What Is a Chromosomal Aberration?

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

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

The number and structure of chromosomes in each organism is relatively constant, but under the influence of natural conditions or artificial factors, the number and structure of chromosomes may change, resulting in biological mutation. Chromosome aberrations include variations in the number of chromosomes and variations in the structure of chromosomes.

Chinese name

Chromosome aberration

Normal person

23 chromosomes into one genome

Number distortion

Chromosome deviation from normal number

Aneuploidy

More or less chromosomes than diploid

Structural distortion

Chromosome structural aberrations

Classification

Number and structural distortion

Overview of chromosome aberrations

Normal human germ cells have 23 chromosomes as a chromosome group, called haploid (n), somatic cells have 46 chromosomes, containing two chromosomes.

Chromosome aberration

The chromosome group is called diploid (2n). Deviating from the normal number of chromosomes is called aberration of chromosome number, which is divided into euploid and non-global changes. The chromosome composition is increased and decreased euploid. Human triploid (3n = 69) and tetraploid (4n = 92) euploids are mostly found in spontaneous abortions. Individuals with more or less chromosomes than diploids are called aneuploidy. Haplotypes, trisomy and quads are visible in humans. Haplotype: The number of chromosomes in a cell is 45, which means one less chromosome. It is mainly found in the haplotype of the X chromosome with a karyotype of 45 and X. Trisomy: The number of chromosomes in a cell is 47, that is, there are 3 chromosomes. With the exception of the trisomy 21, 13, 18, and 22, miscarriages are more common. Sex chromosome trisomy is common. Four-body type: 48 chromosomes, that is, 4 chromosomes. There are mainly four body types of sex chromosomes. For structural aberrations see "chromosomal structural aberrations".

Chromosomal Aberration Discovery

Chromosome structural aberrations were first discovered in Drosophila melanogaster. American geneticist CB Bridges discovered a chromosome deletion in 1917, and issued in 1919

Chromosome aberration

It is repeated, and a translocation was found in 1923. American geneticist AH Stevete found the inversion in 1926. Aberrations in chromosome numbers were first discovered in Drosophila. In 1916, Bridges discovered the phenomenon of one more and one less X chromosome in the study of fruit flies. In 1920, American geneticist AF Blacksley et al. Found in a mandala study that there was a mutant type with one more chromosome than normal plants. Since then, studies on chromosomal aberrations in plants such as tobacco and wheat have been carried out.

As early as the 1930s, K. Sachs and others began to study ionizing radiation-induced chromosomal aberrations in organisms such as purple duckweed. These studies indicate that the chromosomes of animal and plant cells are very sensitive to ionizing radiation, and there is a certain relationship between the dose of radiation and the number of chromosome aberrations under certain conditions. In 1962, MA Bend proposed that the radiation dose received by the human body could be calculated based on the number of dicentric chromosomes and circular chromosomes in human cells, which is called the bioassay of radiation.

In 1928, Stivent and others discovered that X-rays can induce chromosomal translocation in Drosophila. In 1937, Blacksley et al. Obtained polyploids in plants through colchicine treatment, and began to apply research on chromosomal aberrations to animal and plant breeding. In 1959, French clinician J. Legener et al. Reported trisomy 21 of human chromosome 21. With the development of cytogenetics and chromosome banding technology, reports of chromosomal aberrations have been increasing.

Causes of chromosome aberrations

Chromosomal aberration mothers are too old at conception

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

Chromosome aberration

Chromosome aberration

Eggs are related to aging.

Chromosome aberration radiation

Human chromosomes are very sensitive to radiation. After exposure to radiation, pregnant women are at increased risk for chromosomal aberrations.

Chromosome aberration virus infection

Viruses such as infectious mononucleosis, mumps, rubella, and hepatitis can cause chromosome breakage and cause fetal chromosome aberrations.

Chromosomal aberrations

Many chemicals, antimetabolites, and poisons can cause chromosomal aberrations.

Genetic factors of chromosome aberrations

Chromosomal abnormalities may be passed on to the next generation.

Chromosome aberration type

Chromosome aberrations can be divided into spontaneous and induced aberrations according to their causes. According to the nature of the aberration, chromosome aberrations can be divided into number aberrations and structural aberrations.

Chromosome aberration

Chromosome aberration

Most eukaryotic somatic cells have two chromosomes, and such organisms and their somatic cells are called diploids (2 n ). There is only one chromosome in the gametes produced by diploid germ cells after meiosis, which is called haploid ( n ). An increase or decrease in the number of a chromosome is called an aneuploidy change; an increase or decrease in the number of sets of chromosomes is called an aneuploidy change (see Chromosome ploidy). Aneuploidy and euploidy change are collectively called aneuploidy change.

