The chromosomal theory of inheritance argues that Mendelian genes possess specific loci on chromosomes and the latter segregate and assort themselves independently.
The Morgan embryoologist was the first to associate a specific gene with a particular chromosome, and though skeptical of Mendelian theories, convinced through his experiments that the chromosomes were the seat of Mendel's hereditary factors.
For his experiments, Morgan selected a kind of fruit fly, Drosophila melanogaster, and used it to make different mixtures and pairs, all with red eyes, except for a rare case of white-eye fever.
The most common phenotype in natural populations is called wild type, while alternative characters are called mutant phenotypes because they are due to alleles likely to be caused by changes or mutations.
Morgan also understood that the genus responsible for the white-eye character was to be located exclusively in the X chromosome, and since males possess only one X chromosome, there was no wild-type allele able to neutralize the recessive allele.
Gender genes are defined as genes related to sex.
Each chromosome has hundreds or thousands of genes, and these genes tend to be eradicated together in genetic crosses, and this type of genes is called linkage genes.
Genetic recombination is the production of a progeny characterized by new combinations of characters inherited by the two parents.
Generations P are those individuals of progeny who have combinations of characters that do not match those observed in their parents.
The Punnett square allows to predict the prognosis of genotypes and phenotypes of progeny.
The parental type is the offspring inheriting a phenotype equal to one of the two parental phenotypes, and when the progeny manifests new character combinations, they are called recombined.
Recombination between non-concatenated genes occurs due to the random orientation of homologous chromosomes during meiosis I metaphase, which results in the independent assortment of alleles.
Concatenated genes do not follow the law of independent assortment, since they are located on the same chromosome and therefore tend to move together during meiosis and fertilization, but the recombination of conjugated genes occurs.
Crossing over is the exchange of segments between homologous chromosomes that breaks the concatenation between the two genes and is therefore responsible for the recombination of the conjugated genes.
Crossing over occurs during meiosis I and on this occasion non-brooded chromatids can break into corresponding sites, exchanging DNA fragments, and recombined chromosomes can bring new allelic combinations.
Alfred H. elaborated the genetic mapping that lists the sequence of gene loci along a particular chromosome.
Sturtevant hypothesized that more than two genes are distant on a chromosome and the greater the probability that one crosses over, because it increases the number of points where it can occur, and hence the frequency with which the scrambling increases 'Increase the distance separating the 2 genes.
So to test his hypothesis he started mapping the genes into a map based on recombination frequencies called concatenation map (or linkage), and expressed the distance between genes in map units, with the centimorgan measurement unit of 1 % Recombination frequency.
The chromosome recombination frequency so far distant as crossing over is always equal to the maximum value of 50%.
It is also worth remembering that a second crossing over overrides the first, and for this reason the crossing frequency is not quite uniform, so the map units are not an absolute measure.
Other methods allow to cramp the cytological maps of the chromosomes, which locate the genes with respect to the chromosomal characteristics.
Sexual chromosomes
There are 2 types of sex chromosomes, X and Y.
Sex is a fact of chance, who inherits 2 X chromosomes develops the female sex, one X and one Y, male.
The indispensable gene for the development of the testicles is called SRY, also without some genes located on Y chromosomes, males can not exercise their reproductive functions.
Each male receiving the recessive allele from the mother will manifest the character in question, and for this reason males have many more hereditary diseases related to recessive sex alleles.
A sex-related illness is Duchenne's muscular dystrophy, which causes muscle weakening and difficulty in coordination, and finally death.
Hemolysis is another sex-related illness due to the absence of one or more proteins involved in blood clotting.
In mammalian females, one of the 2 chromosomes X is almost always deactivated during embryonic development, so males and females possess the same number of loci genes on chromosome X, and the inactive X condenses forming a structure known as Barr's body , Whose genes, for the most part, are not expressed even if they are active.
If a female is sexually heterozygous, half its cells express an allele and half the other.
