Aneuploidy is a change in the number of individual chromosomes.
ABCGHI
At least 50%.
3n = 21
Because the unbalanced gene dosage in the zygote is often lethal.
Polyploidy is an increase in the number of chromosome sets.
2n = 14
Tension is necessary for the two homologs to separate properly in anaphase I.
Missegregation of chromosomes in meiosis I during oogenesis.
A condition where both chromosomes of a homologous pair are inherited from the same parent.
Gynandromorphs are sexual mosaics where some cells express female traits (XX) and others express male traits (XO).
The presence of a third chromosome.
Exchange between chromosomes 21 and 14.
It is higher in humans, with aneuploidy found in no more than 2% of fertilized eggs in mice.
Aneuploidy is the loss or gain of one or more individual chromosomes.
It usually results in spontaneous abortion.
Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate during meiosis or mitosis.
Homologous chromosomes can pair or fail to pair in three ways.
A diploid (2n) early embryonic cell undergoes replication and separation of sister chromatids without cell division, producing an autotetraploid (4n) cell.
The genetic variant occurs at high frequencies in many human populations and may have been favored by natural selection, although the reason for this is unclear.
Because there is so little information in the Y chromosome.
A mutation that produces a globe-shaped seedcase and is dominant but primarily inherited from the female parent.
The gametes will have two copies of some chromosomes and one copy of others.
About 4%.
21 (3 × 7)
Prader-Willi syndrome is a condition that arises when a paternal copy of a gene on chromosome 15 is missing. Uniparental disomy can cause it when both copies of chromosome 15 are inherited from the mother and no copy is inherited from the father.
27 ((1 × 7) + (2 × 10))
Because it has the same number of chromosomes (six) as both diploid species.
The embryo becomes an XX/XO mosaic, developing a mixture of male and female traits.
Because they are sterile due to unbalanced gametes.
4n = 40
Aneuploidy can arise from the loss of a chromosome subsequent to translocation or from nondisjunction in meiosis or mitosis.
A genetic variant at the PLK4 gene increases the frequency of aneuploidy by disrupting centriole regulation, leading to failure of chromosome separation in mitosis.
45 chromosomes.
It shows a triploid cell (3n) with three almost similar sized X-shaped chromosomes that are green, blue, and red, and three different ways these chromosomes can segregate.
Part a shows the pairing of two of three homologous chromosomes in meiosis 1, where the cell has red and blue pairing together while the green is aligned at a distance.
It allows children affected with a recessive disorder to appear in families where only one parent is a carrier.
Jimson weed (Datura stramonium).
7
The presence of more than two sets of chromosomes.
Normal chromosomes 21 and 14.
Plants.
Approximately 40%.
About 50% of those diagnosed with Turner syndrome are actually mosaics, possessing some 45,X cells and some normal 46,XX cells.
The resulting zygote will have familial Down syndrome.
The resulting zygote will have monosomy 21 and will be spontaneously aborted.
A zygote with trisomy 14 (aborted) and a zygote with monosomy 14 (aborted).
No.
A monosomic (2n-1) zygote with only one blue chromatid.
Trisomy 21 (Down syndrome).
Because the hybrid chromosomes are not homologous and will not usually pair and segregate properly in meiosis.
2n = 20
The translocation of part of chromosome 21 to another chromosome.
An autotriploid can arise when nondisjunction in meiosis produces a diploid gamete that fuses with a normal haploid gamete, or from a cross between an autotetraploid and a diploid.
The gain of two homologous chromosomes, represented as 2n + 2.
The incidence increases with maternal age.
Nondisjunction leads to some gametes or cells containing an extra chromosome and others missing a chromosome.
Because they have two copies of the long arms of chromosomes 14 and 21, and the short arms of these chromosomes, which are lost, carry no essential genetic information.
A. Francis Blakeslee.
A secondary oocyte is produced and meiosis is suspended again.
Chance determines which gamete gets the extra chromosome, differing for each homologous trio of chromosomes.
Trisomy of chromosome 21.
An amphidiploid is an allopolyploid consisting of two combined diploid genomes, functionally diploid with each chromosome having one homologous partner.
The cell produces 2 diploid (2n) gametes with 1 red and 1 green chromatid each.
Triploids (3n), tetraploids (4n), pentaploids (5n), hexaploids (6n), and organisms with even higher numbers of chromosome sets.
One chromatid 21, one chromatid 14, and a hybrid chromatid 14-21.
Normal phenotype.
70% to 80%.
Turner syndrome mosaics typically have some 45,X cells and some normal 46,XX cells.
37 chromosomes.
In even-numbered autopolyploids, homologous chromosomes can theoretically form pairs and divide equally, but this rarely happens.
