7.3 Errors in Meiosis

Two nuclear divisions are included in meiosis, which is preceded by one round of DNA replication.

The four daughter cells are haploid.
The daughter cells are the same as the parent cells.

When chromosomes behave differently during meiosis, there can be inherited disorders.

There are two types of chromosome disorders: abnormality in chromosome number and structural rearrangements.
Small segments of chromosomes can span many genes, which is why chromosomal disorders are often fatal.

The isolation and observation of chromosomes is the primary method by which clinicians detect chromosomal abnormality in humans.

The chromosomes of a female human immune cell are shown in the karyogram.

The karyotype can be used to identify chromosomal abnormality from a single cell.
A person's cells are first collected from a blood sample or other tissue to observe their karyotype.
In the laboratory, the cells are stimulated to divide.
A chemical is applied to the cells.
The cells are put in a slide.

The geneticist stains the chromosomes with one of several dyes to better visualize the banding patterns of each pair.

The chromosomes are viewed using a bright-field microscope.
Each band can be identified by an experienced cytogeneticist.
Size and centromere location are used to identify the chromosomes.
The classic depiction of the karyotype, in which pairs of chromosomes are aligned in numerical order from longest to shortest, can be obtained if the geneticist gets a digital image, identifies each chromosomes, and manually arranges the chromosomes into this pattern.

The karyogram can reveal genetic abnormality in which an individual has too many or too few chromosomes.

Down syndrome, which is identified by a third copy of chromosome 21, and Turner syndrome, which is characterized by the presence of only one X chromosomes in women, are examples of this.
Large deletions can be identified by geneticists.
Jacobsen syndrome, which involves distinctive facial features as well as heart and bleeding defects, is identified by a deletion on chromosome 11.
There are certain cancers that are implicated in translocations.

By observing a karyogram, geneticists can see the chromosomal composition of an individual to confirm or predict genetic abnormality in offspring.

A karyogram shows the most obvious abnormality in the chromosomes.

Alterations in the number of complete sets of chromosomes and the loss of entire chromosomes are disorders of chromosome number.

Two gametes with two copies of the This OpenStax book are available for free.

If sister chromatids fail to separate, the result is one gamete that lacks that chromosome, two normal gametes with one copy of the chromosomes, and one gamete with two copies of the chromosomes.

Each gamete has a single copy of each of the chromosomes.

When chromosomes fail to separate during meiosis, there is a nondisjunction.

Humans with only one copy of essential genes fail to develop to birth because they have only one copy of an autosome.

Children that are duplicated of some of the smaller chromosomes can live for several weeks to many years, even if they fail to develop to birth.
An excess in gene dose is a type of genetic imbalance that trisomic individuals suffer from.
Adding a third copy of the same gene causes the cell functions to be adjusted to the amount of product produced by two copies.
Down syndrome is caused by a trisomy 21.
Individuals with this disorder have physical and cognitive delays.

Maternal age increases the incidence of having a fetus with trisomy 21.

The addition of a chromosome leads to Down syndrome in Humans.

Human females and males can function normally despite carrying different numbers of the X chromosomes.
When a few thousand cells are present in a female mammal embryo, one X chromosomes in each cell inactivates, forming a Barr body.
The genes on the inactive X chromosome are not expressed.
All cells descended from a cell that is inactivated will have the same inactive X chromosomes.
Females compensate for their double genetic dose by this process.

The coat-color variegation is observed in so-called "tortoiseshell" cats.

Females who have an X-linked coat color genes will have two different coat colors over different parts of their body.
If you see a tortoiseshell cat, you'll know it's a female.

The tortoiseshell phenotype in cats is caused by the inactivation of one of two X chromosomes.

In an individual with an abnormal number of X chromosomes, cellular mechanisms will inactivate all but one X in each of her cells.

Mild mental and physical defects are typically associated with X-chromosomal abnormality.
The individual will not develop if the X chromosome is missing.

There are several errors in the sex number.

Individuals with three X chromosomes, called triplo-X, appear female but have delays in development and reduced fertility.
One type of Klinefelter syndrome corresponds to male individuals with small testes, enlarged breasts, and reduced body hair.
The extra X chromosomes are inactivated to make up for the extra genetic dosage.
Turner syndrome corresponds to a female with short stature, webbed skin in the neck region, hearing and cardiac impairments, and sterility.

