chromosome aberration

Changes in the number and structure of chromosomes in biological cells

Collection

zero Useful+1

zero

This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

Chromosomal aberration refers to the changes in the number and structure of chromosomes in biological cells.

The number and structure of chromosomes of each species are relatively constant, but under the influence of natural conditions or artificial factors, the number and structure of chromosomes may change, resulting in biological variation. Chromosome aberration includes chromosome number variation and chromosome structure variation.

Chinese name: chromosome aberration
normal person: 23 chromosomes form one genome
Number distortion: Chromosome deviation from normal number

Aneuploid: One or more chromosomes more or less than diploid
Structural distortion: Chromosomal structural aberration
Classification: Number distortion and structure distortion

catalog

one survey
two find
three reason
▪ The mother was too old at the time of conception
▪ Radiation
▪ viral infection

survey

Announce

edit

chromosome aberration

Normal human germ cell It has 23 chromosomes Chromosome , called haploid (n) Somatic cells have 46 chromosomes, including two genome, called diploid (2n). Chromosome deviation from normal number is called chromosome number aberration, which is divided into euploid and non whole change. The doubling of chromosome composition is Euploid 。 In humans triploid (3n=69) and tetraploid (4n=92) were mostly found in spontaneous abortions. than diploid Individuals with more or less one or more chromosomes are called Aneuploid 。 Monosomy, trisomy and tetrasomy are visible in humans. Haplotype The number of chromosomes in the cell is 45, that is, one chromosome is missing. It is mainly seen in the haplotype of X chromosome, karyotype Is 45, X. Trisomy There are 47 chromosomes in the cell, that is, there are 3 chromosomes in one chromosome. In addition to trisomy 21, 13, 18 and 22, others often lead to abortion. Sex chromosome trisomy is common. Tetrasomy: the number of chromosomes in a cell is 48, that is, there are 4 chromosomes in a chromosome number. Mainly Sex chromosome The tetrad of. See "Chromosome structural aberration" for structural aberration.

find

Announce

edit

chromosome aberration

Chromosome structural aberrations first occurred in Drosophila melanogaster Found in. U.S.A geneticist C. B. Bridges Chromosomes were discovered in 1917 defect Duplicate discovery in 1919, discovery in 1923 translocation 。 American geneticist A. H. Stevient Inversion was discovered in 1926. Chromosome number aberrations were first found in Drosophila melanogaster. In 1916, Bridges found one more and one less in the study of fruit flies X chromosome Phenomenon. In 1920, American geneticist A F. Blakesley et al. found that there is one more chromosome than the normal plant in the study of Datura stramonium Mutant 。 Since then, studies on chromosome aberration of plants such as tobacco and wheat have been carried out.

As early as the 1930s, K. Sax The research on chromosome aberration induced by ionizing radiation in such organisms as Commelina communis was started. These studies show that the chromosome pairs of animal and plant cells ionizing radiation It is very sensitive, and there is a certain relationship between the radiation dose and the number of chromosome aberrations under certain conditions. 1962 M. A. Bender It is proposed that Dicentric chromosome and Ring chromosome It is called radiation biodosimetry to calculate the radiation dose received by human body.

1928 Stevient And found that X-ray can induce drosophila Chromosome translocation 。 1937 Blacksley Wait to pass Colchicine The treatment obtained polyploid in plants and began to apply chromosome aberration research to animal and plant breeding. 1959 France clinician J. Legena When someone's Trisomy 21 syndrome 。 Later with cytogenetics and Chromosome banding technique With the development of chromosome aberration, more and more reports have been reported.

reason

Announce

edit

The mother was too old at the time of conception

The older the mother is, the more likely the offspring will have chromosomal disease, which may be related to the mother

chromosome aberration

Chromosome aberration is related to egg aging.

Radiation

Human chromosomes are very sensitive to radiation. The risk of chromosomal aberration in the offspring of pregnant women exposed to radiation increases.

viral infection

Infectious mononucleosis , Popularity mumps 、 Rubella and hepatitis Such viruses can cause chromosome breakage and fetal chromosome aberration.

