Phenotypes and genotype are both genes concerned with the same characteristic.
Phenotypes and genotypes are both genes concerned with the same characteristic. For example, let us assume that there are only two types of genes that determine the color of fur of dog- one black and one white. Since these dogs have two genes for fur color, there are three possible gene combination, or genotypes- both black, (BB); both white (bb); or one white and one black (Bb). In this case, the gene that determines black fur (B) is said to be dominant. That is, when present in the hereditary makeup, the animal’s fur is always black in color.
The gene that determines white fur (b) is called recessive. Thus, we see that while there are three possible genotypes in this case, there are only two possible physical differences in appearance, or phenotypes. The animals are either black or white, although some of the black animals carry genes for white fur. Animals with two genes for black fur are called homozygous dominant, and are black in color. Those animals with two genes for white fur are called homozygous recessive and are white in color. Those with one gene for white and one gene for black fur are called heterozygous and are always black in color since the black gene is dominant.
These examples of how fur color may be determined in the dog help us understand why offspring need not resemble their parents in physical characteristics. If we mate two black dogs, for example, some of their offspring are expected to be black and some are expected to be white. In this case, we can predict that on the average, three out of every four offspring will be black, and one out of four will be white. We know the odds are that one animal out of four will be homozygous dominant (BB), one will be homozygous recessive (bb), and two will be heterozygous (Bb).
To understand how this occurs, let us see what happens genetically, when a new baby is produced. In normal growth, individual cells divide by a process called mitosis, producing two new cells, each with a full complement of 23 pairs of chromosomes. When the animal matures sexually, however, cells are produced in the ovaries or testes that possess only one member of each pair of chromosomes, for a total of 23 rather than 46 chromosomes, as in mitosis. This process of producing the gametes is called meiosis.
Each parent, then, contributes one chromosome of each pair found in the offspring. If both parents are heterozygous for fur color, both re black in color but possess black and white genes. Each ovum produced by the female may possess either a gene for black color, or a gene for white color, but not both. Each sperm cell produced by the male, similarly, may carry either a white or a black gene, but not both.
We can see that on the average, 50 per cent of the sperm cells produced by the male carry genes for white fur, and 50 per cent carry genes for black; the same is true of ova produce by the female. At the time of fertilization, then, there is a probability of one out of four that a gene for white fur from the mother will be combined with a gene for white fur from the father, and a probability of one out of four that two genes for black fur will combine. In half of these cases, a gene for black fur will combine with a gene for white fur. Thus, we can see the basis for our protection concerning the number of black and white animals produced by two heterozygous parents, and we can see that two parents are able to produce offspring very different from themselves.
Not all characteristics influenced by genetic factors are determined by genes that act in the manner just described. In some cases, two genes may interact in such a way as to create a characteristic intermediate between the two extremes. Thus, two genes for fur color, one white and black, in some species, may be produce an intermediate gray color. In addition, the great majority of physical characteristics are determined by more than a single pair of genes. Some human blood types, for example, are determined by several different genes acting together, and in characteristics such as height and weight, the genetic basis is even more complex. Many physical attributes are apparently determined by a large part of the genetic makeup of the individual, and not by single pairs of genes or even a small number of pairs, as in blood type.
Another complicating factor is that one should not speak of heredity without taking into account the environment in which that heredity is expressed. Because of this, we need to clarify the term environment, and consider the way in which hereditary and environmental factors interact, before going on to discuss the role of hereditary factors in behavior.