What is a recessive trait in biology

Family tree analysis

Family tree analysis is part of genetic research, with which, for example, hereditary diseases can be tracked. We explain how a family tree analysis works here on StudyHelp online learning!

Family tree analysis topics on this page

All people are different from one another and have different combinations of characteristics. The most important prerequisite for this is sexual reproduction and related recombination during meiosis.

This will result in various other combinations and characteristics of other characteristics in the next generation. The first experiments were carried out by Johann Gregor Mendel, which resulted in the Mendelian rules for the formation of characteristics. These findings also play an important role in the development of the family tree analysis of hereditary diseases or genetically determined characteristics.

For an introduction to the topic of family tree analysis, you should watch this tutorial video!


Mendelian rules

The foundations for the formation of characteristics were laid in 1866 by the monk Johann Gregor Mendel. On the basis of cross-breeding experiments with peas, he established various rules for inheritance in his observations:

  1. Uniformity rule: When crossing individuals who differ in one trait (parental generation P), the following generation (branch generation F1) shows this trait phenotypically in the same way.
  2. Split rule: If you cross the F1 generation with each other, you get offspring in the F2 generation, which reflect the characteristics in a certain numerical ratio.
  3. Independence rule: If you cross genetically engineered individuals who differ not only in one but in two traits, the traits in the F2 generation split up independently of one another. They can therefore be freely combined and new combinations are created.

Mendel was not yet aware of the genetic processes involved in inheritance. It was only later that chromosomes were recognized as human genetic material. Nevertheless, his findings are of great importance to this day.

Family tree analysis

The inheritance of traits in humans works in principle in the same way as the inheritance in plants and animals. Human genetics is particularly about studying genetically determined diseases, developing treatment methods and making prognoses. An important part of this is the family tree analysis. A family tree is used to examine how a trait or disease is inherited. However, this family tree analysis only works for monogenic traits, i.e. those that are only caused by a single gene.

The first distinction made in a family tree analysis is between

  • autosomal and
  • gonosomal inheritance

At autosomal inheritance the genes of the trait or disease are on the autosomes, i.e. on chromosomes 1-22.

If, on the other hand, it is one gonosomal inheritance, the genes are on the gonosomes - the sex chromosomes.

The next distinction is whether a

  • more dominant or one
  • recessive inheritance.

At a recessive inheritance a trait or a disease is only manifested phenotypically if both alleles carry the same information or are defective. This inheritance is identified by two small letters.

Becomes a feature inherited dominantly, only one of the two alleles has to carry the characteristic or be defective. Capital letters are used for this.

Individuals in whom both relevant alleles are present in the same expression (aa or AA) have this characteristic homozygous (homo = “equal”). If the two alleles are expressed differently (Aa), one speaks of the fact that the characteristic is heterozygous (hetero = "different", "different").

An autosomal dominant inheritance is shown here. With this inheritance, carriers of traits can be both homozygous (AA) and heterozygous (Aa). Phenotypically healthy individuals, i.e. people who do not carry the trait, are also genotypically healthy.

The inheritance present here is referred to as autosomal recessive. In the case of carriers of traits, the alleles are homozygous (aa), while in the phenotypically healthy persons either heterozygous (Aa) or homozygous (AA) alleles can be present.

This inheritance is referred to as gonosomal recessive or, more precisely, X-linked recessive. Here the relevant gene is on the X chromosome. Y-chromosomal inheritance is very unlikely because the Y-chromosome is very small and contains only a few genes. In the case of an X-linked inheritance, it should always be noted that male individuals only have a single X chromosome, which they get from their mother. The father inherits a Y chromoson to his sons on which the trait cannot be found and his daughters receive the only X chromosome he has.