Chemical factors

Many chemicals, antimetabolic drugs and poisons can cause chromosome aberration.

Genetic factor

Parents with chromosomal abnormalities may be passed on to the next generation.

type

Announce

edit

Chromosome aberration can be divided into Spontaneous distortion and induced aberration 。 Chromosome aberration can be divided into Number distortion and Structural distortion 。

Number distortion

chromosome aberration

Most eukaryotes have two chromosome groups in their somatic cells. Such organisms and their somatic cells are called diploid (2 n )。 The gametes produced by diploid germ mother cells through meiosis have only one chromosome group, which is called haploid ( n )。 The increase or decrease of the number of a chromosome is called Aneuploidy change ； The increase or decrease in the number of complete sets of chromosomes is called Euploidy Change (see Chromosome ploidy ）。 Aneuploidy and Euploidy change Collectively referred to as Heteroploidy Change.

I Aneuploidy

Aneuploidy Chromosome number aberration can be divided into:

① Monomer Some of the diploid cells Homologous chromosome There is only one phenomenon instead of two, that is, 2 n -1。 Most animals and plants Monomer The individual cannot survive. The surviving monomer was first found in wheat. There are 21 different monomers in common wheat and 24 different monomers in common tobacco. They are cytogenetics Useful tools for research (see Gene mapping ）。 In humans, except Turner's syndrome (45, X) attribute chromosome in vitro, autosome Monomer Most embryos die in the womb.

chromosome aberration

② The somatic cells of aneuploid organisms are missing a pair Homologous chromosome Phenomenon of, that is, 2 n -2。 Absent body It was first found in oats. Absorptive individuals generally cannot survive. However, a few species, such as common wheat, have a complete set of artificially preserved anosomic individuals. Absorptivity also exists in malignant tumor cells Cell line 。

③ Trisomy Some of the diploid cells Homologous chromosome There are three phenomena, namely 2 n+1 。 The existence of trisomy was originally in solanaceous plants datura Found in. The karyotype of human patients with Down syndrome is 47, XX or XY, 21, that is, chromosome 21 is one more than that of normal people. The karyotype of patients with Klinefelter's syndrome is 47, XXY, that is, the sex chromosome X is one more than that of normal people. Trisomic individuals generally survive.

④ Multibody Some of the diploid cells Homologous chromosome The number of is more than three. For example, tetrasomy 48, XXXX or tetrasomy 49, XXXXX pentasomy seen in human chromosome diseases are all.

Superdiploidy and Subdiploidy It refers to the phenomenon that the somatic cells of diploid organisms have more or less chromosomes. It also belongs to Aneuploid distortion , common in vitro culture tumor cell 。

chromosome aberration

produce Aneuploidy The main reason of the individual is that Germ mother cell Meiotic formation gamete During or during Zygote Of cleavage It is caused by abnormal replication and distribution of chromosomes during meiosis, and is mainly caused by a pair of homologies at anaphase I Chromosome disjunction or Later stage II Two chromatids of a chromosome No separation Caused by. If no separation occurs gamete In the process of formation n 1 and n -Two types of abnormal gametes. When these gametes and normal gametes（ n ）When combined, it develops into Monomer (2 n -1) Or trisomy (2 n 1) Individual. If it does not occur in the process of fertilized eggs forming early embryos through cleavage, then Monomer And trisomy somatic cell Can exist in the same individual at the same time, thus forming Chimera 。

2、 Euploidy See Chromosome ploidy 。

Structural distortion

Chromatid aberrations There are two forms of structural changes between chromatids or chromatids: ① simple loss, that is, the loss of fragments from monomer breakage; ② Structural rearrangement refers to the exchange of monomers within or between the arms of the same chromosome and that between monomers of different chromosomes. Interchange can be equal or unequal. According to the reconnection mode, the interchange between monomer can be divided into two types. If the broken end Centromere The proximal part of the center is connected with the proximal part, and the distal part is connected with the distal part, which is called U-type interchange 。 If the proximal part of the fracture end is connected with the distal part, it is called X-type interchange 。 U-type exchange is asymmetric exchange, and X-type exchange is symmetric exchange.

chromosome aberration

There are 6 kinds of inter arm and intra arm exchange of the same chromosome, and the exchange of different chromosome monomers can be divided into 12 cases according to the paired homologous chromosomes, the type of exchange, whether the exchange is complete, and the polarity of chromosomes.