Errors and exceptions in chromosomal inheritance
Physical or chemical disorders and errors during the meiosis may damage the chromosomes and alter their number in the cell.
Non-disjunction occurs when members of a pair of homologous chromosomes do not dissociate correctly during meiosis I, or the broth chromatides do not separate in meiosis II, and when this occurs the gametes receive 2 chromosomes of the same type, No one else receives it.
If one of these gametes joins a normal gamete during fertilization, the progeny will have an abnormal number of chromosomes, in a condition known as aneuploidy.
If the chromosome is present in triplicate in a fertilized egg cell, it has the trimosomic aneuploidy, while the cell having a lower chromosome is called monosomy.
If this anomaly occurs during embryonic development, mitosis will transmit it to all cells, and the entire body will have serious problems.
Polyploidosis occurs when organisms possess more than 2 complete chromosomal kits.
It seems that a missing chromosome or one more, disturb the body more than the presence of whole sets of supernumerated chromosomes.
Breaking a chromosome can give rise to 4 different types of alterations:
- The deletion occurs when a chromosomal fragment lacking centromere is lost during cell division.
- Duplication occurs when the deletion fragment is added during meiosis to the chromogenic brother or to a homologous chromosome.
- Reversal occurs when the fragment reconnects on the original chromosome but is oriented in the opposite direction.
- Translocation occurs when the fragment is added to a non-homologous chromosome.
These effects can be deleterious to the body and are often lethal, and inversions and translocations can alter the phenotype.
Genetic diseases due to chromosomal alterations
Non-disunion can cause spontaneous abortion, and another type of aneuploidy can cause Down's syndrome, where there is a supernumerary chromosome 21, so that each somatic cell has 47 chromosomes.
This syndrome involves common facial traits, weakness, infertility, obesity, mental retardation and predisposition to various diseases, and most subjects do not reach the age of three.
Down syndrome is caused by non-disunion in the production of the gametes of one of the two parents, and it seems that the older the pregnant woman is, the greater the chance of having a child Down.
Klinefelter's syndrome occurs when the chromosome X in the male (XXY) is supernatant, in which case the individuals are sterile and often have female characteristics.
In the case of XYY in males, these are usually bigger and more robust.
In XXX women do not have XX differences except for their karyotype.
X monosomy in females, known as Turner's syndrome, occurs when there is only one X chromosome and this entails sterility and non-development of sexual organs.
Cri du chat is when the number of chromosomes is normal but there is a deletion in chromosome 5, which results in mental retardation in the small and small head, and subjects die from small.
CML chronic myeloid leukemia is a cancer that affects the cells that produce white blood cells, which is caused by a reciprocal translocation.
In mammals, some traits and some hereditary dysfunctions depend on which of the parents transmits the alleles related to the character in question.
Prader-Willi's syndrome is caused by mental retardation, obesity, bass, mania and small feet, and it manifests itself if the child inherits the chromosome 15 smeared by the father, while if inherited from the mother has Angelman's syndrome, which involves Uncontrollable rice, convulsive movements and various motor and mental abnormalities.
Genomic imprinting is when a gene located on a chromosome remains silent while its homologue located on the homologous chromosome expresses, and this would explain why the diversity of the effects depending on whether the gene is male or female.
The fragile X syndrome is when the X chromosome has an abnormal appearance, which causes mental retardation.
This syndrome is transmitted more easily by the mother because if a male XY inherits a fragile X chromosome, this must necessarily come from the mother.
Not all genes of a eukaryotic cell are located in nuclear chromosomes, there are also genes in mitochondria, in plants, in plastids.
However, cytoplasmic genes do not follow the Mendelian hereditary picture.
In mammals the mitochondria contained in the zygote are all derived from the egg cell cytoplasm, and mitochondrial DNA mutations cause rare diseases, which usually reduce the amount of ATP produced by the cell.
Organic structures that are most susceptible to energy shortages are the nervous system and the muscular system, and for example, mitochondrial myopathy involves weakness and muscle deterioration; in other cases, these mutations can cause diabetes and heart disease.