Mendel's Second Law: Independent Assortment and Inheritance
Mendel's Second Law: Independent Assortment and Inheritance
Relation to the Law of Segregation
Mendel's Second Law describes the principle of independent assortment and is distinct from Mendel's First Law (the law of segregation). Mendel's First Law states that during gamete formation each allele of a pair is separated from the other member, so each gamete receives only one allele of a gene. This separation (disjunction) during meiosis determines the genetic constitution of gametes.
Mechanism and Gamete Formation
Under the modern interpretation, the two alleles that encode a particular trait are segregated during the production of gametes through meiotic cell divisions. Each gamete therefore contains only one allele for each gene. When gametes fuse at fertilization, maternal and paternal alleles combine in the offspring, producing genetic variation.
Monohybrid Crosses and Phenotypic Ratios
Mendel obtained classic results by crossing individuals that were heterozygous for a single gene (diploid with two allelic variants, e.g., Aa). In his pea experiments he observed that monohybrid crosses often produced a phenotypic ratio of 3:1 (dominant to recessive phenotype), for example, yellow to green seed color in peas. This 3:1 ratio illustrates the law of segregation in monohybrid crosses.
Independent Assortment for Multiple Genes
Mendel's Second Law—independent assortment—applies when genes for different traits are located on different chromosomes (or assort independently). In such cases, alleles of different genes segregate independently into gametes, increasing the number of possible genotype and phenotype combinations in offspring.
Genotypes: Homozygous and Heterozygous
For each characteristic an organism inherits two alleles, one from each parent. In somatic cells, one allele is inherited from the mother and one from the father. These alleles may be homozygous (both alleles identical) or heterozygous (two different alleles).
Applications of Genetics
The best known applications of genetics include the development of biotechnology and genetic engineering. Genetic knowledge is used to create new techniques and to improve existing ones across many fields, such as:
- Agriculture
- Livestock breeding
- Industry
- Medicine
Incomplete Dominance
Incomplete dominance is a form of genetic interaction in which the phenotype of the heterozygote is intermediate between the phenotypes of the two homozygotes. In incomplete dominance, neither allele is completely dominant or completely recessive; instead, the heterozygous phenotype shows a blend or intermediate expression of the two alleles.
Cloning
Cloning (derived from the Greek word κλών klôn, meaning "twig" or "sprout") can be defined as the process by which identical copies of an organism, cell, or molecule are produced, often by asexual means.[1]
Institutions
Universidad de Oriente (UDO), Universidad del Zulia (LUZ), Universidad de Los Andes (ULA), Universidad Nacional Experimental de Los Llanos Ezequiel Zamora, Venezuelan Institute for Scientific Research (IVIC)