Quantitative or polygenic inheritance is a type of gene interaction. It occurs when two or more pairs of alleles add or accumulate their effects, producing a series of phenotypes that are different from each other.
The traits can also be affected by environmental factors, which increases the phenotypic variation.
In quantitative inheritance, the number of phenotypes found depends on the number of alleles involved. The number of phenotypes follows this expression: number of alleles + 1.
Example: If there are 4 alleles involved, 5 phenotypes arise; If there are 6 alleles, 7 phenotypes arise. And so on.
Examples of quantitative inheritance are the height, weight and skin and eye color characteristics of humans.
Inheritance of skin color in the human species
Human skin color follows the pattern of quantitative inheritance, in which the alleles of each gene add their effects.
Skin color classifies people into five basic phenotypes: black, dark mulatto, medium mulatto, light mulatto, and white.
These phenotypes are controlled by two pairs of alleles (Aa and Bb).
Capital alleles (AB) condition the production of large amounts of melanin. Lowercase (ab) alleles are less active in melanin production.
Learn more about the Dominant and Recessive Genes.
According to the interaction between these four genes, located on different homologous chromosomes, we have the following genotypes and phenotypes:
| Genotypes | Phenotypes |
|---|---|
| AABB | Black |
| AABb or AaBB | dark mulatto |
| AAbb, aaBB or AaBb | medium mulatto |
| Aabb or aaBb | light mulatto |
| aab | White |
Human eye color also follows the pattern of quantitative inheritance. Different eye colors are produced due to different amounts of melanin.
A variety of genes influence the production of melanin and, consequently, the color of the eyes.
What differentiates quantitative inheritance from other genetic inheritances?
- Gradual variation of phenotype:
Using skin color as an example, there are two extreme phenotypes: white and black. However, between these two extremes there are several intermediate phenotypes.
- Distribution of phenotypes in a normal or Gaussian curve:
Extreme phenotypes are found to a lesser extent. While intermediate phenotypes are observed more frequently. This distribution pattern establishes a normal curve, called a Gauss curve.
Exercises
1. (FEPECS-DF) The amount of pigment in human skin can increase under the action of the sun's rays. Inheritance of human skin color appears to be determined by at least two pairs of alleles, each located on different pairs of homologous chromosomes. Assuming that the inheritance of human skin color is determined by only two pairs of alleles, the probability of a couple, he is an average mulatto son of a white mother, she is a light mulatto, having a male and white child é:
a) 1/32
b) 1/16
c) 1/8
d) 1/4
e) 1/2
b) 1/16
2. (UCS) Human skin color depends on at least two pairs of alleles, located on homologous chromosomes. The gene interaction that determines color is called __________. However, skin color can vary influenced by the environment, as people who bathe they become tanned, that is, they are darker in color due to an increase in the pigment called ___________.
Tick the alternative that fills in correctly and respectively the blanks above.
a) quantitative inheritance - melanin
b) pleiotropy - serotonin
c) incomplete dominance - erythrocruerin
d) epistasis - serotonin
e) complete dominance - melanin
a) quantitative inheritance - melanin
3. (PUC) The iris color of the eyes in the human species is a QUANTITATIVE INHERITANCE determined by different pairs of alleles. In this type of inheritance, each effective allele, represented by capital letters (N and B), adds the same degree of intensity to the phenotype. Alleles represented by lowercase letters (no and B) are ineffective.
another allele gene THE with independent segregation of the other two mentioned alleles is necessary for the production of melanin and consequent effectiveness of the alleles N and B. individuals yy they are albino and do not deposit melanin pigments in the iris.
According to the information given, it is INCORRECT to state:
a) All descendants of parents who are homozygous for all genes must have the same genotype, even if it is different from the one presented by the parents.
b) Considering only the two pairs of additive alleles, several genotypes are possible, but only five phenotypes.
c) The non-occurrence of preferential crosses in a non-albino population, whose allele frequency N and B be equal, favors a higher percentage of offspring with an intermediate phenotype.
d) The crossing of individuals NnBbAa with nnbbaa can produce eight different phenotypes.
d) The crossing of individuals NnBbAa with nnbbaa can produce eight different phenotypes.
4. (UECE) Knowing that human height is determined by additive genes and assuming that 3 (three) pairs of effective alleles determine the tall phenotype of 1.95m; that height classes vary from 5 cm to 5 cm; that the low phenotype is determined by the same 3 (three) pairs of ineffective alleles, performing the crossing between tri-hybrids, it is expected to find, in the class of 1.85m, a proportion phenotypic of:
a) 3/32;
b) 15/64;
c) 5/16;
d) 1/64.
a) 3/32;