An abnormal diploid egg with a normal haploid sperm would yield a triploid zygote.

Polyploid animals are rare, with only a few examples among the flatworms, crustaceans, and fish.

Triploid animals are sterile because of meiosis.

Polyploidy is very common in the plant kingdom, and polyploid plants tend to be larger and more robust than their euploid counterparts.

Structural rearrangements in the chromosomes have been characterized.

Duplications and deletions can cause offspring to have physical and mental problems.
The French word for "cry of the cat" is cri-du-chat, a syndrome associated with nervous system abnormality and identifiable physical features that results from a deletion of most of the small arm of chromosomes.
The disorder's name is based on the high-pitched cry that the infants emit.

The individual with cri-du-chat syndrome is shown at different ages.

During meiosis, cells can be observed because chromosomes with a rearrangement in one of the pair must contort to maintain appropriate gene alignment and pair effectively.

Inversions only change the orientation of genes and are likely to have more mild effects than aneuploid errors.

Not all structural rearrangements of chromosomes produce infertile individuals.

Sometimes a change can lead to the evolution of a new species.

The evolution of humans is thought to have been caused by an insturment in chromosome 18.
Chimpanzees are our closest genetic relatives.

The divergence from a common ancestor with Chimpanzees is thought to have caused the chromosome 18 inversion in early humans.

It has been suggested that a long stretch of DNA was duplicated on the same part of the human's body as an ancestors, but that it was inverted into a different part of the body.

They are on different parts of the human and chimpanzee chromosomes.

It's possible that one of the inversion breakpoints occurred between the two genes.
It is possible that the chromosome 18 inversion in an ancestral human reset the expression levels of specific genes.

A translocation occurs when a segment of a chromosome splits into two.

Depending on how the positions of genes are altered, translocations can be benign or devastating.
Specific translocations have been associated with a number of diseases.
An (a) inversion occurs when a segment breaks from the chromosomes, reverses its orientation, and then reattaches in the original position.
There is no genetic information lost or duplicated when there is a (b) reciprocal translocation between two nonhomologous chromosomes.

Eukaryotes undergo sexual reproduction.

The variation introduced into the reproductive cells by meiosis appears to be one of the advantages of sexual reproduction that has made it so successful.
Gametes are genetically unique reproductive cells that have half the number of chromosomes as the parent cell.
The diploid condition is restored by the fusion of haploid gametes.
Sexually reproducing organisms alternate between haploid and diploid stages.
There are different ways in which reproductive cells are produced.
There are three main types of life cycles: diploid-dominant, haploid-dominant, and alternation of generations.

Sexual reproduction requires diploid organisms to produce haploid cells that can form diploid offspring.

Meiosis results in haploid cells.
The events that arrange and separate chromosomes into daughter cells are called meiosis.
Each chromosome is duplicated during the interphase of meiosis.
Two rounds of nuclear division result in four nuclei and four haploid daughter cells, each with half the number of chromosomes as the parent cell.
Variations in the daughter nuclei are introduced because of random alignment at metaphase I.
The cells produced by meiosis are unique.

There are similarities between meiosis and mitosis.

There are single nuclear divisions that produce daughter nuclei that are genetically identical and have the same number of chromosomes as the original cell.

Two nuclear divisions produce four daughter nuclei that are genetically different and have different chromosomes than the parent cell.

The first division of meiosis has the main differences between the processes.
A reduction of ploidy level is caused by the separation of the homologous chromosomes.
The second division of meiosis is very similar to the first division.

The number, size, shape, and banding pattern of chromosomes make them easily identifiable in a karyogram and allow for the assessment of many chromosomal abnormality.

Aneuploidies are usually lethal to the embryo, although a few trisomic genotypes are viable.
Sex chromosomes have milder effects because of X inactivation.
Aneuploidies include instances in which segments of a chromosomes are duplicated or deleted.
Inversion or translocation can be used to rearrange chromosome structures.
There can be negative effects on development or death.
Inversions and translocations are associated with reduced fertility because of the likelihood of nondisjunction.

Meiosis creates daughter cells.

The part of meiosis that is similar is inactivation.