Until the mid-nineteenth century, it was widely believed that offspring would receive a blend or average of whatever traits their parents possessed. Australian botanist Gregor Johann Mendel, “The Father of Modern Genetics,” was the first person to figure out the basic principles of heredity. Over the span of eight years, he crossed tens of thousands pea plants in his garden to reveal specific patterns in the inheritance of physical traits.
Mendel decided to experiment on pea plants because they grow relatively quickly and because different varieties have distinct characteristics that can easily be told apart. He began by cross-fertilizing pea plants with opposite characteristics: tall with short, smooth with wrinkled, green seeds with yellow seeds, etc. Finally, after years of growing and observing these plants, Mendel came up with two theories that have now been made into laws: the Law of Segregation and the Law of Independent Assortment.
The Law of Segregation states that:
Genes determine hereditary information.
Organisms inherit one allele from each parent. Two homogeneous parents pass on one allele for the dominant phenotype and one for the recessive phenotype. These two alleles comprise the gene pair.
Gametes (male or female reproductive cells that contain only half of the genetic material of the organism) only carry one allele for each trait.
When the two alleles of a pair are different, one is dominant and the other is recessive.
This was significant progress from the common blending belief during Mendel's time.
The Law of Independent Assortment states that traits are passed on independently of other traits from parent to offspring. For example, in humans, the genotype for height that the child acquires does not affect what color hair that child has (“Gregor Mendel Biography”). When two or more characteristics are inherited, individual hereditary factors are distributed independently during gamete production, so different traits have equal opportunities of occurring together ("Law of Independent Assortment").
The genotype is the actual genetic makeup of an organism, whereas the phenotype is the what is seen or expressed. There are two types of alleles: dominant and recessive. A dominant allele in the genotype, if present, is always expressed, no matter what the other allele may be. They tend to hide or cover up recessive alleles; therefore, dominant traits are more commonly seen than recessive ones. A recessive allele, on the other hand, is only expressed in the phenotype when neither of the alleles present is dominant. Often, the recessive allele is present in the genotype but cannot be seen in the phenotype due to a dominant allele masking it. If an organism carries one dominant and one recessive allele, then the dominant phenotype is the one that is expressed (“Allele”).
Parents pass on their DNA through meiosis. The offspring inherits exactly half their DNA from their mother and the other half from their father. The sperm and egg each contain only half the genetic information of each parent (called haploid), so when they meet, they combine to form a diploid (a fertilized egg with 23 pairs of chromosomes). This makes it possible for a child to have genes from both sides for the family, but to be identical to no one, with the exception of identical twins (“DNA is Passed on From Parent to Child”).
Works Cited
“Allele.” Nature.com. Nature Education, 2014. Web. 31 Oct. 2015.
“DNA is Passed on From Parent to Child.” NHS National Genetics and Genomics Education Centre. Department of Health, 2015. Web. 1 Nov. 2015.
“Gregor Mendel Biography.” The Biography.com. A&E Television Networks, 2015, Web. 1 Nov. 2015.
"Law of Independent Assortment." The American Heritage Science Dictionary. Houghton Mifflin Company, 2015. Web. 01 Nov. 2015.