How does nondisjunction work

Nondisjunction occurring during meiosis II results in 50 percent normal gametes. The Effects of Nondisjunction In the sex chromosomes of a cell, nondisjunction can cause some lesser-known diseases, such as Klinefelter syndrome a male with 2X and 1 Y , Turner syndrome a female with only one X chromosome , or Trisomy X female with 3 X chromosomes instead of 2.

Nondisjunction Produces Abnormal Gametes. What happens if nondisjunction occurs during meiosis? Nondisjunction Produces Abnormal Gametes If nondisjunction occurs during anaphase I of meiosis I, this means that at least one pair of homologous chromosomes did not separate. The end result is two cells that have an extra copy of one chromosome and two cells that are missing that chromosome.

What happens if nondisjunction occurs during mitosis? Nondisjunction in meiosis can result in pregnancy loss or birth of a child with an extra chromosome in all cells, whereas nondisjunction in mitosis will result in mosaicism with two or more cell lines. Aneuploidy may also result from anaphase lag. What causes nondisjunction to occur? They are caused by nondisjunction, which occurs when pairs of homologous chromosomes or sister chromatids fail to separate during meiosis. Nondisjunction occurs when homologous chromosomes meiosis I or sister chromatids meiosis II fail to separate during meiosis.

How does Klinefelter syndrome affect a person? Klinefelter syndrome is a genetic condition that results when a boy is born with an extra copy of the X chromosome. Klinefelter syndrome may adversely affect testicular growth, resulting in smaller than normal testicles, which can lead to lower production of testosterone.

Why does Nondisjunction increase with age? By observing a karyogram, geneticists can actually visualize the chromosomal composition of an individual to confirm or predict genetic abnormalities in offspring even before birth. Of all the chromosomal disorders, abnormalities in chromosome number are the most easily identifiable from a karyogram.

Disorders of chromosome number include the duplication or loss of entire chromosomes, as well as changes in the number of complete sets of chromosomes. They are caused by nondisjunction , which occurs when pairs of homologous chromosomes or sister chromatids fail to separate during meiosis.

The risk of nondisjunction increases with the age of the parents. Nondisjunction can occur during either meiosis I or II, with different results Figure 7. If homologous chromosomes fail to separate during meiosis I, the result is two gametes that lack that chromosome and two gametes with two copies of the chromosome. If sister chromatids fail to separate during meiosis II, the result is one gamete that lacks that chromosome, two normal gametes with one copy of the chromosome, and one gamete with two copies of the chromosome.

An individual with the appropriate number of chromosomes for their species is called euploid; in humans, euploidy corresponds to 22 pairs of autosomes and one pair of sex chromosomes. An individual with an error in chromosome number is described as aneuploid, a term that includes monosomy loss of one chromosome or trisomy gain of an extraneous chromosome.

Monosomic human zygotes missing any one copy of an autosome invariably fail to develop to birth because they have only one copy of essential genes. Most autosomal trisomies also fail to develop to birth; however, duplications of some of the smaller chromosomes 13, 15, 18, 21, or 22 can result in offspring that survive for several weeks to many years.

Trisomic individuals suffer from a different type of genetic imbalance: an excess in gene dose. Cell functions are calibrated to the amount of gene product produced by two copies doses of each gene; adding a third copy dose disrupts this balance.

The most common trisomy is that of chromosome 21, which leads to Down syndrome. Individuals with this inherited disorder have characteristic physical features and developmental delays in growth and cognition.

The incidence of Down syndrome is correlated with maternal age, such that older women are more likely to give birth to children with Down syndrome Figure 7. Concept in Action Visualize the addition of a chromosome that leads to Down syndrome in this video simulation. Humans display dramatic deleterious effects with autosomal trisomies and monosomies. Therefore, it may seem counterintuitive that human females and males can function normally, despite carrying different numbers of the X chromosome.

In part, this occurs because of a process called X inactivation. Early in development, when female mammalian embryos consist of just a few thousand cells, one X chromosome in each cell inactivates by condensing into a structure called a Barr body. The genes on the inactive X chromosome are not expressed. The particular X chromosome maternally or paternally derived that is inactivated in each cell is random, but once the inactivation occurs, all cells descended from that cell will have the same inactive X chromosome.

By this process, females compensate for their double genetic dose of X chromosome. Females heterozygous for an X-linked coat color gene will express one of two different coat colors over different regions of their body, corresponding to whichever X chromosome is inactivated in the embryonic cell progenitor of that region.

When you see a tortoiseshell cat, you will know that it has to be a female. In an individual carrying an abnormal number of X chromosomes, cellular mechanisms will inactivate all but one X in each of her cells. As a result, X-chromosomal abnormalities are typically associated with mild mental and physical defects, as well as sterility.

If the X chromosome is absent altogether, the individual will not develop. Several errors in sex chromosome number have been characterized. Most autosomal trisomies also fail to develop to birth; however, duplications of some of the smaller chromosomes 13, 15, 18, 21, or 22 can result in offspring that survive for several weeks to many years. Trisomic individuals suffer from a different type of genetic imbalance: an excess in gene dose. Individuals with an extra chromosome may synthesize an abundance of the gene products encoded by that chromosome.

This extra dose percent of specific genes can lead to a number of functional challenges and often precludes development. The most common trisomy among viable births is that of chromosome 21, which corresponds to Down Syndrome. Individuals with this inherited disorder are characterized by short stature and stunted digits, facial distinctions that include a broad skull and large tongue, and significant developmental delays.

The incidence of Down syndrome is correlated with maternal age; older women are more likely to become pregnant with fetuses carrying the trisomy 21 genotype Figure 2. Figure 2. The incidence of having a fetus with trisomy 21 increases dramatically with maternal age. Visualize the addition of a chromosome that leads to Down syndrome in this video simulation. Figure 3. As with many polyploid plants, this triploid orange daylily Hemerocallis fulva is particularly large and robust, and grows flowers with triple the number of petals of its diploid counterparts.

An individual with more than the correct number of chromosome sets two for diploid species is called polyploid. For instance, fertilization of an abnormal diploid egg with a normal haploid sperm would yield a triploid zygote. Polyploid animals are extremely rare, with only a few examples among the flatworms, crustaceans, amphibians, fish, and lizards. Polyploid animals are sterile because meiosis cannot proceed normally and instead produces mostly aneuploid daughter cells that cannot yield viable zygotes.

Rarely, polyploid animals can reproduce asexually by haplodiploidy, in which an unfertilized egg divides mitotically to produce offspring. In contrast, polyploidy is very common in the plant kingdom, and polyploid plants tend to be larger and more robust than euploids of their species Figure 3. Humans display dramatic deleterious effects with autosomal trisomies and monosomies. Therefore, it may seem counterintuitive that human females and males can function normally, despite carrying different numbers of the X chromosome.

Rather than a gain or loss of autosomes, variations in the number of sex chromosomes are associated with relatively mild effects. In part, this occurs because of a molecular process called X inactivation.