In addition to simple monomer exchange and inter monomer exchange, some aberrations are caused by multiple exchanges at the chromosome and chromatid levels, such as three-phase exchange. This distortion occurs at an isosite Chromatid aberration And a simple Chromatid It is caused by one exchange between the breaks, or by several complex exchanges between two or more chromatids.

The structural changes of chromosomes are mainly as follows:

chromosome aberration

① Missing Chromosome arm Broken and lost part genetic material Results. One Chromosome arm If a fracture occurs, and the fracture end fails to reconnect with other fracture ends, then a belt Centromere And a fragment without centromere. The latter can not be targeted during cell division and is lost. The segment with centromere becomes a chromosome with terminal deletion. If a chromosome breaks twice and loses the segment without centromere in the middle, the two segments left will be reconnected Middle absence Chromosomes of. If two arms of the same chromosome are broken at the same time, and the sections of the remaining two arms are reconnected Ring chromosome 。 According to the size of the missing chromosome segment, the damage caused by deletion varies. Larger defects often lead to Lethal effect , and small deletions are not fatal. If the missing part includes some Dominant allele , then Homologous chromosome The recessive allele at the position corresponding to this deletion can be expressed, which is called Pseudodominance 。 In maize, specific phenotypic effects can often be produced if the chromosome segments with color determining genes are missing, such as white seedlings and brown midrib. In humans, partial loss of chromosomes often results in Chromosomal disease , such as Catscream syndrome Because Chromosome 5 Caused by partial loss of the short arm of.

chromosome aberration

② Two or more copies of a certain part of a chromosome are repeated. Repetitions connected end to end are called cohesive repetitions or serial repetitions; Repetitions connected in the opposite direction from head to tail are called inverted cohesion repetition or inverted repetition. The duplicated part can appear in the adjacent position of the same chromosome, or in other positions of the same chromosome or on other chromosomes. The repeated heterozygote has a characteristic meiotic image, and its chromosomes are in progress Federation Repeat the clip when Homologous chromosome No corresponding structure can be found on the, thus forming a circular protuberance called a repeating ring. Similar images can be seen in the salivary gland chromosomes of Drosophila's repeated heterozygotes. Similar images can also be seen in the missing heterozygote cells Missing ring 。 Duplicate genetic effect It is more moderate than lack, but too much repetition will also affect the individual's vitality, and even cause individual death. Repetition of certain regions on the chromosome can produce specific phenotypic effects, such as Dominant gene Bar eye (B) is the result of repetition. The main phenotypic effect is the reduction of the number of single eyes in the compound eye, which makes the compound eye stick rather than the usual oval. On the salivary gland chromosome of the Drosophila melanogaster, the obvious cross striated duplication on the X chromosome can be seen (see Position effect ）。 However, for general chromosomes, it is difficult to detect duplication without banding.

③ Inversion Two breaks appear on one chromosome at the same time, and the middle segment is twisted 180 °, reconnecting to reverse the sequence of genes in this segment of cells with homologous chromosomes. Reversed clips include Centromere The reverse of is called Arm to arm inversion ； The inversion excluding centromere is called Arm inversion 。 Two Breaking point And Centromere With unequal distances between Arm to arm inversion It is easy to identify, and the inversion of equal distance is not easy to detect unless the band display technology is applied. Inversion heterozygotes also have characteristic Meiosis Image, its inverted chromosome and normal Homologous chromosome Federation Appears when Inverted ring 。。 Arm inversion If heterozygote Inverted ring A chromatid with two centromeres and a segment without centromeres are formed after one internal exchange, so that chromosome bridges and segments without centromeres will appear at the later stage of meiosis, and the latter often cannot enter the progeny nucleus Medium; When the bridge with two centromeres is pulled apart, although the two chromosomes can enter the daughter cell separately, the broken position is different, which often leads to loss and gamete death.