An autotetraploid is produced by nondisjunction of all chromosomes in mitosis in an early 2n embryo, doubling the chromosome number.
A person who has a translocation of chromosome 21 to another chromosome, such as 14, and is phenotypically normal but at increased risk for producing children with Down syndrome.
It results in improper segregation and can lead to aneuploidy.
Normal female phenotype with red eyes and wild-type wings.
Sometimes both chromosomes are inherited from the same parent, a condition termed uniparental disomy.
As trisomies, where a trisomic embryo can survive if one of the three chromosomes is lost early in development.
Uniparental disomy results.
Three homologs are present, and one may fail to align with the other two, leading to random segregation of chromosomes.
When a trisomic embryo loses one of the triplicate chromosomes early in development.
He demonstrated that the 12 mutants were trisomics.
An amphidiploid undergoes normal meiosis to produce balanced gametes with six chromosomes each.
The cell produces 2 haploid (1n) gametes with 1 blue chromatid each.
One cell has only the red chromosome and another cell has blue and green chromosomes.
The cell produces 2 haploid (1n) gametes with 1 red chromatid each.
Pair a: 14-21 chromatid and two chromatids 21 and 14; Pair b: 14-21 chromatid and 21 chromatid, and 14 chromatid; Pair c: 14-21 chromatid and 21 chromatid, and 21 chromatid.
A translocation carrier and a normal zygote.
A zygote with Down syndrome and a zygote with monosomy 21 (aborted).
GHI
Chromosomes must segregate to one gamete and one chromosome to the other.
Trisomy of chromosome 21.
The inefficiency of crossover maturation in females may account for the high levels of aneuploidy.
Through the development of molecular techniques that facilitate the identification of specific DNA sequences.
Because any X-linked recessive genes present in the cells with a single X chromosome (XO) will be expressed.
X-chromosome inactivation ensures that males and females receive the same functional dosage for X-linked genes.
Because additional X chromosomes are inactivated, reducing the impact of aneuploidy.
2n + 2 + 2.
Immediately before the nuclei of egg and sperm unite to form a zygote.
Different cells within the same individual organism have different chromosome constitutions.
If a child inherits two defective cystic fibrosis alleles from a heterozygous parent and no normal allele from the other parent, it can lead to cystic fibrosis.
Because it has a tendency to run in families.
20% to 30% of Prader-Willi syndrome cases arise from uniparental disomy.
Genetic mosaicism is a condition where regions of tissue have different chromosome constitutions due to nondisjunction in a mitotic division.
This segregation produces abnormal gametes; half will have two functional copies of chromosome 21 (one normal and one attached to chromosome 14) and the other half will lack chromosome 21.
More than 30%.
Autopolyploidy is caused by accidents of mitosis or meiosis that produce extra sets of chromosomes, all derived from a single species.
Aneuploidy can arise through the loss of a chromosome during mitosis or meiosis, loss of a small chromosome generated by a Robertsonian translocation, or through nondisjunction.
Sex is determined independently in each cell during development; XX cells express female traits, and XO cells express male traits.
No, translocation carriers do not have Down syndrome.
Aneuploidy disrupts gene dosage and often has severe phenotypic effects.
Aneuploidy usually produces serious developmental problems that result in spontaneous abortion, with only about 2% of fetuses with a chromosome mutation surviving to birth.
1 child in 2000 births.
Because the cells are XO, missing the wild-type X chromosome, allowing the white and miniature alleles to be expressed.
1 child in 100 births.
Aneuploidies involving the sex chromosomes.
12 different seed case mutants.
Turner syndrome and Klinefelter syndrome.
Because there is no mechanism of dosage compensation for autosomes.
Allopolyploidy can arise when nondisjunction during meiosis leads to a 2n gamete, which fuses with a 1n or 2n gamete from a different species to produce a polyploid individual.
Georgii Karpechenko.
Karpechenko wanted to produce a plant with cabbage leaves and radish roots so that no part of the plant would go to waste.
The cell produces 2 diploid (2n) gametes with 1 blue and 1 green chromatid each.
Wheat, oats, cotton, potatoes, and sugarcane.
No, it is less common in animals.
The cell produces 2 gametes with no chromosomes.
An allopolyploid arises from hybridization between two species followed by chromosome doubling.
They result in spontaneously aborted embryos.
Few live for more than a year after birth.
About 1 in 15,000 live births.
About half die within the first month of life, and 95% die by the age of three.
Most are mosaics, having some cells with three copies of chromosome 8 and other cells with the usual two copies.
The resulting zygote has different numbers of chromosomes, creating unbalanced gene dosage, which is often lethal.
Yes, it can also be caused by translocations between chromosome 21 and other chromosomes, such as 15.
XO means that the cell has a single X chromosome and no Y chromosome.