I. Aneuploidy

Aneuploidy chromosome aberrations can be divided into:

There is only one rather than two homologous chromosomes in haploid diploid cells, that is, 2 n -1. Most animal and plant haploid individuals cannot survive, and the surviving haplotypes were originally found in wheat. There are 21 different monomers in common wheat and 24 different monomers in common tobacco, which are useful tools for cytogenetics research (see Gene Mapping). In humans, with the exception of Turner syndrome (45, X) chromosomal monomers, autosomal embryos often die in the womb.

The phenomenon that a somatic cell of a diploid diploid organism lacks a pair of homologous chromosomes, that is, 2 n -2. Defects were first discovered in oats. Degenerate individuals also generally cannot survive. However, a few species, such as common wheat, have artificially-preserved sets of deficient individuals. Malignant

Chromosome aberration

There are also deficient cell lines in tumor cells.

There are three homologous chromosomes in trisomy diploid cells, that is, 2 n + 1 . The existence of the trisomy was first discovered in the nightshade plant Datura. The karyotype of human Down's syndrome patients is 47, XX or XY, 21, that is, chromosome 21 has one more than normal people. The karyotype of patients with Creutzenberg's syndrome is 47, XXY, that is, one more sex chromosome X than normal. Trisomy individuals generally survive.

The phenomenon that there are more than three homologous chromosomes in polyploid diploid cells. For example, 48, XXXX tetras or 49, XXXXX pentoses, etc. are seen in human chromosomal diseases.

Superdiploidy and hypodiploidy refer to the phenomenon in which somatic cells of diploid organisms have more or fewer chromosomes, and also belong to aneuploidy aberrations, which are common in tumor cells cultured in vitro.

The main reason for the generation of aneuploidy individuals is during the meiosis of the germ cells to form gametes or during the cleavage of fertilized eggs

Chromosome aberration

It is caused by the abnormal replication and distribution of the body, and is mainly caused by the non-separation of a homologous chromosome pair in the late meiosis or the two chromatids of a chromosome in the late . If non-separation occurs during gamete formation, then two types of abnormal gametes, n 1 and n -1, are formed. When these gametes are combined with normal gametes ( n ), they develop into monosomic (2 n -1) or trisomy (2 n 1) individuals. If it does not leave in the process of zygote formation of fertilized eggs into early embryos, then monosomic and trisomy somatic cells can coexist in the same individual, thereby forming a chimera.

2. See ploidy for chromosome.

Structural aberration

Chromatid aberrations There are two forms of structural changes between chromatids or chromatids: simple deletion, that is, the fragment that is broken by the monomer is lost; structural rearrangement, that is, occurs within or between the arms of the same chromosome Inter-monomer swapping and swapping between monomers on different chromosomes. Swaps can be equal or unequal. The interchange between monomers can be divided into two types according to the reconnection method. If the near-centre part with the centromere as the center of the fracture end is connected with the near-centre part, and the telecentric part is connected with the telecentric part, it is called a U-shaped interchange. If the near-central part of the fracture end and the telecentric part meet, it is called X-type interchange. U-type swap is asymmetric swap, and X-type swap is symmetric swap.

Chromosome aberration

There can be 6 ways of inter-arm and intra-arm swaps on the same chromosome, and swaps between different chromosome monomers are based on the paired homologous chromosomes, the type of swap, whether the swap is complete, and the polarity of the chromosome. Divided into 12 cases.

In addition to simple intra-monomer and inter-monomer swaps, some aberrations are caused by multiple swaps at the chromosome and chromatid levels, such as three-phase swaps. This aberration occurs due to an interchange between an allelic chromatid aberration and a simple chromatid break, or several complex inter-monomer occurrences between two or more chromatids. Caused by the swap.

