Monk Gregor Mendel discovered in 1857 the fundamentals of heredity by making an experimental study on pea plants.
According to genetics, character is the set of inherited aspects that vary according to individuals, and each variant of a character is defined as stroke.
The pure lines are those generations of children who keep their characters even in later generations.
The hybridization intersection is when two pure lines are distinguished by one character, and parents belonging to pure lines are defined as parental generation, while the hybrid offspring is the F1 generation (first branch generation), and the individuals generated They are called generation F2 (second branch generation).
Alternative forms of a gene are called alleles, they are variants of the same gene, and each gene is located in a particular locus of a given chromosome, and the DNA of that locus may exhibit some nucleotide sequence variation.
Mendel hypothesized that:
- Alternative versions of alleles are responsible for the different versions of an inherited character.
- For each character, a body inherits 2 alleles, one coming from each parent.
- If the two alleles are not identical then one of these is the dominant allele, ie it fully expresses the phenotype of the organism, the other being the recessive allele, and does not exert any obvious effect.
- The 2 alleles of each character are separated during the formation of the gametes, and each egg cell and each sperm receive only 1 of the 2 alleles.
Mendel's segregation law is when there is the segregation of alleles in different gametes.
The Punnett square is a practical way to predict the result of a genetic cross between individuals with known genotype.
An individual possessing a pair of identical alleles for a given character is called homozygote for the gene that controls that character.
Individuals with different alleles for a particular locus are called heterozygotes for that gene.
The external appearance of an organism (both physiological and physical) is called a phenotype, while its genetic constitution is called a genotype.
Crossing or testcross is Mendel's intersection of a recessive homozygote with an organism from the dominant phenotype but from the unknown genotype.
Crosses of individuals that differ from a single character in F1 are called mono hybrids, and when they are heterozygous for both characters, they are called hybrid.
Independent segregation of each pair of alleles during gamete production is known as Mendel's independent assortment law.
The laws of segregation and assortment explain the inherited variations in terms of alternative forms of genes that are transmitted from one generation to another based on the likelihood of the sum and product.
Post-Mendelian genetics
Some genes exhibit incomplete dominance, ie F1 hybrids exhibit a phenotypic aspect somewhat intermediate to the 2 parental varieties.
Full dominance is when the phenotypes of heterozygote and dominant homozygous are indistinguishable.
Coexistence is when 2 alleles act on the phenotype in distinct and distinct ways.
An allele is not said to be dominant because it is able to subdue the recessive allele, the 2 alleles do not interact with each other, but domination and recessivity express rather in the pathway leading from the genotype to the phenotype.
Moreover, the fact that an allele of a particular character is dominant does not necessarily mean that it is more common in a population than its recessive allele.
Most genes are present in populations in more than two formative forms, for example, in humans the ABO blood group is an example of multiple alleles.
Pleiotropy is the ability of a gene to affect the body in many ways.
Epistaxis causes a locus gene to alter the phenotypic expression of a gene located in a second locus.
Multigenic inheritance has on quantitative characters (characters that vary gradually and continuously) when there is an additive effect of 2 or more genes on a single phenotypic character.
The phenotype of an individual depends on both its genetic constitution and environmental factors.
For example, power affects the height, sun exposure on the tan.
The product of a given genotype is not a rigidly defined phenotype, but a range of phenotypic possibilities (the reaction norm) on which environmental factors may be affected.
There are cases where the reaction norm has no amplitude and therefore a given genotype defines an extremely specific phenotype.
Multifactorial characters are the set of factors, genetic and environmental, that affect the phenotype altogether.
The term phenotype is also used to describe an organism as a whole, both anatomically, physically and behaviorally.
The term genotype can be used for the whole genetic constitution of the organism.
The phenotype of an organism reflects both the overall genotype and its environmental history.
Mendelian genetics in the human species
Modern techniques in molecular biology have made many important discoveries, however, Mendelian genetics remains the foundation of human genetics.
