MENDELIAN LAWS OF HEREDITY, MONOHYBRID AND DIHYBRID CROSSES

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.

• *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.

• 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.

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).

• 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.

MENDEL’S LAW OF SEGREGATION

1. 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.

1. 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.

1. 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.

MONOHYBRID AND DIHYBRID CROSSES

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.

DIHYBRID CROSS

• A cross between two individuals identically heterozygous at two loci for example, AaBb/AaBb.

Example of a CrossThe 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.