Primary oocytes are suspended in the diplotene substage of prophase I of meiosis.
Translocation carriers have an increased chance of producing children with Down syndrome.
Missegregation of chromosomes in meiosis I during oogenesis
A genetic variant at the Polo-like Kinase 4 (PLK4) gene increases the frequency of aneuploidy in humans.
The phenotypic characteristics of familial Down syndrome are the same as those of primary Down syndrome.
There is no similar age effect seen in males because sperm are produced continuously after puberty, with no long suspension of the meiotic divisions.
A condition where regions of tissue have different chromosome constitutions due to nondisjunction in a mitotic division.
Turner syndrome and Klinefelter syndrome result from aneuploidy of the sex chromosomes.
Edwards syndrome
Severe intellectual disability, a small head, a sloping forehead, small eyes, a cleft lip and palate, extra fingers and toes, and numerous other problems.
It usually results in spontaneous abortion.
In XX/XO mosaics, X-linked recessive genes present in cells with a single X chromosome will be expressed.
A translocation carrier has 45 chromosomes with the long arm of chromosome 21 attached to another chromosome, such as chromosome 14.
The incidence of Down syndrome starts to significantly increase around age 35.
Prader–Willi syndrome
The most common aneuploidies seen in living humans are those that involve the sex chromosomes.
Two-thirds of their offspring will have a normal phenotype.
Spontaneous nondisjunction during egg formation
No, it has little hereditary tendency.
37 chromosomes
The second meiotic division takes place immediately before the nuclei of egg and sperm unite to form a zygote.
Down syndrome is caused by trisomy of chromosome 21.
Genetic mosaicism is a condition where different cells within the same individual organism have different chromosome constitutions.
Molecular techniques have made it possible to determine the parental origins of chromosomes by identifying specific DNA sequences.
Sex-chromosome aneuploidies are more common than autosomal aneuploidies because X-chromosome inactivation prevents problems of gene dosage for X-linked genes.
Some 45,X cells and some normal 46,XX cells.
Primary Down syndrome
The resulting zygote will have familial Down syndrome.
No, translocation carriers do not have Down syndrome and are phenotypically normal.
For mothers aged 20 years, 1 child in 2000 suffers from Down syndrome.
Many cases of uniparental disomy probably originate as trisomies, where a trisomic embryo loses one of the three chromosomes early in development.
Down syndrome results from three functional copies of chromosome 21, either through trisomy (primary Down syndrome) or a Robertsonian translocation (familial Down syndrome).
Aneuploidy of chromosome 21 occasionally results in live births because it is small and carries relatively few genes, making the presence of extra copies less detrimental.
Meiosis I
About half die within the first month of life, and 95% die by the age of three.
Sex is determined independently in each cell during development; XX cells express female traits, and XO cells express male traits.
About 4% of people with Down syndrome are not trisomic for a complete chromosome 21.
A normal phenotype.
For mothers aged 40 years, 1 child in 100 suffers from Down syndrome.
Uniparental disomy violates the rule that children affected with a recessive disorder appear only in families where both parents are carriers, as it can result in a child inheriting two copies of a chromosome with a recessive mutation from one parent.
Trisomy 21
Half of the gametes will have the translocation chromosome and no other copies of chromosomes 21 and 14, leading to a translocation carrier; the other half will be normal, each with a single copy of chromosomes 21 and 14.
The fruit fly develops as an XX/XO mosaic, exhibiting a mixture of male and female traits.
This pattern is rare and results in all abnormal gametes: half with monosomy 14 and half with trisomy 14, all of which are spontaneously aborted.
The left side of a gynandromorph fruit fly exhibits a normal female phenotype with red eyes and wild-type wings.
The incidence of primary Down syndrome increases with maternal age, with a significant rise starting around age 35 and a steep increase from age 40 to 50.
For mothers aged 30 years, 1 child in 900 suffers from Down syndrome.
A trisomic embryo can survive if one of the three chromosomes is lost early in development, potentially resulting in uniparental disomy.
Cystic fibrosis can occur due to uniparental disomy if a person inherits two copies of chromosome 7 carrying the cystic fibrosis mutation from one parent, even if only one parent is heterozygous for the mutation.
