GENETICS

© A.C.E. (1989, pp. 14-17) Used by permission

Construction workers follow blueprints that determine specific characteristics of houses they build. There is a blueprint that determines your characteristics. Obviously, you inherited these characteristics from your parents, who inherited them from their parents, and so on all the way back to Adam and Eve. Where is the blueprint that determined your characteristics? The answer to this question can be found in a study of genetics.

Gregor Mendel was an Austrian monk who become fascinated by the way plants handed down their characteristics to the next generation. Experimenting in the monastery garden, he bred and crossbred thousands of pea plants during a seven year period. He worked with traits such as tall and short plants, green and yellow pods, and wrinkled and round peas. Because he used plants with different expressions of the same trait, the offspring of his cross pollinations were called hybrids.

Mr. Mendel carefully observed the characteristics of each generation of plants and kept accurate records of his observations. Although he published his discoveries in a scientific journal, no one paid much attention to his research on plant heredity. He eventually abandoned his work, but it laid the foundation for the science of genetics -- the study of inherited characteristics.

Gregor Mendel's crossbreeding method was simple and can easily be duplicated. He pollinated the pistil of one plant with the pollen from another. When he used plants with yellow and green peas for his first crosspollination, Mr. Mendel discovered that all the offspring had yellow peas. He next allowed those "first generation" hybrid plants to self pollinate and was surprised to discover a ratio of 3:1 -- three-fourths of the offspring of those yellow plants had yellow peas, but one-fourth had green.

Even though Gregor Mendel knew nothing about chromosome pairs, he made some conclusions based on his experiments. He first concluded that when plants are crossbred, the offspring inherit pairs of possible characteristics. When one of the paired characteristics revealed itself in the appearance of the plant, he called that characteristic dominant. The hidden characteristic he called recessive. In the first generation of pea plants, Mr. Mendel concluded, the yellow trait was dominant because all the peas were yellow, and the green trait was recessive.

To make it easier to chart inherited characteristics, geneticists use capital and lower-case letters to identify the traits. In the example of the yellow and green peas, the capital letter Y indicates the dominant yellow trait; and the lower-case letter y indicates the corresponding recessive green trait. An allele is one of the two or more forms of a gene, so Y and y are both alleles of the Y gene.

Mr. Mendel began his experiments with a pure green-pea and a pure yellow-pea plant. If the alleles for a trait are exactly the same, that organism is called homozygous. Since the cells in the original plant with yellow peas contained the genes YY and cells in the green plant contained the genes yy, those plants were homozygous. Every gamete in the yellow-pea plant contained one Y of its pair, and every gamete in the green-pea plant contained one y.

When fertilization occurred between these two gametes, the offspring received one Y and one y chromosome. An organism which has two different genes in the pair for a particular trait is said to be heterozygous. Because these offspring had two different genes in each pair that determined pea color (Yy), those plants were heterozygous. (In heterozygous pairs, the capital letter always precedes the lower-case letter.)

All these first generation offspring had yellow peas because they contained the dominant yellow trait (Y). The trait that is evident in the appearance of the organism is called a phenotype. Since the first generation plants had yellow peas, the phenotype of these plants was yellow peas. The genotype is the complete hereditary or genetic makeup, dominant and recessive, of an organism. The genotype of these first generation plants was Yy, because they had one yellow and one green gene in each cell even though only the yellow trait was evident. (You can more easily remember what these terms mean if you remember that phenotype = physical characteristic; and genotype = genetic make-up.)

We can chart Mr. Mendel's first crossbreeding like this:

The two alleles of the yellow-pea plant are separated and listed at the top of the box; the two alleles of the green-pea plant are listed on the left side. Each of the small squares includes two alleles -- one inherited from the yellow-pea parent and one from the green-pea parent. The phenotype of all four new plants is yellow peas, and their genotype is Yy.

We can also show the results of pollinating two Yy plants:

Looking at the second chart, we observe a ratio of 3:1 -- the phenotype of three plants is yellow peas, and the phenotype of one plant is green peas. The genotypes of the four plants are YY, Yy, Yy, and yy. Remember, anytime that a dominant gene appears in the genotype, that trait will be evident in the phenotype. An organism will only show a recessive trait if its genotype contains only recessive genes. In other words, if a plant contains a Y gene, it has yellow peas; only the yy plant has green peas.

Because some genes are neither dominant nor recessive, both alleles of a pair may be evident in the phenotype. This is called incomplete dominance. If, for example, a pure red four-o'clock flower (rr) is crossed with a pure white four-o'clock flower (ww), the first generation will be pink (rw). Then, if those pink flowers are allowed to self-pollinate, the second generation will contain one red, two pink, and one white phenotype. The genotypes will be rr, rw, rw, and ww.

The study of human genetics is fascinating and becomes even more so when you consider that a small part of the genetic material contained in each of your cells can be traced back to Adam. God never created another human being from dust, and all of us have descended from Adam. When you consider that within your body is a small part of Adam's physical body, it is easier to understand what the Bible means when it states that we have also inherited Adam's sinful nature.

Contained within each of your cells is your total genetic code. If your eyes are blue, every cell contains the genes for blue eyes. Those genes are only evident in the blue irises of your eyes -- your hair and fingernails are not blue. How amazing our bodies are! How difficult it would be to believe that we just "happened" by accident or are descended from apes! Only God could create such an intricate being!

Even your sex was determined by a gene. The male sex chromosome is called the Y chromosome; the female is the X chromosome. Because all female body cells contain a homozygous pair of X chromosomes, each female gamete contains a single X chromosome. Male body cells contain heterozygous sex chromosomes -- one of the pair is an X chromosome and the other is a Y chromosome. Male gametes, therefore, contain either a single X or a single Y chromosome. If an X sperm fertilizes the ovum, the child is a girl. If a Y sperm fertilizes the ovum, the child is a boy.

Some human characteristics are called sex-linked characteristics. The genes for these characteristics are located on the X chromosome and are recessive. If a male's X chromosome carries the recessive gene for a trait, he will have that trait since there is no corresponding X chromosome (remember, he is XY) which might carry the dominant gene for the trait. A female who has the dominant characteristic on one X chromosome and the recessive characteristic on the other X chromosome is called a carrier. Although the characteristic is not evident, she will pass it on to her children. In the following example, the male parent is "normal," and the female is a carrier of the red-green "color blind" trait. (People with red-green color blindness find it difficult to distinguish between red and green since both appear as shades of gray.) Notice that of their possible children, one is a normal boy, one a normal girl, one a colorblind boy, and one a carrier girl.



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A.C.E. 1989, Science: Biology 1107, Rev. Ed., Accelerated Christian Education Inc.