The pedigree is a descendant tree that describes relationships between parents and children throughout generations.
We also recall that in the various representations, the lower case letter (eg f) is represented by the recessive alleles, with uppercase the dominant ones (es F).
Many of the genetic diseases follow the Mendelian laws of heredity, and pedigree trees are very useful for their study and prevention.
Thousands of genetic diseases are inherited as simple recessive characters.
In recessive disorders, heterozygotes have a normal phenotype, because even a single copy of the normal allele produces sufficient amount of the protein in question, so recessive hereditary disorder manifests itself only in homozygous individuals inheriting a recessive allele from each parent.
If we indicate the genotype of these individuals as aa, while people who do not exhibit the disease have AA or Aa genotype, Aa heterozygotes that are phenotypically normal are defined as carriers of the disease.
Most recessive disorder patients have normal phenotypic parents, but both bearers of the disease (a pair of carriers corresponds to a Mendelian cross F1 AaxAa where zygote has a probability of 1/4 of receiving recessive alleles).
Cystic fibrosis is a very dangerous genetic disease that strikes in the US 1 white baby on 25.
Tay-Sachs disease is caused by the disorder of a recessive allele, a defective enzyme that is not capable of degrading a particular class of lipids.
The most common genetic disease among color people is sickle cell anemia, which is caused by the replacement of a single red blood cell hemoglobin amino acid, and this results in a decrease in oxygen in the blood.
People who have in common a recent ancestor have a greater chance of wearing the same recessive alleles than people who do not have any kinship constraints, so it is more likely that a relationship between relatives produces a homozygous progeny for a recessive trait, possibly damaging (For this reason in several countries it is forbidden to marry relatives).
Many disorders are also caused by dominant alleles, such as acroplasia, a form of dwarfism.
Unfortunately, when symptoms of lethal dominant alleles manifest themselves, they may already be too late, and may have already been transmitted to progeny.
Huntington's Corea is a degenerative disease of the nervous system that does not show any apparent phenotypic effect before 35-45 years, but once the degeneration of the nervous system has begun, this is irreversible and causes death.
There are also diseases that have a multifactorial basis, characterized by a genetic component and an environmental component.
In many cases the hereditary component is multigenerational, or many genes can affect health, and lifestyle can also affect the course of the disease (eg those who smoke can have more cardiovascular problems, not just because they are predisposed).
Sometimes genetic diseases can be predicted, estimating the risk of some disease occurring prior to conception or in the early stages of pregnancy.
Several tests are available to check if parents are carriers of the alleles of various diseases.
However, when they are expected to be illnesses that may occur, healthy carriers may be discriminated against by life insurance, employers, etc ...
Amniocentesis is an analytical technique that can determine from the 14th-16th week of pregnancy if the fetus is affected by Tay-Sachs disease.
This technique involves the insertion of a needle into the maternal uterus and the extraction of amniotic fluid from the fetus, which allows the detection of certain substances.
The technique of choral wildlife analysis involves the insertion of a thin cannula through the cervix to the uterus and the aspiration of a small sample of fetal tissue from the placenta.
This last analysis is more efficient than the previous one because the results can be achieved in 24 hours, unlike the several weeks needed for amniocentesis, and also wildlife analysis can be done already since the eighth week of pregnancy.
As opposed to wildlife analysis, it does not detect diseases that require amniotic fluid analysis, and not all hospitals are equipped to do so.
The method of ultrasound allows you to examine the fetus through sound waves.
Fetoscopy, on the other hand, involves the introduction into the uterus of a thin probe containing a fiber optic lens.
Amniocentesis and fetoscopy are cmq risky for the fetus and can kill it (occurs in 1% of cases), and for this reason they are only used when the risk of disease is really high.
Infant screening is used to detect hereditary phenylketonuria (PKU) disease.
This disease does not allow the proper degradation of aminoacid phenylalanine, which can accumulate in the blood until it reaches toxic levels.
If the defect is detected in the infant, it can prevent this from causing mental retardation through a special diet.