chromosome aberration

④ Transposition Chromosome arm One section of is moved to another Nonhomologous chromosome Structural distortion on the arm of. Two Nonhomologous chromosome The exchange of chromosome fragments between Reciprocal translocation 。 Reciprocal translocation Chromosome segments can be equal or unequal in length. commonly gene Changing its position on the chromosome does not change its function, but it is found in drosophila and other organisms that if the position is Autosome The gene of Heterochromatin When you are near, its function will be affected and show mottle Position effect Phenomenon. Inversion may also have the same effect. The translocation homozygote has no obvious cytological characteristics, and its pairing during meiosis will not be abnormal, so the translocation chromosome can be passed from one cell generation to another. However, translocation heterozygotes are different. Due to the pairing of homologous parts of normal chromosomes and translocation chromosomes, a unique cross shaped image can be seen at the metaphase of meiosis. With the process of splitting, the cross shape gradually opens, and the adjacent two Centromere It tends to the same pole or two poles, forming a ring or figure 8 image. The former mode of chromosome departure is called adjacent departure, and the latter is called interactive departure. Reciprocal translocation About 50% of the pollen mother cells of the heterozygote are in a ring shape, with adjacent genera separated, 50% in a figure of 8, and the genera separated alternately. This shows that the orientation of the four centromere towards the poles is random and the action is independent. The result of adjacent departure gamete It contains duplicated or missing chromosomes, forming lethal Unbalanced gamete 。 Interactive departure leads to a non lethal balance gamete The chromosomes of half of the gametes are normal, and half of the gametes have balanced translocation chromosomes, which means that although translocation occurs, translocation does not bring gene Increase or lack of. The reciprocal departure makes two translocation chromosomes enter one gamete cell, and two non translocation chromosomes enter another gamete cell. So this separation method makes Nonhomologous chromosome On gene The free combination of genes on different chromosomes is restricted, which leads to the linkage of genes on different chromosomes. This phenomenon is called False linkage (See Gene mapping ）。

Occurs between two or more chromosomes Reciprocal translocation If the proximal ends of these chromosomes meet, a double Centromere Body or Polycentric body 。 double Centromere The soma has two functional centromeres at the same time, and they tend to two poles respectively in the anaphase of cell division, so that anaphase bridges appear. If this bridge is pulled off, it often leads to cell death. However, according to the study of maize endosperm cells, it is believed that although the chromosome bridge can be pulled off, the two centromeres at both ends can still smoothly enter the two poles of the cell and participate in Daughter cell And their fracture ends are still open and can be reconnected. When they are connected again, another bridge will be formed at the later stage of the next split, and this "fracture fusion bridge" cycle can last for many times cell generation 。

Whole arm transposition It is a translocation between the whole arm (or almost the whole arm). The result of this translocation can produce two new chromosomes with different structures. There is another special case in the whole arm translocation, that is, two homologous (or non homologous) Proximal centromere chromosome The centromeres of the chromosomes fuse with each other to form a central (or subcentric) centromeric chromosome, resulting in a reduction in the number of chromosomes but a constant number of arms. This whole arm displacement is called Robertson's translocation , is W R. B. Robertson discovered it in 1916. For example, in the mouse genus, the most common karyotype is 40 metacentric chromosomes, but in some wild mouse cells there are several double arm chromosomes. These double arm chromosomes are formed by centromere fusion. It is generally believed that in the evolution of mammalian karyotype, Centromere fusion It is the most common form.

⑤ Ring chromosome: if the long arm and the short arm of the chromosome break at the same time, the broken ends of the long arm and the short arm containing the centromere segment are connected to form a ring chromosome. This abnormality is genetically unstable, because its chromosome ring must be opened once along with chromosome replication.

⑥ Isochromosome: When the centromere of a chromosome splits laterally instead of vertically, it is formed by making one progeny accept two long arms and the other accept two short tubes. Isoarm chromosome is the most common structural abnormality of chromosome.

