Sheryl A. DeCaprio
In 1865 a botanist by the name of Gregor Mendel presented his theories of genetics to a group of prominent scientists. They were unimpressed. It was many years before the genius of Mendel’s work was recognized. Through careful study, precise records, and controlled experiments, Mendel discovered that certain characteristics were transmitted from one generation to another in a predictable, mathematical fashion. Through his research and study of plant growth and seed shape, Mendel deduced the following two laws of genetics:
1. Mendel’s First Law
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A simple genetic trait is determined by a pair of separable factors (called alleles of a gene). An offspring will receive one allele from each parent.
2. Mendel’s Law of Segregation
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Paired alleles of parents segregate during the formation of egg and sperm cells so that only one of the two alleles is included in each egg or sperm.
Knowledge of the process of meiosis is basic to the under. standing of how genetic information is transferred from parent to offspring. A child receives genetic information from each parent. During meiosis, the process in which chromosomes in a cell’s nucleus are separated, each egg and sperm cell receives 23 chromosomes or one half of the genetic material needed to create a new organism. A normal child receives 23 chromosomes from each parent, a total of 46 chromosomes. Consider a chromosome as a collection of specific genetic material or genes. All individuals have like chromosomes containing information that will describe one’s physical traits. For example, all individuals have a set of sex chromosomes (XX for females, XY for males) and the genetic information that constitutes that set of chromosomes determines the sex of the individual. During meiosis, each parent passes some of their genetic material to their children. Each gene that is transferred from parent to child carries the material that will allow a specific trait to be expressed. For example, a mother may pass a gene that determines a particular hair coloring to her child. The father, in a like manner, passes along a similar gene which expresses hair color. The pairing of these two genes, describing the same trait, hair coloring, is the new genetic material of their offspring, or a child’s
genotype
with respect to hair color. (For a more detailed description of meiosis see additional units in this volume.)
Through experimentation and careful analysis, Mendel discovered that genes exist as pairs in the cell. Each gene is represented by 2
alleles
(an allele is one of several possible forms of a gene). An egg or sperm cell will receive only one of the alleles from each parent during this period of meiosis. When fertilization then occurs, the separate alleles from both parents join to create the new genotype, or genetic material, of the offspring.
Mendel also discovered that if one knew the genotype of the parents, one could accurately predict the probable genotypes of the offspring, and in the proper ratios. He found that certain traits are always expressed (seen in an organism) as long as one allele of this trait is present in the gene pairing. He described this characteristic as
a dominant
trait. The traits expressed only if both the alleles were identical were then called
recessive
traits
.