Crash Course: Biology

Genetics: Cytogenetics

a/n: this is my last section on genetics, but someone should remind me to explain chi-square probability analysis

What the heck is cytogenetics?

Well, it's genetics of the cell. mostly chromosomal disorders, the topic of my last chapter on Genetics.

Well, first, what's a karyotype?

Okay. So, do you remember when I talked about Metaphase of Mitosis and I went into all this unnecessary detail?

Let me quote myself: "If you were to make a karyotype, the best time to capture the chromosomes is in metaphase"

Right, so what is a karyotype? A karyotype is made up of chromosomes photographed during metaphase, cut out and arranged in pairs according to size, banding, and centromere position. As you can see from the little picture in the sidebar, all the chromosomes are arranged into pairs, except the sex chromosomes (I'll get to why there's three 21s in a minute).

Now that that's out of the way, how do scientists obtain cells for prenatal diagnosis of chromosomal abnormalities?

I bet that sentence flew straight over your head, so I'll rephrase: How do scientists know if your child has Down Syndrome/whatever?

One method is Fluorescent In Situ Hybridization (FISH). FISH uses fluorescent markers that bind and attach to certain cells different. For example, Chromosome 21. If you have two, you're normal. I'll get to the third one later, shush. These cells are usually obtained from the fetus through amniocentesis (inserting the hollow needle and withdrawing suspended fetal cells and fluid) or chorionic villus sampling (inserting the catheter through the vagina or abdominal wall into the uterus).

So, uh, three 21s?

You brought it upon yourself.

Trisomy 21, also known as Down Syndrome. A genetic disorder caused by nondisjunction(chromosomes fail to seperate properly during anaphase, happens often when the mother get older) in meiosis where a child is born with three copies of the 21st chromosome. the eyelids of a child with downsyndrome may contain an epicanthic fold, and physical growth, behavior, and mental development are retarded. Out of all aneuploidies, Down Syndrome is the most common.

Wait, we're not stopping there.

Trisomy 13, also known as Patau Syndrome. As you can probably infer, affected individuals have three copies of chromosome 13. The phenotype includes a cleft lip/palate, eye defects, and polydactyly. Internally, they often have severely malformed brains and nervous systems. Half of all affected individuals die within the first month of birth. I'll save you the horror of seeing an image. Google if you wish.

Trisomy 18, otherwise known as Edwards Syndrome almost always feature heart malformations, clentched fists, with the second and fifth fingers overlapping the third and fourth fingers. Infants with this disorder are small at birth, grow extremely slowly, and are mentally retarded. The average survival time for an infant with Edwards Syndrome is 2-4 months. For some unknown reason, 80% of all Edwards Syndrome births are female.

Turners Syndrome, where the child only has one X chromosome and no Y chromosome. They are classified as female, and are often sterile, short, wide-chested, and have underdeveloped brains. There is no mental retardation associated with Turner's Syndrome, but there has been a correlation between having Turner's and reduced skills in interpreting spatial relationships. In ~75% of all cases, the nondisjunction originates from the father.

Klinefelter Syndrome, where the child has an extra X chromosome and a Y chromosome, resulting in and XXY karyotype. Scientifically, people with Klinefelter's are identified as male, and suffer no problems until puberty, where they begin to grow breasts (in 50% of the cases), and show poor sexual development and have very low fertility rates.

XYY Syndrome, where individuals are above average in height, have personality disorders, and 77% of which have subnormal intelligence.

So, those are all the syndromes, right?

Wrong. There are more, except that people are never born with them.

Triploidy Where the child has three or more copies of every chromosome, and three sex chromosomes.

Tetraploidy: Where the child has four copies of every chromosome, and four sex chromosomes.

Trisomy 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, you get the point: Where the child has three copies of the specified chromosome.

Monosomy: Where the child has one copy of a chromosome.

Is there more?

Heck, yes. Blame yourself for being curious.

Cri-du-chat Syndrome: Caused by a deletion on the p-arm of chromosome 15. The affected child is mentally retarded, has defects in facial development, gastrointestinal malformations, and an abnormal development on the glottis and larynx, which cases their cry to sound like a cat mewoing, hence the name cri-du-chat, meaning Cry of the Cat.

Prader-Willi Syndrome: caused by a deletion on the q-arm of Chromosome 15. As infants, the children do not feed well due to poor sucking abilities. However, once age 5 is reached, victims literally eat themselves to death if left untreated. 

Robertsonian Translocation: There's more to Down Syndrome?

Well, yes. A Robertsonian translocation is the breakage in the short arms of acrocentric chromosomes (often 21), followed by fusion of the long parts into a single chromosome.