BIOL 2500 - Topic 7 (part 5)

Trisomy 21

Aka Down syndrome, which is the most common autosomal aneuploidy in humans

Cause of trisomy 21

90% is a result of nondisjunction during meiosis I, 10% occur via Robertsonian translocation

Chromosome distribution in trisomy 21

They have 2 maternal and 1 paternal copy of chromosome 21

Trisomy 21 is linked to…

1.) It is linked to the age of the mother, as a result of the pause that occurs during prophase I

2.) The older you get, the longer the pause, the more likely nondisjunction occurs

When does the pause in prophase I stop?

Monthly cycles reinitiate meiosis, allowing eggs to be released into the fallopian tubes after meiosis I, but meiosis II won’t begin until after fertilization with a sperm

Blakeslee and Belling

They worked on Jimson weed and recognized that aneuploidy alters gene dosage, resulting in a variety of phenotypes

Gene dosage effect

Refers to the fact that aneuploidy changes the gene dosage of all the genes on the affected chromosomes, therefore leading to an imbalance in gene products

Normal gene dosage in diploid organisms

100% (anything less or more leads to phenotypic differences)

Gene dosage in monosomic vs. trisomic individuals

1.) Monosomic (2n-1) = 50%

2.) Trisomic (2n+1) = 150%

Effect of gene dosage in animals vs. plants

Animals are usually more affected by it, while plants are more tolerant of the changes it causes, as they have different developmental programmings than animals

Effect of gene dosage on animals

Most of the time it is lethal, as it causes developmental delays and/or nervous system underdevelopment

X-inactivation mosaicism

It is when one X chromosome in female somatic cells randomly inactivates, causing different cells to have slightly different phenotypes

Can nondisjunction happen in mitosis

Yes, but it only affects the embryos body, as it occurs during embryogenesis early in development, and does not get passed down to future progeny

X-inactivation mosaic Turner syndrome

25-30% of cases regarding turner syndrome can result in a mosaic of XO monosomy (C45), while the rest are XX (C46), but some individuals also have XXX cells (C47)

Uniparental disomy

It is when both copies of the homologous chromosome pair are derived from the same parent

Uniparental disomy can cause…

1.) Angelman syndrome —> has paternal copies of chromosome 15

2.) Prader-Willi syndrome —> has maternal copies of chromosome 15

Rare cause of uniparental disomy

Nondisjunction occurs at the same chromosome in both the egg and sperm, where females result in XX and the male have 00 (but this is super rare)

Normal cause of uniparental disomy

1.) Nondisjunction occurs in one parent that results in an aneuploid gamete that has two copies of the chromosome and then it gets fertilized by a normal parental gamete

2.) Trisomy rescue occurs, but sometimes it ends up tossing out the normal gamete chromosome instead of the extra chromosome, resulting in the two copies of the chromosome from the same parent

Trisomy rescue

It is when the extra copy of a chromosome is tossed out during embryonic development