⑦ Chromosome duplication: A part of a chromosome is duplicated, and the newly duplicated chromosome can be located in the same chromosome, attached to another chromosome, or become an independent segment. In fact, the incidence of chromosome duplication is higher than that of chromosome deletion, but because there is no loss of genetic material, phenotypic abnormalities are uncommon and may be ignored.^[1]

In addition to the above structural aberrations, other morphological changes of chromosomes can also be seen under the light microscope. For example, chromosome adhesion, the number of chromosomes stuck together can be two or more. Many physical and chemical factors can induce chromosome adhesion during cell mitosis and meiosis, and some mutant genes can also promote chromosome adhesion. In addition, environmental factor The violent effect of can also have various effects: Chromosome smashing Chemical (high frequency fracture) haplochromatization (metaphase chromosomes exist in monosomic form) Different steps (The formation speed of chromosomes in the same cell is inconsistent) Despinning (The metaphase chromosome is unscrewed and loosened), etc.

Cause disease

Announce

edit

Congenital foolishness (trisomy 21 syndrome)

Children with congenital foolishness (trisomy 21 syndrome)

The child has special craniofacial malformation, small and round head, flat occipital bone, small eyes but too wide eye distance, high outside of both eyes but low inside, round face, flat nose, half open mouth, tongue often protrudes out of the mouth, cracked tongue, low ear position, often through hand, stunted development, low intelligence, and short average life span, about 1/3 of patients died by the age of 10. The cause is that there is a small one Chromosome 21 。 Most patients have congenital heart disease at the same time.

Patau syndrome (trisomy 13 syndrome)

With additional Chromosome 13 , recorded as 47, XX or XY, 13, with an incidence of about 1/5000. The number of female patients was significantly more than that of male patients, and the deformity and other clinical features of the children were more serious than those of trisomy 21, including small head, cleft lip and/or palate Congenital heart disease Severe mental retardation. 90% died within 6 months after birth.

Edwards syndrome

The karyotype is 47, XX or XY, 18 due to additional 18 chromosomes (group E). Another 8% - 10% cases are 48, XXX or XXY, 18, double Aneuploidy 。 These children not only have an additional chromosome 18, but also an additional X chromosome. It can lead to severe malformations and death shortly after birth, with an average age of only 71 days. Because its malformation covers almost all organ systems, in addition, 95% of children have congenital heart disease, which is an important cause of infant death.

Catscream syndrome

The loss of chromosome 5 caused the crying sound of the patient as a cat when he was young. Hence the name Catscream syndrome 。

Two hypotheses

Announce

edit

Fracture reconnection hypothesis

from 50. J. Stadler It was proposed in 1931. The other is exchange hypothesis , by S. H. Revere It was proposed in 1959. The breakage reconnection hypothesis believes that the primary damage leading to the change of chromosome structure is breakage. Such breaks can occur spontaneously or as a result of mutagenic factors. The consequences of fracture include the following three types: ① Most fractures (90~99%) are reconnected (healed) in place through the repair process, so that cytology It cannot be identified on ② The reconnection of different breaks is called reconnection, which changes the structure of chromosomes, so most of them can be found. ③ The broken end is still free, becoming a stable state of chromosome structure, such as the deletion of the end part.

exchange hypothesis

It is believed that the fundamental cause of chromosome structural aberration is that there are unstable parts on the chromosome, and all structural aberrations are the result of the exchange between two unstable parts close together. The occurrence of exchange can be divided into two stages: the first stage is a relatively stable state secondary to the fracture of unstable parts, which is called the beginning of exchange. The second stage is the mechanical exchange and connection process. If the two primary damages cannot interact, these damages can be repair 。

application

Announce

edit

Changes in the structure or number of chromosomes can occur spontaneously or induced. Chromosome aberration generally refers to a large range of structural changes in chromosomes, which can be identified under the light microscope. prokaryote Although the structural change of chromosome in is indistinguishable under the light microscope, the aberration similar to that of eukaryotes can be seen under the electron microscope Heterozygote Image.

