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Mendel's Laws


Mendel's 1st Law: Factors Segregation Law

The confirmation of the dominance and recessivity hypothesis in Mendel's various experiments later led to the formulation of his 1st law:

“Each feature is determined by two factors that separate in the formation of gametes, where they occur in a single dose”that is, for each male or female gamete only one factor is addressed.

Mendel had no idea of ​​the constitution of these factors, nor where they were located.

The cell bases of segregation

The rediscovery of Mendel's work in 1900 brought the question: where are the hereditary factors and how do they segregate?

In 1902, while studying the formation of gametes in locusts, US researcher Walter S. Sutton noted striking similarities between the behavior of homologous chromosomes, which separated during meiosis, and the factors imagined by Mendel. Sutton hypothesized that the hereditary factor pairs were located on homologous chromosome pairs in such a way that separation of homologues led to factor segregation.

Today we know that the factors Mendel referred to are the genes (from the Greek genos, originate, originate), and which are actually located on chromosomes, as Sutton had proposed. The different forms in which a gene can present itself are called alleles. The yellow color and green color of the pea seed, for example, are determined by two alleles, that is, two different forms of the seed color gene.

Example of Mendel's first law in an animal

Let's study an example of the application of Mendel's first law to an animal, taking advantage of the modern terminology used in genetics. The characteristic we chose was the color of the guinea pig coat, which can be black or white. According to a widely accepted convention, we will represent by B the dominant allele, which conditions the color black, and by b the recessive allele, which conditions the color white.

A simple technique of combining gametes produced by individuals from F1 to obtain the genetic makeup of individuals from F2 It is the assembly of Punnet Square. This consists of a table, with number of rows and columns corresponding respectively to the types of male and female gametes formed at the intersection. Punnet's square for the crossing of heterozygous guinea pigs is:

B

Parent Gametes

B

Maternal Gametes

B b

BB

black

BB

black

BB

black

BB

White