In the case of the human species, where experiments with directed breeding cannot be performed, the determination of the inheritance pattern of traits depends on a survey of the family history in which certain traits appear.
This allows the geneticist to know whether or not a given trait is inherited and how it is inherited. This survey is done in the form of a graphical representation called heredogram (from Latin heredium, inheritance), also known as genealogy or Genealogy.
Constructing a heredogram consists of representing, using symbols, kinship relationships between individuals in a family. Each individual is represented by a symbol indicating their particular characteristics and kinship with others.
Males are represented by a square, and females by a circle. Marriage, in the biological sense of procreation, is indicated by a horizontal line joining the two members of the couple. The children of a marriage are represented by vertical lines joined to the horizontal line of the couple.
The main symbols are as follows:
Mounting a heredogram follows a few rules:
1st) In each couple, the man should be placed on the left and the woman on the right, whenever possible.
2nd) Children should be placed in order of birth from left to right.
3ª) Each succeeding generation is indicated by Roman numerals (I, II, III, etc.). Within each generation, individuals are indicated by Arabic numerals from left to right. Another possibility is to indicate all individuals of a heredogram by Arabic numerals, starting with the first left, of the first generation.
Interpretation of Heredograms
Analysis of heredograms may allow one to determine the inheritance pattern of a certain trait (whether autosomal, dominant or recessive, etc.). It also allows us to discover the genotype of the people involved, if not all, at least part of them. When one of the members of a genealogy manifests a dominant phenotype, and we cannot determine whether it is dominant or heterozygous, its genotype is usually indicated as THE_, B_or Ç_, for example.
The first information sought in the analysis of a heredogram is whether the character in question is conditioned by a dominant or recessive gene. To do this, we must look in the heredogram for couples who are phenotypically equal and who had one or more children different from them. Whether the trait remained hidden in the couple, and manifested in the child, can only be determined by a recessive gene. Phenotypically equal parents with a different child indicate that the character present in the child is recessive!
Once we find out which is the dominant gene and which is the recessive one, let's now locate the recessive homozygotes, because they all manifest the recessive character. After that, we can start discovering other people's genotypes. We must remember two things:
1st) In a pair of allele genes, one came from the father and the other came from the mother. If an individual is a recessive homozygote, he must have received a recessive gene from each ancestor.
2nd) If an individual is a recessive homozygote, he sends the recessive gene to all of his children. Thus, as in a puzzle, the other genotypes are being discovered. All genotypes must be indicated, even in their partial form (THE_, for example).
In such a tree, women are represented by circles and men by squares. Marriages are indicated by horizontal lines connecting a circle to a square. The Roman numerals I, II, III to the left of the genealogy represent the generations. Three generations are represented. In the first there is a married woman and a man, in the second, four people, three female and one male. Individuals bound to a horizontal line by vertical lines are a brotherhood. In the second generation, a woman is married to a man of a brotherhood of three.