Gregor Mendel conducted experiments breeding pea plants to discover the basic principles of heredity. He found that organisms have discrete factors (now known as genes) that determine traits, which exist in two versions (alleles). During reproduction, parents contribute one of each allele to offspring randomly. Mendel also discovered that traits are inherited independently and that dominant alleles mask recessive alleles when both are present. His work formed the basis of classical genetics and established the laws of segregation and independent assortment.

1. Mendel’s experiment
• Gregor Mendel discovered the basic principle of heredity
by breeding pea plant in carefully planned experiment.
• Mendel chose to work with peas because there are many
varieties, short life span and large number of offspring
from each mating.
• A heritable feature that varies among individuals, such as
flower color, is called a character.
• Each variant for a character, such as purple or white color
for flowers, is called a trait.
• Mendel chose to track only those characters that
occurred in two distinct, alternative forms.
• He also made sure that he started his experiments
with varieties that were true breeding.

3. Mendel’s conclusion
• Organisms have discrete factors that determine its
features (these ‘factors’ are now recognized as genes).
• organisms possess two versions of each factor
(these ‘versions’ are now recognized as alleles).
• Each gamete contains only one version of each factor
(sex cells are now recognized to be haploid).
• Parents contribute equally to the inheritance of
offspring as a result of the fusion between randomly
selected egg and sperm.
• For each factor, one version is dominant over another
and will be completely expressed if present.

4. Mendel’s laws
• Law of segregation: states that genes have alternative
forms, or alleles which are separated during gamete
formation so that each gamete carries only one allele for
each gene.
• Law of Independent assortment: states that the pair of
alleles for a given gene segregates into gametes
independently of the pair of alleles for any other gene.
This law does not hold true for genes located on the
same chromosome.
• Principle of Dominance: states that expression of the
dominant allele masks the phenotypic effect of the
recessive allele. Not all genes show a complete
dominance some show co-dominance or incomplete
dominance.

5. Concept of recessiveness and
dominance
• Recessive alleles only show their effect if the
individual has two copies of the allele (also known as
being homozygous). For example, the allele for blue
eyes is recessive, therefore to have blue eyes you
need to have two copies of the 'blue eye' allele.
• Dominant alleles show their effect even if the
individual only has one copy of the allele (also
known as being heterozygous). For example, the
allele for brown eyes is dominant, therefore you only
need one copy of the 'brown eye' allele to have brown
eyes (although, with two copies you will still have
brown eyes).

6. Example

7. Comparison of Recessive and
Dominant Allele

8. Gene mapping
• Gene mapping is the process of establishing the
locations of genes on the chromosomes. Early gene
maps used linkage analysis. The closer two genes are to
each other on the chromosome, the more likely it is that
they will be inherited together. By following inheritance
patterns, the relative positions of genes can be
determined. More recently, scientists have used
recombinant DNA (rDNA) techniques to establish the
actual physical locations of genes on the chromosomes.

9. Types of gene mapping

10. Uses of Gene mapping
1. Identify genes responsible for diseases.
• Heritable diseases
• Cancer
2. Identify genes responsible for traits.
• Plants and Animals
• Disease resistance
• Meat or milk production