From hybridization between two species, resulting in a polyploid carrying chromosome sets from two or more species.
n (species I) + 2n (species II) = 7 + 20 = 27 or 2n (species I) + n (species II) = 14 + 10 = 24
Through nondisjunction in mitosis or meiosis.
Maternal nondisjunction.
In the diplotene substage of prophase I of meiosis.
Just before ovulation.
An individual organism has two copies of a chromosome from one parent and no copy from the other.
1 child in 12 births.
Because there is a mechanism of dosage compensation for sex chromosomes.
12 pairs of chromosomes (2n = 24).
Because many of the extra chromosomes in the trisomics were lost in meiosis, leading to fewer than 50% of the gametes carrying the extra chromosome.
Because the pollen containing an extra chromosome was less successful in fertilization, and trisomic zygotes were less viable.
No, it affects the number of gene copies but not their nucleotide sequences.
John Langdon Down in 1866.
Through nondisjunction in meiosis I, meiosis II, or mitosis.
All three chromosomes move to the same side in the cell during Anaphase 1.
The cell produces 2 (3n) gametes with 1 blue, 1 red, and 1 green chromatid each.
Two gametes with 2 chromatids each and two gametes with no chromatids.
The incidence increases among children born to older mothers.
A type of polyploidy in which chromosome sets are from two or more species.
Spontaneous nondisjunction during egg formation.
X-chromosome inactivation prevents problems of gene dosage for X-linked genes.
It produces 2 normal diploid (2n) zygotes with one red and one blue chromatid each.
The loss of both members of a homologous pair of chromosomes, represented as 2n - 2.
Crossover formation in females is often inefficient compared to that in males, leading to fewer mature crossovers and more crossovers likely to lead to missegregation of chromosomes during meiosis.
The chromosome may be lost because the spindle microtubules cannot attach, preventing it from moving to the spindle pole and being incorporated into a nucleus after cell division.
Colchicine disrupts spindle formation and is often used to induce polyploidy in agriculturally and ornamentally important plants.
48 chromosomes.
Around age 35.
Because the cells are XX and the recessive alleles on one X chromosome are masked by the presence of wild-type alleles on the other.
Because all chromosome sets are homologous and attempt to align in prophase I of meiosis, leading to irregular segregation.
Aneuploidy of the sex chromosomes.
12 different mutants.
Uniparental disomy occurs when both copies of a chromosome are inherited from one parent and no copy is inherited from the other parent.
Rolled, Glossy, Buckling, Elongate, Echinus, Cocklebur, Microcarpic, Reduced, Spinach, Poinsettia, Globe, and Ilex.
Most are spontaneously aborted.
Chromosome 21.
Aneuploidy alters the dosage for some, but not all, genes.
The hybrid had 2n = 18 chromosomes.
They produce aneuploid zygotes.
A nucleus with 2 X-shaped chromosomes fails to separate, resulting in one nucleus with 2 chromosomes and one with none.
The viable and fertile plants were allotetraploids with 2n chromosomes.
They are at increased risk for producing children with familial Down syndrome.
Some of the embryos with Down syndrome are spontaneously aborted.
Many tumor cells have extra chromosomes or missing chromosomes, or both.
The sterile hybrid cannot produce viable gametes through meiosis because it has non-homologous chromosomes that do not pair and segregate properly.
Crossovers connect the two chromosomes, creating tension when the homologs are pulled in opposite directions, ensuring proper segregation.
The frequency of aneuploidy increases with maternal age.
The result is the formation of an autotriploid (3n) zygote when the diploid gamete fuses with a normal haploid gamete.
The individual has an extra copy of each of two different (nonhomologous) chromosomes, represented as 2n + 1 + 1.
2n - 1 - 1.
He observed unusual ratios of progeny, such as only 25% of progeny having the globe phenotype when globe mutants self-fertilized.
A person with cystic fibrosis may inherit two copies of chromosome 7 carrying the cystic fibrosis mutation from one parent.
Older mothers are more likely to give birth to a child with Down syndrome than younger mothers.
Two homologous chromosomes pair, whereas the other segregates randomly.
A condition where individuals have 46 chromosomes but an extra copy of part of chromosome 21 is attached to another chromosome through a translocation.
40 (4 × 10)
They are the same.
It arises in offspring whose parents are carriers of chromosomes that have undergone a Robertsonian translocation.
Chromosome 21 and chromosome 14.
The long arm of chromosome 21 and the short arm of chromosome 14 exchange places.
Half of the gametes will have the translocation chromosome and no other copies of chromosomes 21 and 14, and the other half will be normal.
It gives rise to a translocation carrier.
Three of the six types of gametes.
A type of polyploidy in which all chromosome sets are from a single species.
