESThey cause death and modify the normal proportions of phenotypes and genotypes in the offspring

4.- QUANTITATIVE INHERITANCESome particular traits can have a number of alternative expressions

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

CHROMOSOME MAPPING

DETERMINING SEXSEX 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.

MUTATIONOccasionally, gees can be altered due to unexpected and random changes called mutations.Mutations can sometimes be harmful to individuals.

TYPES OF MUTATIONS1. 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