The study of chromosome aberration can be used to reveal the rule and mechanism of chromosome structure change; It can be used to draw cytological map; It can be used to explore the mechanism of speciation; stay behavioral genetics It can be used to obtain chimeras in research; It can be used to detect mutagenesis and Carcinogen Indicators; It can be used as the basis for diagnosis and prevention of human chromosome diseases; It can also be used to cultivate excellent animal and plant varieties.

Many chemicals can induce chromosomal aberrations, but the DNA damage caused by these chemicals is only shown as chromosomal aberrations after passing the DNA replication stage. If on cell cycle When exposed to chemicals at other stages of cell division, the damage caused by exposure to chemicals has been repaired before cell division, and chromosome aberration can no longer be detected. Compared with chromosome aberration analysis, sister chromatid exchange (SCE) SCE ）The frequency measurement is more accurate and has been widely used in monitoring environmental Mutagen And carcinogens (see Toxicological genetics ）。

The study of chromosome aberration has also been applied to the diagnosis of human chromosomal diseases (including prenatal diagnosis) and the exploration of the causes of chromosomal diseases. For example, with the increase of mother's age, the birth rate of children with trisomy 21 syndrome increases. It can be seen that the incidence of chromosome non departure is significantly increased in the process of germ cell formation in older women, which is related to the decline of ovarian function.

Chromosome aberration is also widely used in the breeding of new varieties of animals and plants. stay Autopolyploid The use of seedless watermelon can be used as an example (see Chromosome ploidy ）。 stay Allopolyploid Chinese geneticist Bao Wenkui When they have cultivated Triticale 。 Haploid breeding can also be used to cultivate excellent crop varieties. For example, for China anther culture Tobacco and wheat varieties were bred by chromosome doubling technique. In addition, using Chromosome structure variation It can also breed new varieties. For example, the sex chromosome of male silkworm is ZZ, and that of female silkworm is ZZ Sex chromosome It is ZW. By induction Chromosome translocation You can set Autosome The fragments with egg color genes on the T chromosome translocated to the W chromosome. Such ZW eggs can be distinguished from ZZ eggs without this egg color gene, which can easily distinguish male and female eggs and eliminate them as soon as possible female egg So as to improve the yield and quality of silk.

The study of chromosome aberration has also been applied to gene mapping. For example, from 21 different monomers of wheat, 21 different Absent body 。 If a new recessive mutation is found in wheat gene , we can combine this mutant with 21 Absent body hybridization The chromosome to which this gene belongs is judged according to the character ratio of the hybrid offspring. Gene location can also be determined by using various strains with chromosome arm deletion (see Gene mapping ）。

The mechanism of chromosome aberration is still unclear. Although the change of the number can be attributed to the non separation of chromosomes, it is not very clear which factors lead to non separation and why non separation occurs. The two hypotheses that explain the change of chromosome structure cannot explain some facts. For example, according to the exchange hypothesis, even the simple end missing should also be the product of exchange, but it is recently used SCE technology It is revealed that SCE will also occur without seeing SCE Missing end 。 Another example is why chromosome breakage and deletion always occur in specific parts of certain chromosomes, which also needs further research.

influence

Announce

edit

Chromosome aberration and Gene mutation It can occur naturally or induced. Common inducing factors include radiation (such as r ray, ultraviolet ray, etc.), viruses (such as rubella virus, cytomegalovirus, hepatitis virus, HIV, etc.) and chemical substances (such as some pesticides, antibiotics, food additives, lead, mercury, benzene, cadmium, etc.). In addition, the old age of pregnant women is also one of the reasons for the formation of 21 trisomy and other trisomy chromosomal abnormalities.

chromosomal aberration syndromes

Announce

edit

Diseases caused by abnormal number or structure of chromosomes are called chromosomal diseases, which usually show a series of clinical symptoms and involve abnormalities in the morphological structure and function of many organ systems, so they are also called chromosome aberration syndrome or chromosome syndrome.

According to different aberrant chromosomes, chromosome syndrome can be divided into two categories: autosomal syndrome (1-22 pairs of autosomal abnormalities) and sex chromosome syndrome (X, Y abnormalities of sex chromosomes). They all include diseases caused by chromosome number or structural aberration.

