Mutations at the genomic level
The genomic mutations are variations in the normal number of chromosomes of a species. They are usually produced by an unequal distribution of chromosomes during meiosis in the formation of gametes, so that some gametes are left with more chromosomes and others with fewer chromosomes.
Genomic mutations are classified into:
They are produced by the non-separation of homologous chromosomes during meiosis.
They are mutations consisting of an increase in the normal number of "sets of chromosomes", and can be triploid (3n), tetraploid (4n) and, in general, polyploid. They can be caused by an error in the reductive division of meiosis or by polyspermia, if an egg is fertilized by more than one sperm.
- Autopolyploids: If all the sets of chromosomes come from the same species.
- Alopolyploids: If the "extra games" come from hybridization, that is, from the crossing of two different species. In plants, hybridization is frequent and their offspring are not sterile.
It is mutations in which there is only one game n of chromosomes.
The haploid is normal in living beings haplontes or haplodiplontes cycles. In animals it is rarer, but it also occurs in some species that reproduce by parthenogenesis.
The haploidía not very important in evolution when it occurs spontaneously, but man has used the haploid individuals with the intent to improve plants. If you manage to duplicate a haploid you get a homozygous diploid individual for all your genes, much faster than trying to get it through heterozygous crosses.
They are mutations that affect only the number of chromosomes in a pair, but without reaching the complete set.
Aneuploidy can occur by defect or by excess of chromosomes, depending on whether a chromosome is missing or has too many. Aneuploidy occurs frequently in nature. They are less harmful in plants than in animals. In humans they are also relatively common and cause disorders such as Down syndrome and others that affect the sex chromosomes.
The aneuploidy may be produced by:
- Nondisjunction in meiosis I, leaving two disomic gametes (with two homologous chromatids) and two nullisomic gametes (without chromatids).
Video: No disjunction at anaphase I.
- Nondisjunction in meiosis II, creating two gametes normal (one chromatid), a gamete disomic and other nullisomic.
Video: Nondisjunction in anaphase II.
After fertilization, nullisomic gametes will give rise to zygotes with mosomy (2n-1) and disomic gametes, to zygotes with trisomies (2n+1).
The only viable monosomy in humans is that of the X chromosome, Turner syndrome.
The Turner syndrome affects only women, caused by the absence of one chromosome X.
Two of the 46 chromosomes in humans determine sex (the sex chromosomes). Women normally have two of the same sex chromosomes (XX), while men have one X chromosome and one Y chromosome (XY). In women with Turner syndrome, the cells are missing all or part of an X chromosome. Most commonly, the patient has only one X chromosome; others may have two X chromosomes, but one of them is incomplete.
The absence of the Y chromosome determines the female sex of all affected individuals, and the absence of the second X chromosome determines the lack of development of primary and secondary sexual characteristics. This gives women with Turner syndrome a childlike appearance and sterility for life. This syndrome appears in approximately 1 in 2,500 girls.
A trisomy is the existence of an extra chromosome in a diploid organism. Instead of a homologous pair of chromosomes, you have a triplet (2n+1 chromosomes).
It can affect both autosomes and sex chromosomes, being more frequent and less serious in the latter.
- Trisomies affecting autosomes:
- Trisomies that affect sex chromosomes:
Children have severe developmental malformations, cleft lip, absence of the olfactory bulb, cardiac, urinary, digestive malformations, and polydactyly. The average survival of these children is about three months. The average age of the parents of affected children is older than that of the parents of normal children, but it is not as high as the average maternal age in Down syndrome. This case only occurs in 1 in every 19,000 births.
Video: Trisomy 13 or Patau syndrome.
The phenotype of these children illustrates that the presence of an extra autosome (18) produces congenital malformations and a shorter life expectancy. These children are smaller than normal. Their skulls are elongated along the anteroposterior axis and their ears are deformed and located lower than normal, they present a wide neck, congenital dislocation of the hip and a depressed chin. The average survival is less than four months. The death is usually caused by pneumonia or heart failure. The mean maternal age is high, babies with Edwards syndrome are predominantly female.
It appears with a frequency of 1 in 8000 births.
People with Down syndrome have short stature, a rounded head, a high, flattened forehead, and a dry, fissured tongue and lips. They present epicanthus, a fold of skin in the inner corner of the eyes. The palms of the hands show a single transverse crease, and the soles of the feet have a crease from the heel to the first web space (between the first two fingers). In many cases they suffer from congenital heart disease and tend to develop leukemia. The intelligence quotient ranges from 20 to 60 but under an intervention and early stimulation program these individuals can achieve significant cognitive development.
The overall incidence of Down syndrome is approximately 1 in 700 births, but the risk varies according to the age of the mother. The incidence in 25-year-old mothers is 1 in 1250 live births; in 30-year-old mothers it is 1 per 1000; in 35-year-old mothers it is 1 in 400; in 40-year-old mothers it is 1%; in 45-year-old mothers it is 1 in 30. Amniocentesis and chorionic villus biopsy can be used to detect the chromosomal abnormality during the prenatal period.
This hereditary anomaly is produced by an error in the separation of the 21 chromosome pair in meiosis, which is why it presents three chromosomes because two chromosomes go together in the same gamete. Therefore, people with Down syndrome have 47 chromosomes instead of 46 (the normal number for the human genome) in all of their cells.