The structural changes of chromosomes are as follows:

The result of the broken chromosome arm is broken and part of the genetic material is lost. A chromosomal arm was broken, and this broken end failed to rejoin another broken end, so a fragment with centromere and a fragment without centromere were formed. The latter during cell division

Chromosome aberration

Cannot be targeted and lost. The centromere becomes a chromosome with a terminal deletion. If a chromosome is broken twice and the fragment without centromere is lost, the remaining two fragments are rejoined to become a chromosome with an intermediate deletion. If the two arms of the same chromosome are broken at the same time, and the cross sections of the remaining two arms are rejoined, a circular chromosome is formed. Depending on the size of the missing chromosome segment, the harm caused by the deletion varies. Larger deletions often have a lethal effect, while smaller deletions do not. If the deletion includes certain dominant alleles, the recessive allele at the position corresponding to the deletion on the homologous chromosome is manifested. This phenomenon is called pseudodominance. In maize, if a segment of the chromosome with a color-determining gene is deleted, it can often produce specific phenotypic effects, such as white seedlings and brown midribs. In humans, partial deletion of chromosomes often leads to chromosomal diseases, such as Meow Syndrome, which is caused by partial deletion of the short arm of chromosome 5.

Repeat two or more copies of a certain part of a chromosome. End-to-end repeats are called concatenated repeats or concatenated repeats; repeats connected in opposite directions are called upside-down concatenated repeats or inverted repeats. Duplicates can appear adjacent to the same chromosome, or they can

Chromosome aberration

Appears elsewhere on the same chromosome or on other chromosomes. The complex zygote has a characteristic meiotic image. When the chromosomes of the chromosome are connected, the corresponding fragments cannot find the corresponding structure on the homologous chromosome, so a circular protrusion called a repeating loop is formed. Similar images can be seen in the salivary gland chromosomes of the complex complex of Drosophila. Similar deletion loops can also be seen in deletion-hybrid cells. The genetic effect of repetition is more moderate than that of deletion, but too much repetition can also affect the vitality of the individual and even cause the individual to die. Repeats in certain regions of the chromosome can produce specific phenotypic effects. For example, if the fly's dominant gene, Bar eye (B), is the result of the duplication. The main phenotypic effect is a reduction in the number of single eyes in the compound eye, which makes the compound eye stick-shaped instead of the usual oval shape. On this salivary gland chromosome of this fruit fly, you can see obvious horizontal stripes repeat on X chromosome (see position effect). However, for ordinary chromosomes, it is difficult to detect duplicates without using the banding method.

Inversion Two chromosomes have two breaks at the same time, the middle segment is twisted 180 °, reconnected, and the sequence of genes in this segment of cells with homologous chromosomes is reversed. The inversion that includes the centromere inversion is called interarm inversion; the inversion that does not include the centromere is called intra-arm inversion. Inter-arm inversions with unequal distances between the two breakpoints and the centromere are easy to identify, and equidistant inversions are generally not easily detectable unless the banding technique is applied. The inversion heterozygote also has a characteristic meiotic image, and an inversion loop appears when its inverse chromosome meets with a normal homologous chromosome. . Intra-arm inverse heterozygosity, if there is an exchange in the inversion loop, a chromatid with two centromeres and a fragment without centromere will be formed, so chromosomal bridges and no The centromere fragments, the latter often cannot enter the daughter cell nucleus; and after the two centromere bridges are broken, although the two chromosomes can enter the daughter cells separately, they are often lost due to the different break locations. Causes gamete death.

Structural aberrations in which a segment of one chromosomal arm is transferred to the arm of another non-homologous chromosome. The exchange of chromosomal fragments between two non-homologous chromosomes is called mutual translocation. Mutually translocated chromosome fragments can be of equal length or unequal length. Generally, when a gene changes its position on a chromosome, it does not change its function. However, it has been found in organisms such as Drosophila that if an autosomal gene is located near heterochromatin through translocation, its function will be affected. The effect of variegated position appears. Inversion may also bring 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 cell generation. However, the translocation heterozygote is different. Due to the pairing of the homologous parts of the normal chromosome and the translocation chromosome, a unique cross-shaped image can be seen in the middle of meiosis. With the progress of the splitting process, the cross shape gradually opens, and the two adjacent centromeres tend to the same pole or to the poles, forming a ring-shaped or figure-eight image. The former method is called proximity leave, and the latter method is called interactive leave. Reciprocal heterozygous pollen mother cells

Chromosome aberration

About 50% of the images are in a ring shape, which belongs to the adjacent departure, and 50% is a figure 8, which is an interactive departure. This shows that the orientation of the four centromeres toward the poles is random and their actions are independent. As a result of the proximity leaving, the gamete contains duplicate or missing chromosomes, forming a lethal imbalance gamete. Interleaved leads to non-lethal balanced gametes, in which half of the gametes have normal chromosomes and half of the gametes have balanced translocation chromosomes. This means that although translocations occur, translocations do not result in the addition or absence of genes. Interleaved leaves two translocated chromosomes into one gamete cell and two non-translocated chromosomes into the other gamete cell. Therefore, this separation method restricts the free combination of genes on non-homologous chromosomes, and causes genes that are originally on different chromosomes to appear linked. This phenomenon is called false linkage (see Gene Mapping).