Chromosomes are carriers of nuclear genes. There are about 50000~60000 structural genes on human haploid chromosomes (i.e. 22 autosomes and X and Y sex chromosomes), and on average there are at least thousands of genes on each chromosome. Therefore, chromosome aberration usually involves the loss or increase of many genes. And because of the pleiotropy of genes, it can cause abnormalities of various body traits, with various clinical manifestations, that is, syndrome. The general clinical manifestations of autosomal syndrome are mental retardation, growth retardation, and multiple congenital malformations. The common clinical characteristics of sex chromosome syndrome are sexual agenesis or hermaphroditism. Some patients only show decreased fertility, secondary amenorrhea and poor intelligence. However, not all chromosomal abnormalities will have obvious clinical symptoms or phenotypic abnormalities, such as balanced translocation or inversion carriers with normal phenotypes, but only after marriage may cause abortion, stillbirth, neonatal death, birth of congenital malformations or mental retardation.^[2]

Autosomal dominant disease (AD)

Autosomal dominant genetic diseases refer to diseases caused by dominant pathogenic genes located on autosomes. Human somatic cells have 22 pairs of autosomes and 1 pair of sex chromosomes. Paired autosomes have alleles at the same locus, and they can be divided into dominant (A) and recessive (a). Since the dominant trait A gene is a pathogenic gene, as long as individuals with A gene are patients, including those with AA and Aa genotypes, individuals with genotype aa are normal. The pedigree of autosomal dominant diseases has the following characteristics:

1. One of the patient's parents is often a patient. Because the disease genes are passed from parents. Both parents have no disease, and children generally do not get sick unless mutation occurs.

2. Most patients in the pedigree are heterozygotes (Aa), so about 1/2 of the children of patients are patients. Sometimes such a proportion can not be reflected in small families, but if the population of many small families is statistically analyzed, an approximate proportion will be seen.

3. Because it is a dominant gene that determines the onset of disease, the disease will occur to whoever it is transmitted to. It can be found that patients appear in successive generations, that is, successive generations of inheritance.

4. Because the pathogenic gene is on the autosome and has nothing to do with sex, men and women have equal opportunities for disease.

Most dominant inheritance belongs to complete dominance, that is, the heterozygote (Aa) shows the same phenotype as the dominant homozygote (AA). For example, genotype BB and Bb are both short fingered, with no difference in clinical performance. In a few cases, dominant inheritance also has some special manifestations: incomplete dominance or semi dominance; Irregular dominance; Codominant inheritance; Delayed dominance; Inherit from sex.

Autosomal recessive disease (AR)

Autosomal recessive genetic disease refers to that the recessive pathogenic gene located on the autosome only comes on when it is homozygous (aa). When it is heterozygous (Aa), Aa does not come on because the role of the normal dominant gene A can mask the role of the pathogenic gene a. This kind of individuals with disease causing genes but without disease are called disease causing gene carriers. The carrier Aa can pass the A gene to the next generation. If one couple is AA and the other is AA, then their offspring are carriers Aa; When both couples are carriers Aa, about 1/4 of their offspring are patients aa, 1/2 are carriers Aa, and 1/4 are normal people AA, showing a ratio of 1:2:1.

The genealogy of autosomal recessive inheritance has the following characteristics:

1. The patient's parents are usually disease-free, but they are all carriers of recessive pathogenic genes (Aa).

2. About 1/4 of the patients' siblings suffer from the disease, but it is rare to see 1/4 of them in a small family. If several parents are all families of Aa, the possibility of aa in offspring patients is close to 1/4. Because the pathogenic gene is located on the autosome, men and women have equal opportunities for disease.

3. There is no successive inheritance in the pedigree, which is shown as sporadic inheritance. It is likely that patients only appear in one generation.

4. The risk of recessive genetic diseases in offspring of inbreeding is much higher than that of non inbreeding.

Novice on the road

Growth task Getting Started with Editing Edit Rule Edit by myself

I have questions

Content query Online Service Official post bar Feedback

Complaints and suggestions

Report bad information Failed to appeal through entries Complaint of infringement information Blocking query and unblocking

Jinggong Network Anbei No. 1100000200001