2. Mendel's Laws of Heredity
• is a set of primary tenets that underlie much of
genetics developed by Gregor Mendel in the
latter part of the 19th century.
• Mendel (1822-1884), an Austrian monk, was
interested in understanding variances in plants,
and between 1856 and 1863 cultivated and tested
some 28,000 pea plants.
3. • *His experiments brought forth two
generalizations which later became known as
Mendel's Laws of Heredity or Mendelian
inheritance.
• his paper "Experiments on Plant Hybridization"
that was read to the Natural History Society of
Brunn on February 8 and March 8, 1865, and
was published in 1866.
4. • the prevailing theory of
biological inheritance was that of blending
inheritance, in which the sperm and egg of
parent organisms contained a sampling of
the parent's "essence" and that they
somehow blended together to form the
pattern for the offspring.
5. MENDEL’S LAW OF
INDEPENDENT ASSORTMENT
Also known as Mendel's Second Law
• The most important principle of Mendel's law of independent
assortment is that the emergence of one trait will not affect the
emergence of another. While his experiments mixing one trait always
resulted in a 3:1 ratio (Fig. 1) between dominant and recessive
phenotypes, his experiments with two traits showed 9:3:3:1 ratios
(Fig. 2). Mendel concluded that each organism carries two sets of
information about its phenotype. If the two sets differ on the same
phenotype, one of them dominates the other. That way, information
can be passed on through the generations, even if the phenotype is
not expressed (F1 generations, figures 1 and 2).
6. • The most important principle of Mendel's
law of independent assortment is that the
emergence of one trait will not affect the
emergence of another.
7. MENDEL’S LAW OF
SEGREGATION
• Alternative versions of genes account
for variations in inherited characters.
This is the concept of alleles. Alleles are
different versions of genes that impart the
same characteristic. Each human has a
gene that controls height, but there are
variations among these genes in
accordance with the specific height the
gene "codes" for.
8. • For each character, an organism
inherits two genes, one from each
parent. This means that when somatic
cells are produced from two gametes, one
allele comes from the mother, one from
the father.
9. • If the two alleles differ, then one, the
dominant allele, is fully expressed in
the organism's appearance; the other,
the segregate during gamete
production. This is the last part of
Mendel's generalization. The two alleles
of the organism are separated into
different gametes, ensuring variation.
11. MONOHYBRID CROSS
• hybridization using a single trait with two
alleles
• A cross between two individuals
identically heterozygous at one
gene pair for example, Aa x Aa.
12. DIHYBRID CROSS
• A cross between two
individuals identically
heterozygous at two loci for
example, AaBb/AaBb.
13. Example of a Cross
The following dihybrid cross involves two true breeding pea plants, where two factors are looked at, the shape of the seed and the colour of the seed.
