If one of the two alleles of the chromosome pair has more power to express itself than the other, it is called the dominant allele. The recessive allele is the one that has less strength, and only manifests itself if there is no other dominant allele to hide it. Thus, if there is a dominant allele and a recessive allele, the character determined by the dominant allele is expressed. Only if both alleles are recessive will that character be expressed.
For example, the gene that determines eye color has an allele for dark eyes that dominates over the allele that determines light eyes. The individual will manifest the dark-eyed phenotype if at least one of the two alleles is dominant. For the phenotype to be light-eyed, both alleles must be recessive, so the individual must be homozygous recessive.
It can also happen that two alleles have the same strength and neither dominates over the other, being co-dominant alleles. In this case, a new intermediate phenotype will appear. Two cases can occur:
Being a carrier means not having the disease, but having a copy of the altered gene. In some cases, females show mild symptoms of the disease. Males have one X and one Y chromosome (XY) so if one of the genes on the X chromosome has an alteration they do not have another copy of the gene to compensate for it. This means that they will be affected by the disease.
Diseases that are inherited in this way are called X-linked recessive diseases. Examples of such diseases are hemophilia A and Duchenne muscular dystrophy.
If a female carrier has a daughter, she can pass on to her the X chromosome with the normal gene, or the X chromosome with the altered gene. Each daughter therefore has a 50% chance (1 in 2 chance) of inheriting the mutated gene. If this occurs, the daughter may be a carrier, like her mother.
If a man has an X-linked disease he will always transmit the mutated gene to his daughters, this is because a man has only one X chromosome which is the one that determines female sex when it is inherited. All his daughters, therefore, will be carriers, i.e. they will not develop the disease but are at risk of having affected children. If a man who has an X-linked disease has a son, he will never inherit the mutated gene (located on the X chromosome). This is because men always transmit the Y chromosome to their sons (if they transmitted the X chromosome they would have a daughter).
Types of Mendelian inheritance
Each chromosome has many genes, and the position that each gene occupies along the chromosome is called locus (from Latin, place). Each gene then has its homologous copy at the equivalent locus on the other chromosome in the pair. Each of the two copies of the gene is called an allele.
Let us say, then, that a human cell has two alleles for each of its 20,000 distinct genes. Not all genes are expressed (i.e., transcribed and translated) at the same time and in the same place. In a given tissue or cell type, a subset of the set of all genes is expressed. One of the key points in the regulation of gene expression is the control of transcription. This control is not only concerned with “turning on” or “turning off” genes (all-or-none effect), but also with regulating the amount of product (RNA or protein) of the “turned on” genes.
Surprisingly, 50% of our genome consists of repeated sequences mostly of viral origin. Many of these sequences, mostly located in intergenic regions and introns, correspond to so-called mobile elements, or transposons, “jumping” DNA segments, i.e. they can be duplicated and inserted into other regions within the same genome.
There are several types of genetic inheritance
Genetic inheritance is the process by which the characteristics of individuals are transmitted to their descendants. How many types of genetic inheritance are there? Below we show you all of them and explain what each one consists of.
Genetic inheritance refers to the information that DNA is capable of transmitting to subsequent generations. As you know, the genetic map of a person is composed of the inheritance received from his predecessors, these genetic codes are in turn mediated by the thousands of previous generations.
For today’s science, analyzing genetic inheritance is key to countless practical applications, from preventing diseases to determining the degree of probability of certain cases, most of which occur through genetic inheritance.
The most obvious characteristics are those that are perceived in the physique, which we will review below, however, there are many other factors that are directly inherited from previous generations, and that can have a decisive influence throughout our lives.