The study of inheritance patterns, the way in which genes are transmitted from parents to offspring, is one of the main fields of study in genetics. As a precursor of this scientific discipline, Gregor Johann Mendel and the experiments he carried out during the second half of the 19th century are often cited.
In diploid organisms, such as the human being, each body cell, or somatic cell, contains two copies of the genome, that is, it contains two copies of each chromosome. The exception is the sex chromosomes, since the male individual has a single copy of the X chromosome and a single copy of the Y chromosome (the female individual has two copies of the X chromosome). The reproductive cells or gametes are haploid with a single copy of each chromosome.
Chromosomes that are not involved in sex determination are called autosomesto differentiate them from sex chromosomes. The human being has a total of 46 chromosomes; Since there are two copies of each gene, it has 22 pairs of autosomes and one pair of sex chromosomes (XY or XX).
Each gene occupies a specific place on a chromosome, the locusand the different variations of the same gene are known as allele. Therefore, in diploid organisms, each individual has two alleles of each autosomal gene, one inherited from the mother and one inherited from the father.
The entire set of alleles is the genotypeand it is the genotype, together with environmental factors, that determines the characteristics that an individual develops, characteristics whose set forms the phenotype. The study of inheritance patterns covers both the inheritance of genes and the inheritance of phenotypic characteristics, since having two alleles of the same gene, the associated phenotypic characteristic will depend on which of them is expressed.
Inheritance patterns also apply to inherited genetic diseasesbecause after all they are also phenotypic characteristics associated with a certain genotype.
1. Mendelian patterns of inheritance
Within a population, many alleles of the same gene can be found, although each person only has two copies. If the two copies are the same, the individual is said to be homozygous for that gene. On the other hand, if each allele is different, the individual is said to be heterozygous.
Homozygous individuals for a gene, having the same two alleles, will present the characteristic associated with that type of allele. But in heterozygous individuals, the phenotypic characteristic developed will depend on which of the two copies is expressed; the allele that is expressed is said to be dominantwhile the other is the allele recessive. For example, the gene for eye color has alleles for different shades, with brown being dominant over blue or green.
Based on these characteristics, various inheritance patterns can be described depending on whether they are on autosomal chromosomes or sex chromosomes, and whether they are dominant or recessive alleles. These patterns are known as mendelian patterns for having been studied for the first time by Mendel.
autosomal dominant pattern
If the phenotype associated with a certain autosomal allele is observed when the individual has only one copy of that allele, then it has been inherited in an autosomal dominant manner. The phenotype can be observed in both heterozygous and homozygous individuals.
Autosomal dominant inheritance usually implies that the characteristic is present in at least one of the two parents. If the parent with the dominant allele is homozygous, he will always pass on that allele to 100% of his offspring; but if this parent is heterozygous (possesses the dominant allele and the recessive allele), there will be a 50% chance that this characteristic will be observed in the offspring, depending on which of the two alleles was present in the sperm or in the egg from which the individual arose.
autosomal recessive pattern
Recessive alleles are those that are not expressed in the presence of a dominant allele. Therefore, for the characteristic associated with this allele to be observed, the individual must be homozygous for that allele. Heterozygous individuals would not express this allele, but they carry it and can pass it on to their offspring.
For an individual to be homozygous, it must receive the same allele from both parents and, therefore, both parents must have it. If both are homozygous, 100% of their offspring will show the associated phenotype, 50% if one parent is homozygous and the other heterozygous, and 25% if both are heterozygous.
When an individual is heterozygous and exhibits the characteristics associated with both alleles, it is said to be codominant. The individual may show mixed characteristics of the two alleles or intermediate characteristics. One of the most typical examples of codominance is the inheritance of the blood groups of the ABO system.
Inheritance linked to sex chromosomes
On the sex chromosomes, the X and Y, being different, contain genes specifically linked to them. Also, in male individuals there will not be two copies of these genes, since they have only one chromosome of each type.
Dominant genes linked to the X chromosome will be expressed in 100% of the offspring, in both male and female individuals. On the contrary, the characteristics associated with recessive genes linked to the X chromosome appear in all male offspring, since they do not have another copy of the X chromosome, and only in homozygous female offspring, which is not very common for recessive alleles.
A characteristic of X-linked inheritance is that parents cannot pass it on to male offspring, because male offspring always receive the X chromosome from their mothers. Instead, female offspring receive one copy of the X chromosome from the father and one from the mother.
Genes linked to the Y chromosome, being present only in the male sex, can only be transmitted from parents to male offspring.
2. Non-Mendelian patterns of inheritance
Some features have a continuous variability which cannot be easily divided into categories of dominant and recessive inheritance. For example, although there are genes that specifically influence the height that an individual can reach, there are many environmental factors that influence the height that an individual eventually develops, such as nutrition, but height also depends on the presence and interaction with other genes.
The phenotypes that can vary within a wide range depending on the gene-environment or gene-gene interaction are known as complex or multifactorial phenotypes.
Another non-Mendelian pattern of inheritance is mitochondrial DNA inheritance. Mitochondria are organelles whose origin were protobacteria that maintained a symbiotic relationship with the first eukaryotes. These organelles still maintain their own genetic material and follow a rhythm of reproduction and transfer. different from the genetic material of the nucleus.
Mitochondrial inheritance has a very peculiar characteristic, which is that it is only transmitted by mothers. This is because eggs contain mitochondria while sperm lose them at fertilization. The mitochondria of the future embryo will all come from the egg. Mitochondrial DNA-associated diseases will affect 100% of the offspring if the mother is the carrier, while no offspring will be affected if the father is the carrier.