AGB 121: Principles of Animal Genetics and Population Genetics (2+1)

LAW OF SEGREGATION

• Results of Mendel’s’ First Experiment on Seven pairs of characteristics in the Garden Pea.

Characteristics	F2 Results		Ratio
Form of seed Colour albumen Colour of seed-coats Form of pods Colour of pods Position of flowers Length of stem All characteristics combined	5474 round 6022 yellow 705 grey-brown 882 inflated 428 green 651 axial 787 tall 14,889 dominant	1850 wrinkled 2001 green 224 white 299 constricted 152 yellow 207 terminal 277 dwarf 5010 recessive	2.96: 1 3.01: 1 3.15: 1 2.95: 1 2.82: 1 3.14: 1 2.84: 1 2.98: 1

How are traits passed from parents to offspring?

• After a careful study of his experimental results Mendel formulated what is now known as Mendel's law of segregation.
• There must be two hereditary units in the body cells of a mature organism (he called them as factors, we now call them genes or alleles), which were responsible for the transmission of characteristics.
• The pair of alleles of each parent separated into gametes during reproduction. Equal numbers of gametes were produced that contained each allele.
• Both parents contributed equally to the factors of heredity in the offspring.
• Gametes randomly unite at fertilization.
• When the two alleles of a pair are different (hybrid), one is dominant and the other is recessive.
• Due to non-mixing of alleles in the hybrid, the masked recessive trait reappears in the next generation.
• Mendel’s first principle, the law of segregation, referring to the non-mixing of alleles in the hybrid and their subsequent segregation or separation in the gametes in equal frequencies, may be considered as the most important contribution of Mendel to heredity, since there no contrary experimental evidence as to the prevalence of any mixing of alleles in the hybrids.
• So Mendel’s first law is universally applicable.

The law of segregation or the law of purity of gametes

It states that when a pair of factors / allelomorphs (alleles) is brought together in a hybrid (F₁) they remain together without contaminating each other and they separate or segregate from each other into a gamete in a complete and pure form during the formation of gametes.