The hybrid has one chromatid each of A, B, C, G, H, and I, with a total of 2n=6.
Meiosis I.
It produces a monosomic (2n - 1) zygote with only 1 blue chromatid.
No, the couple’s relatives are not more likely to have a child with primary Down syndrome.
2n (species I) + 2n (species II) = 14 + 20 = 34
Male phenotype with white eyes and miniature wings.
It produces a 2n gamete that fuses with a 1n gamete to produce an autotriploid (3n) zygote.
1 child in 900 births.
Calvin Bridges in 1913.
2n.
10
Abnormal gene dosage.
Because it alters the number of gene copies, leading to abnormal gene dosage.
Completely normal.
Because the translocation chromosome can segregate in a way that leads to an unbalanced set of chromosomes in the offspring.
A broad, flat face, a small nose, and oval-shaped eyes.
The initial hybrid was sterile.
He noticed that one of his hybrid plants produced a few seeds, which grew into viable and fertile plants.
About 1 in 700 human births.
One-third should be translocation carriers, one-third should have familial Down syndrome, and one-third should have a normal set of chromosomes.
Normal separation of chromosomes in meiosis I, followed by nondisjunction in meiosis II.
Approximately 1 in 8000 live births.
The allotetraploid has a pair of each of chromosomes A, B, C, G, H, and I, with a total of 4n=12.
No, the failure of the chromosomes to divide has little hereditary tendency.
On rare occasions, nondisjunction takes place in a mitotic division, leading to a doubling of chromosome number and the formation of an allotetraploid.
The loss of a single chromosome, represented as 2n - 1.
24 ((2 × 7) + (1 × 10))
31 ((3 × 7) + (1 × 10))
It may separate from the normal chromosomes 14 and 21.
Trisomy 21.
Down syndrome (trisomy 21).
It results in offspring with a normal set of chromosomes.
One-third of their offspring will have Down syndrome.
Primary oocytes may remain suspended in diplotene for many years before ovulation takes place and meiosis recommences.
Because sperm are produced continuously after puberty, with no long suspension of the meiotic divisions.
No, most children with Down syndrome are born to unaffected parents.
The sterile hybrid may be able to perpetuate itself through mitosis (asexual reproduction).
Intellectual disability, contracted fingers and toes, low-set malformed ears, and a prominent forehead.
47 chromosomes.
34 ((2 × 7) + (2 × 10))
37 ((1 × 7) + (3 × 10))
This pattern produces abnormal gametes: half result in monosomy 14 and the other half in trisomy 14, all of which are spontaneously aborted.
Two-thirds of their offspring will have a normal phenotype.
Down syndrome.
Primary Down syndrome.
Non-disjunction leads to a doubling of all chromosomes, producing an allotetraploid (4n=12).
Some types of tumors are consistently associated with specific chromosome mutations, including aneuploidy and chromosome rearrangements.
Severe intellectual disability, a small head, a sloping forehead, small eyes, a cleft lip and palate, extra fingers and toes, and numerous other problems.
Ranging from about 1 in 25,000 to 1 in 50,000 live births.
The gain of a single chromosome, represented as 2n + 1.
One chromosome includes the long arms of chromosomes 14 and 21, and another very small chromosome consists of the short arms of chromosomes 21 and 14.
It is generally lost after several cell divisions.
It creates unbalanced gametes with various numbers of chromosomes.
Hybridization between two diploid species (2n=6) produces a hybrid with six non-homologous chromosomes that do not pair and segregate properly in meiosis, resulting in unbalanced, non-viable gametes.
The breakdown of cohesin components of the spindle and other structures required for proper chromosome segregation during the long arrest of meiosis.
Severe intellectual disability, low-set ears, a short neck, deformed feet, clenched fingers, heart problems, and other disabilities.
A normal gamete has one blue chromatid.
Chromosome pairing and segregation are normal, producing balanced gametes.
Down syndrome results from three functional copies of chromosome 21, either through trisomy (primary Down syndrome) or a Robertsonian translocation (familial Down syndrome).
45 chromosomes.
Some invertebrates, fishes, salamanders, frogs, and lizards.
A trisomic (2n+1) zygote with 2 red and one blue chromatid.
Approximately 92%.
The two possible outcomes are gametogenesis producing non-viable gametes or non-disjunction at an early mitotic cell division producing an allotetraploid.
It produces a trisomic (2n + 1) zygote with 2 red and 1 blue chromatids.
No, they have only a slightly higher risk compared to other couples of similar age.
A somatic clone of monosomic cells (2n - 1) and a somatic clone of trisomic cells (2n + 1) are produced.
About 75%.
A nucleus with 1 red chromatid and a nucleus with one red and 2 blue chromatids are produced.
44 chromosomes.