Approximately 92%
Unaffected parents
At least 50%
Each pair of homologous chromosomes must undergo at least one crossover to segregate properly during meiosis.
The PLK4 gene plays a role in regulating the centriole, the disruption of which can lead to failure of the chromosomes to separate properly in mitosis.
Uniparental disomy may arise when a trisomic embryo loses one of the triplicate chromosomes early in development.
Uniparental disomy is a condition where both chromosomes of a homologous pair are inherited from the same parent.
Many tumor cells have extra chromosomes or missing chromosomes, or both; some types of tumors are consistently associated with specific chromosome mutations, including aneuploidy and chromosome rearrangements.
About 50%
Down syndrome (Trisomy 21)
Patau syndrome
No, they are not more likely.
Sexual mosaics in fruit flies are called gynandromorphs.
Yes, it can also be caused by translocations between chromosome 21 and other chromosomes, such as 15.
The right side of a gynandromorph fruit fly has white eyes and miniature wings because the cells are missing the wild-type X chromosome (are XO), allowing the recessive alleles for white eyes and miniature wings to be expressed.
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 translocation carrier or a normal zygote, forming two-thirds of live births.
For mothers aged 50 years, 1 child in 12 suffers from Down syndrome.
The incidence increases among children born to older mothers.
Most are mosaics, having some cells with three copies of chromosome 8 and other cells with the usual two copies.
More than 30%
Tension during metaphase I is necessary for the two homologs to separate properly in anaphase I.
A zygote with Down syndrome, forming one-third of live births.
Older mothers are more likely to give birth to a child with Down syndrome due to the increased incidence of aneuploidies with advancing maternal age.
Most embryos with autosomal aneuploidies are spontaneously aborted.
Approximately 1 in 8000 live births.
No, they have only a slightly higher risk compared to other couples of similar age.
The resulting zygote will have monosomy 21 and will be spontaneously aborted.
Exchange between chromosomes 21 and 14.
The long arm of chromosome 21 and the short arm of chromosome 14 exchange places, producing one chromosome that includes the long arms of chromosomes 14 and 21 and another, very small chromosome that consists of the short arms of chromosomes 21 and 14.
It is aborted.
It is aborted.
About 1 in 700 human births
Intellectual disability, contracted fingers and toes, low-set malformed ears, and a prominent forehead.
The frequency of aneuploidy increases with maternal age.
One-third should be translocation carriers, one-third should have familial Down syndrome, and one-third should have a normal set of chromosomes.
The long arm of chromosome 21 is attached to another chromosome, such as chromosome 14.
Uniparental disomy is a condition where an individual has two copies of a chromosome from one parent and no copy from the other.
Familial Down syndrome arises in offspring whose parents are carriers of chromosomes that have undergone a Robertsonian translocation, most commonly between chromosome 21 and chromosome 14.
It is aborted.
Autosomal aneuploidies resulting in live births are less common because there is no mechanism of dosage compensation for autosomes.
Severe intellectual disability, low-set ears, a short neck, deformed feet, clenched fingers, heart problems, and other disabilities.
The translocation chromosome may separate from the normal chromosomes 14 and 21.
XO means that the cell has a single X chromosome and no Y chromosome.
No more than 2%
Only about 2% of all fetuses with a chromosome mutation survive to birth.
Primary oocytes may remain suspended in diplotene for many years before ovulation takes place and meiosis recommences.
One-third of their offspring will have Down syndrome.
About 75%
About 1 in 15,000 live births.
Crossover formation in females is often inefficient compared to that in males, leading to fewer mature crossovers and more crossovers that are likely to lead to missegregation of chromosomes during meiosis.
Familial Down syndrome is a condition where individuals have 46 chromosomes, but an extra copy of part of chromosome 21 is attached to another chromosome through a translocation.
The breakdown of cohesin components of the spindle and other structures required for proper chromosome segregation during the long arrest of meiosis can lead to more aneuploidy in children born to older mothers.
20% to 30%
Aneuploidy of the human sex chromosomes is better tolerated than aneuploidy of autosomes because there is a mechanism of dosage compensation for sex chromosomes.
John Langdon Down first noticed the characteristic features of Down syndrome in 1866.
Ranging from about 1 in 25,000 to 1 in 50,000 live births.