When two or more chromosomes are translocated with each other, if the proximal ends of these chromosomes meet, dicentrics or polycentrics are formed. Bicentrics have two functional centromeres at the same time, and they each tend to the poles in the later stages of cell division, resulting in late bridges. If this bridge is broken, it often results in cell death. However, according to the study of corn endosperm cells, although the chromosomal bridge can be pulled off, the two centromeres at both ends can still smoothly enter the cell's poles and participate in the formation of daughter cells, and their broken ends are still open and accessible. Reconnect. When connected again, another bridge will form in the later stage of the next division. This "break-fusion-bridge" cycle can last for many cell generations.

Whole arm translocation is a translocation between the entire arm (or almost the entire arm). The result of this translocation can produce two new chromosomes with different structures. In the whole arm translocation, there is another special situation, that is, two homologous (or non-homologous) proximal centromeric chromosomes fuse with each other to become a central (or sub-central) centromere Chromosomes, which results in a reduction in the number of chromosomes, but the number of arms does not change. This whole-arm translocation is called the Robertsonian translocation and was discovered by WRB Robertson in 1916. For example, in the genus Mouse, the most common karyotype is 40 proximal centromeres, but in some wild rat cells, several double-arm chromosomes appear. These two-arm chromosomes are formed by centromere fusion. Centromeric fusion is generally considered to be the most common form of mammalian karyotype evolution.

Ring chromosome (ring): If the long and short arms of the chromosome are broken at the same time, the long and short arms containing the centromere segment are connected at the ends to form a circular chromosome. This anomaly is genetically unstable because its chromosomal loop must be opened once as the chromosome replicates.

Isochromosome (isochromosome): When the centromere of the chromosome does not divide vertically, but lateral division occurs, one progeny cell receives two long arms and the other receives two short tubes. Isoarm chromosomes are the most common structural abnormalities of chromosomes.

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

In addition to the above-mentioned structural distortions, other morphological changes of chromosomes can be seen under the light microscope. For example, chromosome adhesion, the number of chromosomes that are stuck together can be two or more. Many physical and chemical factors can induce chromosome adhesion during mitosis and meiosis, and certain mutant genes can also promote chromosome adhesion. In addition, the violent effects of environmental factors can also have various effects: chromosome shredding (high frequency breaks), singulation (metaphase chromosomes exist as monomers), and unsynchronization (the formation of chromosomes in the same cell has different speeds) , Unwinding (metaphase chromosomes are untwisted and loosened) and so on.

Chromosome aberrations cause diseases

(21) Congenital chromosomal aberration (trisomy 21)

The child has a special craniofacial deformity, with a small and round skull, flat occipital bone, small eyes with too wide eye distance, high lateral and low medial eyes, round face, flat nose, half-open mouth, tongue often sticking out of the mouth, tongue There are cracks, low ears, often through hands, stunted development, mental retardation, and short life expectancy. About one-third of patients have died by the age of 10. The cause is due to the addition of a small chromosome 21. Most patients have congenital heart disease at the same time.

Patau(13) Chromosome aberrations Patau syndrome (trisomy 13)

With an additional chromosome 13, marked 47, XX or XY, 13, the incidence is about 1/5000. There were significantly more women than men in the patients, and the deformities and other clinical features of the children were more severe than that of trisomy 21. The small head, rabbit lips and / or cleft palate, congenital heart disease, and severe mental retardation. 90% died within 6 months of birth.

Edwards Chromosomal Aberration Edwards Syndrome

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

Meow Syndrome

The deletion of chromosome 5 caused the patient to cry like a meow in early childhood. It is called feline syndrome.

Two hypotheses of chromosome aberrations

- Chromosome aberration break-rejoin hypothesis

Proposed by LJ Stadler in 1931. The other is the swap hypothesis, proposed by SH Revell in 1959. The break-rejoin hypothesis considers that the primary damage that causes a change in chromosomal structure is a break. This break can occur spontaneously, or it can be the result of mutagenic factors. The consequences of fracture are nothing more than the following three: The vast majority of fractures (90-99%) are reconnected (healed) in situ through the repair process so that they cannot be identified cytologically. Reconnections at different breaks are called reconnections. Reconnections cause structural changes in chromosomes, so most of them can be found. The broken end is still free, and it becomes a stable state of the chromosome structure, such as the deletion of the terminal part.

