Genetics and Your Health
- Medical science is increasingly coming to understand the genetic root of many maladies in humans.
- Medical doctors and the public need to know more about these genetic problems.
- So, here are some highlights…
Nondisjunction
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Causes:
- Sometimes homologous chromosomes do not separate in meiosis I (illustrated on the left).
- Sometimes sister chromatids do not separate in meiosis II (illustrated on the right).
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Effect:
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Aneuploidy: an incorrect number of chromosomes (either too many or too few)
- An: without
- Eu: good/true
- Ploidy: chromosomes
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If fertilized eggs have three chromosomes for a particular homologous pair, they are said to be trisomic for that chromosome.
- Assuming this only happens once, the gamete's haploid number will be n + 1.
- Thus, the fertilized egg's diploid number will be 2n + 1.
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If fertilized eggs have only one chromosome for a particular homologous pair, they are said to be monosomic for that chromosome.
- Assuming this only happens once, the gamete's haploid number will be n - 1.
- Thus, the fertilized egg's diploid number will be 2n - 1.
Down Syndrome
- A result of trisomy in chromosome 21.
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Symptoms:
- Characteristic facial features
- Short stature
- Heart defects
- Susceptibility to respiratory infections
- Mental retardation
- Often, sexually underdevelopment and sterility
Sex Chromosome Aneuploidy
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XXY (Klinefelter's syndrome)
- Sterile males (have male sex organs)
- May have female characteristics (some breast development).
- Have normal intelligence.
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XO (Turner's syndrome)
- Look like females (are females since they have no Y chromosome).
- Are short (average height is 4'7" tall).
- Fail to develop secondary sexual characteristics (e.g. breasts).
- Are sterile.
- Have normal intelligence.
- The only known viable example of monosomy in humans!
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XYY (no syndrome)
- Tend to be taller than normal.
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XXX (no syndrome)
- Do not differ in appearance from XX.
Polyploidy
- Polyploidy is a condition in which an organism has more than 2 complete sets of chromosomes.
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A quick review of what is considered normal:
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Somatic cells in most organisms have two sets of chromosomes; they are diploid (2n).
- The chromosomes come in homologous pairs, with one chromosome from each set.
- Sex cells in all organisms have one set of chromosomes; they are haploid (n).
Examples
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Triploidy (3n)
- Nondisjunction of all chromosomes leaves an egg with 2n chromosomes. This egg is then fertilized by a normal haploid (n) sperm.
- Alternatively, two sperm can fertilize one egg, resulting in a triploid cell (double fertilization).
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Tetraploidy (4n)
- Caused by the failure of a zygote to divide after it has replicated its chromosomes (2n → 4n).
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Very common in plants
- In fact, it is believed that over 50 % of speciation in plants is due to polyploidy!
- Plant tetraploidy can be induced by the drug cholchicine.
Prenatal Diagnosis Techniques
- Amniocentesis
- Amniotic fluid is removed; the cells are cultured and then karyotyped.
- Chorionic villus sampling
- A narrow tube is inserted through the cervix and a small amount of the placenta is removed; the cells are then karyotyped.
Guinness Book of World Records for Chromosome Number
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Fewest chromosomes
- An Australian bulldog ant, Myrmecia pilosula, has 1 pair of chromosomes per cell.
- Males have only 1 chromosome, because—like all male Hymenopterans—they are haploid.
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Most chromosomes
- There is a species of fern, Ophioglossum reticulatum, that has 1260 chromosomes per cell!
- 630 pairs of homologous chromosomes!
- These numbers are approximate.
Alteration in Chromosome Structure
- Deletion
- Removal of a segment of the chromosome
- Duplication
- Repetition of a segment
- Inversion
- Reversal of a segment
- Translocation
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- Movement of a segment from one chromosome to another non-homologous chromosome.
- If it were to a homologous chromosome, it would be crossing over.
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Two types:
- Reciprocal translocation: trading between chromosomes
- Non-reciprocal translocation: one-way movement (no trading)
Genomic Imprinting
- In genomic imprinting, the phenotypic effects of some genes depend on whether they were inherited from the mother or the father.
- Genes are “imprinted” (manifested) in a different way each generation, depending on whether the gene came from the mother or father.
- Genomic imprinting is only seen in mammals.
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Example: deletion on chromosome 15
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If the abnormal chromosome is inherited from the father, the result is Prader–Wili syndrome.
- Mental retardation
- Obesity
- Short stature
- Small hands and feet
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If the abnormal chromosome is inherited from the mother, the result is Angelman syndrome.
- Spontaneous uncontrollable laughter
- Jerky body movements
Barr Bodies and X-Inactivation in Female Mammals
- Each female somatic cell has two X chromosomes (XX).
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One X chromosome in each cell becomes almost completely inactivated during embryonic development.
- Condenses into a compact object called a Barr body.
- Lies along the inside of the nuclear envelope.
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Example
- The X chromosome in cats has a gene controlling fur color—one allele for black, one allele for orange.
- Male cats (XY) only inherit one or the other and so are either black or orange.
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Female cats (XX) can inherit both and become calico.
- Their embryonic cells are a mosaic, some having active paternal Xs and some having active maternal Xs.
- When X-inactivation occurs, these patterns are frozen.
- All future mitotic descendents of these embryonic cells will take on their parent cell's color.
- Thus, the color will vary throughout the cat depending on which X chromosome was active in its embryonic ancestor cell.
- There are no male calico cats!
