What Is It Called When a Baby Is Born With an Extra X Chromozone?
two.5: Karyotypes Draw Chromosome Number and Structure
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Karyograms are images of real chromosomes
Each eukaryotic species has its nuclear genome divided among a number of chromosomes that is feature of that species. For example, a haploid human nucleus (i.e. sperm or egg) normally has 23 chromosomes (northward=23), and a diploid homo nucleus has 23 pairs of chromosomes (2n=46). A karyotype is the complete set of chromosomes of an individual. The prison cell was in metaphase and then each of the 46 structures is a replicated chromosome even though information technology is hard to see the two sister chromatids for each chromosome at this resolution. As expected there are 46 chromosomes. Note that the chromosomes have different lengths. In fact, human chromosomes were named based upon this characteristic. Our largest chromosome is called one, our next longest is ii, and so on. By convention the chromosomes are arranged into the pattern shown in Figure \(\PageIndex{15}\) and the resulting epitome is chosen a karyogram. A karyogram allows a geneticist to determine a person's karyotype - a written description of their chromosomes including anything out of the ordinary.
Figure \(\PageIndex{15}\): Karyogram of a normal human male karytype.(Wikipedia-NHGRI-PD)
Various stains and fluorescent dyes are used to produce characteristic banding patterns to distinguish all 23 chromosomes. The number of chromosomes varies betwixt species, but at that place appears to exist very little correlation betwixt chromosome number and either the complication of an organism or its total corporeality genomic Dna.
Autosomes and Sex Chromosomes
In the figure above note that most of the chromosomes are paired (same length, centromere location, and banding pattern). These chromosomes are chosen autosomes. However notation that two of the chromosomes, the 10 and the Y do not await alike. These are sexual practice chromosomes. In humans males have ane of each while females have 2 10 chromosomes. Autosomes are those chromosomes present in the same number in males and females while sex chromosomes are those that are not. When sex activity chromosomes were start discovered their function was unknown and the proper name X was used to indicate this mystery. The next ones were named Y, then Z, and so West.
The combination of sex activity chromosomes within a species is associated with either male person or female individuals. In mammals, fruit flies, and some flowering plants embryos, those with two X chromosomes develop into females while those with an X and a Y become males. In birds, moths, and butterflies males are ZZ and females are ZW. Because sex chromosomes take arisen multiple times during evolution the molecular mechanism(s) through which they make up one's mind sex differs amongst those organisms. For case, although humans and Drosophila both have X and Y sex activity chromosomes, they take different mechanisms for determining sexual activity .
In mammals, the sex activity chromosomes evolved merely after the divergence of the monotreme lineage from the lineage that led to placental and marsupial mammals. Thus near every mammal species uses the aforementioned sex activity determination organisation. During embryogenesis the gonads volition develop into either ovaries or testes. A gene present only on the Y chromosome called TDF encodes a poly peptide that makes the gonads mature into testes. XX embryos practice non accept this gene and their gonads mature into ovaries instead (default). Once formed the testes produce sexual activity hormones that direct the rest of the developing embryo to go male person, while the ovaries make different sex hormones that promote female development. The testes and ovaries are as well the organs where gametes (sperm or eggs) are produced.
How practise the sexual activity chromosome behave during meiosis? Well, in those individuals with 2 of the aforementioned chromosome (i.e. homogametic sexes: XX females and ZZ males) the chromosomes pair and segregate during meiosis I the same as autosomes do. During meiosis in XY males or ZW females (heterogametic sexes) the sex activity chromosomes pair with each other (Figure \(\PageIndex{sixteen}\)). In mammals the consequence of this is that all egg cells volition carry an 10 chromosome while the sperm cells will carry either an 10 or a Y chromosome. Half of the offspring will receive two X chromosomes and get female while half will receive an X and a Y and become male.
Effigy \(\PageIndex{16}\): Meiosis in an XY mammal. The stages shown are anaphase I, anaphase II, and mature sperm. Annotation how half of the sperm contain Y chromosomes and half contain X chromosomes. (Original-Harrington-CC:AN)
Aneuploidy - Changes in Chromosome Number
Analysis of karyotypes tin can identify chromosomal abnormalities, including aneuploidy, which is the improver or subtraction of a chromosome from a pair of homologs. More specifically, the absenteeism of one member of a pair of homologous chromosomes is called monosomy (but one remains). On the other hand, in a trisomy, in that location are three, rather than 2 (disomy), homologs of a particular chromosome. Different types of aneuploidy are sometimes represented symbolically; if 2n symbolizes the normal number of chromosomes in a cell, and then 2n-1 indicates monosomy and 2n+1 represents trisomy. The addition or loss of a whole chromosome is a mutation, a modify in the genotype of a cell or organism.
The near familiar homo aneuploidy is trisomy-21 (i.e. 3 copies of chromosome 21), which is one cause of Down syndrome. About (only not all) other human aneuploidies are lethal at an early on phase of embryonic development. Note that aneuploidy usually affects only one ready of homologs within a karyotype, and is therefore distinct from polyploidy, in which the unabridged chromosome set is duplicated (run into beneath). Aneuploidy is almost ever deleterious, whereas polyploidy appears to be beneficial in some organisms, especially many species of food plants.
Aneuploidy can arise due to a non-disjunction result, which is the failure of at least one pair of chromosomes or chromatids to segregate during mitosis or meiosis. Non-disjunction will generate gametes with extra and missing chromosomes.
Chromosomal abnormalities
Structural defects in chromosomes are another type of aberration that can exist detected in karyotypes (Fig two.17). These defects include deletions, duplications, and inversions, which all involve changes in a segment of a unmarried chromosome. Insertions and translocations involve two non-homologous chromosomes. In an insertion, Dna from ane chromosome is moved to a not-homologous chromosome in a unidirectional manner. In a translocation, the transfer of chromosomal segments is bidirectional and reciprocal – a reciprocal translocation.
Effigy \(\PageIndex{17}\): Structural abberations in chromosomes.(Wikipedia-Zephyris-GFDL)
Structural defects affect only part of a chromosome (a subset of genes), and so tend to be less harmful than aneuploidy. In fact, in that location are many examples of ancient chromosomal rearrangements in the genomes of species including our own. Duplications of some pocket-sized chromosomal segments, in item, may accept some evolutionary advantage past providing extra copies of some genes, which can then evolve in new, potentially benign, ways.
Chromosomal abnormalities arise in many different ways, some of which can exist traced to rare errors in natural cellular processes such every bit DNA replication. Chromosome breakage as well occurs infrequently as the event of concrete damage (such as ionizing radiation), move of some types of transposons, and other factors. During the repair of a broken chromosome, deletions, insertions, translocations and even inversions can be introduced.
Source: https://bio.libretexts.org/Bookshelves/Genetics/Book%3A_Online_Open_Genetics_(Nickle_and_Barrette-Ng)/02%3A_Chromosomes_Mitosis_and_Meiosis/2.05%3A_Karyotypes_Describe_Chromosome_Number_and_Structure
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