Chromosome aberration swap hypothesis

It is believed that the root cause of structural aberrations of chromosomes is that there are unstable parts on the chromosome. All structural aberrations are the result of swapping between two unstable parts that are close together. The occurrence of interchange can be divided into two stages: the first stage is a more stable state secondary to the instability after the fracture, which is called the initiation of interchange. The second stage is the mechanical interchange and connection process. If two primary injuries cannot interact, these injuries can be repaired.

Chromosome aberration applications

Changes in the structure or number of chromosomes can occur spontaneously or induced. Chromosome aberrations generally refer to a large-scale structural change of chromosomes that can be identified under a light microscope. Although the chromosome structure changes in prokaryotes cannot be distinguished under the light microscope, aberration heterozygous images similar to eukaryotes can be seen under the electron microscope.

The study of chromosomal aberrations can be used to reveal the rules and mechanisms of chromosome structural changes; it can be used to draw cytological maps; it can be used to explore the mechanism of species formation; it can be used to obtain chimeras in behavioral genetic research; it can be used to detect the environment It can be used as a basis for the diagnosis and prevention of human chromosomal diseases; and it can be used to cultivate excellent animal and plant varieties.

Many chemicals can induce chromosomal aberrations, but the DNA damage caused by these chemicals only manifests itself after passing through the DNA replication stage. If exposed to chemicals at other stages of the cell cycle, the damage caused can no longer be detected because chromosomal aberrations are repaired before they enter the cell division. Compared with chromosomal aberration analysis methods, the measurement of sister chromatid exchange (SCE) frequency is more accurate and has been widely used to monitor mutagenic and carcinogenic agents in the environment (see Toxicology).

The study of chromosomal aberrations has also been applied to the diagnosis of human chromosome diseases (including prenatal diagnosis) and to explore the causes of chromosome diseases. For example, as mothers age, the birth rate of children with trisomy 21 increases. It can be seen that the incidence of chromosome not leaving is significantly increased during the formation of germ cells in older women, which is related to the decline of ovarian function.

Chromosome aberrations are also widely used in the cultivation of new animal and plant varieties. An example of the use of autopolyploids is seedless watermelon (see Chromosome ploidy). In terms of heteropolyploids, Chinese geneticist Bao Wenkui and others have developed triticale. The use of haploid breeding methods can also produce excellent crop varieties. For example, China has cultivated tobacco and wheat varieties using anther culture combined with chromosome doubling technology. In addition, the use of chromosomal structural variations can also breed new breeds. For example, the sex chromosome of male silkworm is ZZ, and the sex chromosome of female silkworm is ZW. By inducing chromosomal translocation, the fragment with the egg color gene on the autosome is translocated to the W chromosome. Such ZW eggs can be distinguished from ZZ eggs without this egg color gene, which can easily distinguish the male and female sex of eggs, and eliminate female eggs early, thereby improving silk yield and quality.

The study of chromosomal aberrations has also been applied to gene mapping. For example, 21 different deficient bodies can be obtained from 21 different monomers of wheat. If a new recessive mutant gene is found in wheat, this mutant can be crossed with 21 kinds of lacking bodies, and the chromosome to which this gene belongs can be judged based on the trait ratio of the hybrid offspring. Gene position determination can also be performed using strains with various chromosomal arm deletions (see Gene Mapping).

The mechanism of chromosomal aberrations is still unknown. Although the change in numbers can be attributed to the non-segregation of chromosomes, it is not well understood which factors cause non-segregation and why it does not occur. The two hypotheses that explain changes in chromosome structure also fail to explain some facts. For example, according to the swap hypothesis, even simple terminal deletions should also be the products of swaps. However, the recent application of SCE technology reveals that terminal deletions also occur in the absence of SCE. Another reason is that chromosome breakage and deletion always occur in specific parts of certain chromosomes, etc., which need to be further studied.

Chromosome aberration effect

Chromosome aberrations and gene mutations can occur naturally or by induction. Common triggering factors are: radiation (such as r-rays, ultraviolet rays, etc.), viruses (such as rubella virus, cytomegalovirus, hepatitis virus, HIV, etc.) and chemicals (such as certain pesticides, antibiotics, food additives and lead) , Mercury, benzene, cadmium, etc.). In addition, the advanced age of pregnant women is also one of the reasons for the formation of trisomy 21 and other trisomy chromosomal abnormalities.

