Question

Mendel derived four principles of inheritance from his genetics experiments with pea plants. Explain these four postulates in your own words. Using the chromosomes pictured in Figure 2-10 on pages 22-23 in your textbook, relate his Law of Segregation and Law of Independent Assortment to your knowledge of meiosis. Be sure to describe the stages of meiosis that demonstrate these laws.

Answer 1

Postulate I : Law of paired factors

This law states that the various hereditary characters are controlled by discrete units called factors which occur in pairs i.e., there is a pair of factors for each character.

Factors representing the alternate or same form of a character are called alleles. An individual receives one allele from each parent, and passes on one of the two alleles at random to each offspring.

Postulate II : Law of dominance

This law states that when the two contrasting forms of one factor (gene) comes together in an individual, one of them gets completely expressed (dominant allele) and the other (recessive allele) remains as hidden discrete units.

According to this law,

• Character is monogenic
• Genes are biallelic
• There will be complete dominance for any one allele of a gene

Postulate III : Law of segregation

This law states that during the formation of gametes, the two alleles of a gene will segregate from each other so that one gamete receives any one allele of the gene. And also during formation of new zygote, alleles unite randomly.

Postulate IV : Law of independent assortment

This law states that during the inheritance of more than one character at a time, the segregation of two alleles of one gene is independent of the segregation of two alleles of other genes. Thus traits are transmitted to offsprings independent of one another.

The paternal and maternal chromosomes of each homologous pair segregate during anaphase I independently of other chromosomes. So, anaphase I is the stage in meiosis that corresponds to law of independent assortment.