Biology (A)

Mendel’s Laws and Genetics

You might think that Mendel’s discoveries would have made a big impact on science as soon as he made them. But you would be wrong. Why? Mendel never published his work. Charles Darwin published his landmark book on evolution in 1869, not long after Mendel had discovered his laws, but Darwin knew nothing of Mendel’s discoveries. As a result, Darwin didn’t understand heredity. This made his arguments about evolution less convincing to many people. This example shows why it is important for scientists to communicate the results of their investigations.

Rediscovering Mendel’s Work

Mendel’s work was virtually unknown until 1900. Then, in that year, three different European scientists—named DeVries, Correns, and Tschermak—independently arrived at Mendel’s laws. All three had done experiments similar to Mendel’s. They came to the same conclusions that he had drawn almost half a century earlier. Only then was Mendel’s actual work rediscovered. As scientists learned more about heredity over the next few decades, they were able to describe Mendel’s ideas about inheritance in terms of genes. In this way, the field of genetics was born. At the link that follows, you can watch an animation of Mendel explaining his laws of inheritance in genetic terms. http://www.dnalc.org/view/16182-Animation-4-Some-genes-are-dominant-.html

Genetics of Inheritance

Today, we known that characteristics of organisms are controlled by genes on chromosomes (see Figure below). The position of a gene on a chromosome is called its locus. In sexually reproducing organisms, each individual has two copies of the same gene. One copy comes from each parent. The gene for a characteristic may have different versions. The different versions are called alleles. For example, in pea plants, there is a purple-flower allele (B) and a white-flower allele (b). Different alleles account for much of the variation in the characteristics of organisms.

During meiosis, homologous chromosomes separate and go to different gametes. Thus, the two alleles for each gene also go to different gametes. At the same time, different chromosomes assort independently. As a result, alleles for different genes assort independently as well. In these way, alleles are shuffled and recombined in each parent’s gametes.

Genotype and Phenotype

When gametes unite during fertilization, the resulting zygote inherits two alleles for each gene. One allele comes from each parent. The alleles an individual inherits make up the individual’s genotype. The two alleles may be the same or different. As shown in Table 6.2, an organism with two alleles of the same type (BB or bb) is called a homozygote. An organism with two different alleles (Bb) is called a heterozygote.

Genetics of Flower Color in Pea Plants

Alleles	Genotypes	Phenotypes
	BB (homozygote)	purple flowers
B (purple)	Bb (heterozygote)	purple flowers
b (white)	bb (homozygote)	white flowers

Table 6.2 There are two alleles, B and b, that control flower color in pea plants. This results in three possible genotypes. Why are there only two phenotypes?

The expression of an organism’s genotype produces its phenotype. The phenotype refers to the organism’s characteristics, such as purple or white flowers. As you can see from Table 6.2, different genotypes may produce the same phenotype. For example, BB and Bb genotypes both produce plants with purple flowers. Why does this happen? In a Bb heterozygote, only the B allele is expressed, so the b allele doesn’t influence the phenotype. In general, when only one of two alleles is expressed in the phenotype, the expressed allele is called the dominant allele. The allele that isn’t expressed is called the recessive allele.

CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/