Chromosome aberration syndrome

A disease caused by an abnormal number or structure of chromosomes is called a chromosome disease, which usually displays a series of clinical symptoms that involve abnormalities in the morphological structure and function of many organ systems.

According to the different aberrant chromosomes, chromosome syndromes can be divided into two major categories: autosomal syndromes (1-222 pairs of autosomal abnormalities) and sex chromosome syndromes (sex chromosomal abnormalities X and Y). They all include diseases caused by chromosome number or structural aberrations.

Chromosomes are carriers of nuclear genes. There are about 50,000 to 60,000 structural genes on human haploid chromosomes (ie, 22 autosomes and X and Y sex chromosomes). There are at least thousands of genes on each chromosome, so Chromosome aberrations usually involve the deletion or increase of more genes. And because of the pleiotropic nature of genes, it can cause abnormalities in various traits of the body, and has a variety of clinical manifestations, namely, syndromes. The general clinical manifestations of autosomal syndrome are mental retardation, retarded growth and development, and multiple congenital malformations. The common clinical features of sex chromosome syndrome are sexual dysgenesis or hermaphroditism, and some patients only show reduced fertility, secondary amenorrhea, and poor intelligence. However, not all chromosomal abnormalities will have obvious clinical symptoms or phenotypic abnormalities. For example, the phenotype of normal translocation or inversion carriers is normal, but just after marriage may cause miscarriage, stillbirth, neonatal death or congenital malformation. Children or children with mental retardation. ^[2]

Autosomal dominant genetic disease (AD)

Autosomal dominant genetic disease refers to a disease caused by a dominant pathogenic gene located on an autosome. Human somatic cells have 22 pairs of autosomes and 1 pair of sex chromosomes. There are alleles at the same loci of the paired autosomes, and they are divided into dominant (A) and recessive (a). Because the dominant trait A gene is a disease-causing gene, as long as individuals with the A gene are patients, including those with two genotypes of AA and Aa, individuals with genotype aa are normal. The genealogy of autosomal dominant genetic diseases has the following characteristics:

1. One of the patient's parents is often the patient. Because the disease-causing gene came from the parents. The parents are not diseased, and the children are generally not affected unless a mutation occurs.

2. Most of the patients in the genealogy are heterozygous (Aa), so about 1/2 of the patient's children are patients. Sometimes this ratio is not reflected in small families, but if you analyze the population of many small families, you will see an approximate ratio.

3 Because it is a dominant gene that determines the onset of disease, whoever it is passed on to will have the disease. Can be found in successive generations of patients, that is, successive generations of heredity.

4 Because the disease-causing gene is on the autosome and has nothing to do with gender, men and women have equal chances of developing the disease.

Most dominant inheritance is completely dominant, that is, heterozygotes (Aa) exhibit the same phenotype as dominant homozygotes (AA). For example, both genotypes BB and Bb are short fingers, and there is no difference in clinical manifestations. In a few cases, dominant inheritance also has some special manifestations: incomplete dominant or semi-dominant; irregular dominant; co-dominant inheritance; delayed dominant; inherited inheritance.

Autosomal recessive genetic disease (AR)

Autosomal recessive disease refers to a recessive pathogenic gene located on an autosome that occurs only when homozygous (aa), and in the state of heterozygosity (Aa), it can be masked by the role of the normal dominant gene A The role of disease gene a, so Aa does not develop. Such individuals who carry a disease-causing gene but do not develop disease are called carriers of the disease-causing gene. Carrier Aa can pass the a gene to the next generation. If a couple is aa and the other is a normal person AA, then their offspring are carriers Aa; when the couple are both carriers Aa, about 1/4 of their offspring are patients aa, 1 / 2 is the carrier Aa, and 1/4 is the normal person AA, showing a ratio of 1: 2: 1.

Summarizing the pedigree of autosomal recessive inheritance, it has the following characteristics:

1. The patient's parents are often disease-free, but they are both carriers of the recessive pathogenic gene (Aa).

2. About 1/4 of the siblings of the patients develop the disease, but it is rare to see 1/4 in the small families. If several parents with Aa are combined to analyze, the probability of the offspring patients aa is close to 1 / 4. Because the disease-causing gene is on the autosome, men and women have equal chances of developing the disease.

3. There is no continuous generation inheritance in pedigree, which is manifested as sporadic inheritance. It is likely that patients will only appear in one generation.

4. The risk of recessive genetic disease in the offspring of intimate marriages is much higher than that of non-intimate marriages.