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11.2.3. Mutation at the genomic level

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:

Euploidy

The mutation affects the number of complete sets of chromosomes relative to the normal number of chromosomes for the species.

They are produced by the non-separation of homologous chromosomes during meiosis.

Polyploidy

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.

In animals, polyploidy is often incompatible with life. In plants, it is frequent. Polyploid individuals can be:

  • 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.

Haploidy or monoploidy

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.

Aneuploidy

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).

Monosomies

The monosomy occurs when a missing chromosome of a pair of homologous (2n-1). They are rare because they have lethal effects on their wearers.

The only viable monosomy in humans is that of the X chromosome, Turner syndrome.

Turner syndrome (X0)

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.

Video: Turner syndrome part 1 and Turner syndrome part 2 .

Trisomies

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.

Some examples:

Trisomy 13 or Patau syndrome

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.

Trisomy 18 or Edwards 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.

Trisomy 21 or Down syndrome

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.

Curiosity: Marthe Gautier and Down syndrome trisomy 21

Marthe Gautier (born September 10, 1925) is a French doctor and researcher who discovered the chromosomal abnormality of Down syndrome. As the laboratory in which he works did not have a microscope that could photograph the preparations for making a publication, he asked Jérôme Lejeune to help him take the photographs in another better equipped laboratory. In August 1958, photographs identified an extra chromosome in people with Down syndrome, but Lejeune did not return the preparations and said the discovery was his.

In April 1960, this anomaly was officially called trisomy 21.

Curiosity: John Down and trisomy 21

The first documented case of a person with Down syndrome was from Étienne Esquirol in 1838, where a child is diagnosed with "cretinism" or " furfuraceous idiocy ". In 1886, P. Martin Duncan described "a girl with a small, rounded head, with slanted eyes, who let her tongue hang and spoke only a few words".

That same year, in 1886, the director of the Earlswood Asylum for the Mentally Retarded in Surrey, the English physician John Langdon Down, published in London Hospital Reports an article entitled: "Observations in an ethnic group of mentally retarded" where he described in detail the physical characteristics of a group of patients who showed many similarities, also in their capacity for imitation and their sense of humor.

Down attributed the origin of this syndrome to some diseases of the parents, which caused the child to have an involution or retreat to a more primitive phylogenetic state.

He also had a theory that tuberculosis could "break the species barrier" and enable Western parents to have Eastern children. Down's most popular contribution is the description of the category he called " Mongoloid idiocy ," in which he called these people Mongolians because of their similarity in physical appearance to nomadic groups in central Mongolia.

Do you think John Down deserves that the disease suffered by people (whom he called idiots or Mongols) with trisomy 21 syndrome be named after him? What will they think of him in Mongolia? We better call it trisomy 21, right?

Among the trisomies that affect the sex chromosomes are:

Klinefelter syndrome (XXY)

The Klinefelter syndrome is a chromosomal abnormality consisting of the existence of two chromosomes X and one Y chromosome affects men and causes hypogonadism (genital underdeveloped), various malformations and generalized metabolic problems. It worsens between the ages of 14 and 21. Symptoms are more pronounced when there are more than two X chromosomes. Some men have no symptoms and do not know they have this condition until adulthood when they encounter infertility problems.

In Klinefelter syndrome, the individual has a total of 47 chromosomes: 44 + XXY. The two X chromosomes cancel out the Y, and the individual, although he has testes and therefore is male, functionally is not, since they are atrophied.

Curiosity: Carlos II of Spain, "the Bewitched".

Press: "I'm not weird; I have Klinefelter".

Árbol genealógico de Carlos II del España, "el Hechizado"

Carlos II de España, además del síndrome de Klinefelter tenía otras enfermedades a consecuencia, probablemente, de los sucesivos matrimonios consanguíneos de sus antepasados.

Duplo Y syndrome (XYY) or superman syndrome

Some authors do not consider it a syndrome, since the phenotype is normal, (they do not suffer from major disorders) since the vast majority of men with XYY do not know their karyotype.

They have slightly less IQ than their siblings, although within normal limits. Developmental delay and behavior problems are also possible, although they are not especially aggressive.

Formerly it was thought that they were taller and more aggressive, with low IQ, but this is not the case in most cases.

Triple X syndrome

The syndrome Triple XXXX or the superhembra occurs in women who have an extra X chromosome. These are female individuals with atrophied sexual organs, limited fertility, and low IQ. Its prevalence is approximately 1 in 1500 girls. Parents or affected girls will probably not become aware of the presence of this disease, unless they undergo the appropriate medical examinations.

Girls and women with XXX syndrome have three X chromosomes instead of two, which is common. This means that there are 47 chromosomes instead of 46.

Video: Triple X syndrome.