Pedigree analysis.

Date: 2015-10-07; view: 531.

THE BASIC THEORETICAL ITEMS OF INFORMATION

Pedigree analysis is an examination method that includes building and analysis of genealogical (pedigree) maps. There are two steps (phases) of the method:

The first phase is building of the pedigree map. The second one is analysis of the map

General principles of building the family tree (pedigree map) are the following:

• family tree is built starting with proband (propositus). Proband is man, who comes to physician-geneticist.
• family tree is built from offspring to forefathers.
• family tree has to involve alliance of second relation degree at least.
• family tree has to involve alliance of three generations at least.
• the examination of family tree members or analysis of medical records is needed to right building of pedigree map. More seldom the family tree is built according of the anamnesis data (information).
• while family tree is being built the generally accepted legend (notation conventions) are used (figure 30.1).

Figure 30.1 The generally accepted legend (notation conventions) are used to build of family tree

Generations are marked by numerals (more often roman) from top to bottom.

Within the generation sibs (siblings or brothers and sisters) are placed from left to right on the measure of decreasing an age (in the order of birth) and are marked by Arabic numerals.

During an analysis of family tree the following problems are solved:

1) It is determined if there is hereditary disease or unheritable disease (character).

2) The type or variant of inheritance is cleared up

3) The prognosis of birthing of sick children is ascertained,

4) Genotypes of members of family tree are determined,

5) The penetrance of genes is calculated in some cases.

If there are some sick members of pedigree (or some one's have the character) it means this sickness (character) has hereditary nature.

It is possible to define the type of inheritances using such traits as:

• Which of sex have the character (disease) more often.
• Who (daughters or sons) take the character after mother or father?
• Is there presence or absent?
• What is the probability of manifestations of character in posterity (descendants).

There are five types of inheritance.

For instance:

1. In the case of disease is inherited by the autosomal dominant type of inheritance (figure 30.2) is typical:

Frequency of cases among men and women is alike,

The character (disease) is inherited both after fathers and mothers both by daughters and sons,

Probability of manifestation of disease in the posterity of heterozygote parents is equal of 75%,

Vertical type of inheritance is presence.

2. In the case of disease is inherited by the autosomal recessive type of inheritance (figure 30.3) is typical:

Frequency of cases among men and women is alike,

The character (disease) is inherited both after fathers and mothers both by daughters and sons,

Probability of manifestation of disease in the posterity of heterozygote parents is equal of 25%,

Vertical type of inheritance is absent.

3. In the case of disease is inherited by the X-linked dominant type of inheritance (figure 30.4) is typical:

Frequency of cases among the women is more than among the men,

The character (disease) is inherited after fathers by daughters only, and after mothers it is inherited both by daughters and sons,

Vertical type of inheritance is presence.

4. In the case of disease is inherited by the X-linked recessive type of inheritance (figure 30.5, 30.6) is typical:

Frequency of cases among the men is more than among the women,

The character (disease) is inherited after fathers by daughters only, and after mothers it is inherited both by daughters and sons,

Vertical type of inheritance is absent.

5. In the case of disease is inherited by the Y-linked type of inheritance (figure 30.7) is typical:

There is among the men only,

The character (disease) is inherited after fathers by sons only,

Vertical type of inheritance is presence.

Vertical type of inheritance means that the sick parents have the sick children. This type of inheritance is typical to dominants types of inheritance.

There is horizontal type of inheritance to. This type of inheritance is typical to recessive types of inheritance. In this case there are a lot of sick people within one generation.

Genotypes of pedigree members determine according the rules that are based on laws of inheritance, especially on the laws after G. Mendel.

For instance, if there is autosomal dominant type of inheritance and father is sick and heterozygote (Aa), and mother is healthy (Aa) the probability genotypes of their children are Aa (frequency 50%) and aa (frequency 50%).

If there is autosomal recessive type of inheritances and parents are healthy, but their child is has a hereditary disease (aa) hence parents are heterozygotes (Aa).

The prognosis of birthing of sick children in the case of monogenic disease (character) is ascertained according the based on laws of inheritance (G. Mendel's laws). For example, the probability of manifestation of disease in the posterity of heterozygote parents is equal of 25% in the case of autosomal recessive type, or 75% in the case of autosomal dominant type of inheritance.

The special methods are used to define the prognosis of birthing of sick children when there are chromosomal disorders. For instance, in the case of healthy parents, those have a child with the simple type of Down's (trisomy 21) syndrome probability of birthing the next sick child is very little (about 0%). But if mother has a big age (more than 35 years) this probability is increased. And the probability is very high in the case of translocation variant of Down's syndrome.

Figure 30.2

Figure 30.3

Figure 30.4

Figure 30.5

Figure 30.6

Figure 30.7