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Topic 3: Genetics (3.2 Chromosomes (Some prokaryotes also have plasmids…

- - - - Some cells have a haploid nucleus, which has only one chromosome of each type. Two haploid gametes fuse together during fertilization to produce one diploid cell.
  - - - Two common uses of karyograms:
        
        Deducing the sex of an individual
        If there are two X chromosomes the person is female and if there is one X and one Y they are male.
        
        Diagnosing conditions due to chromosome abnormalities:
        e.g.Down syndrome is due to having 3 copies of chromosome 21.
- - - - The ABO blood group system in humans is an example of co-dominance. It is of great medical importance: before blood is transfused, it is vital to find out the blood group of a patient and ensure that it is matched.
        
        One gene determines the ABO blood group of a person. The genotype IAIA gives blood group A and the genotype IBIB gives group B. Neither IA not IB is dominant over the other allele and a person with the genotype IAIB has a different blood group, called AB. There is a third allele of the ABO blood group gene, usually called i. A person with the genotype ii is in blood group O.
  - - - A small proportion of genetic diseases are caused by a dominant allele. It is not possible to be a carrier of these diseases. If a person has one dominant allele then they themselves will develop the disease. 50% that child will inherit. EX. Huntington's disease
      - A very small proportion of genetic diseases are caused by co-dominant alleles. Sickle-cell anemia is EX.
        Most genetic diseases affect males and females in the same way but some show a different pattern of inheritance in males and females. This is called sex linkage. The causes of sex linkage and 2 examples, red-green colour-blindness and hemophilia, are described later in this sub-topic.
      - Sex-linked genes: The pattern of inheritance is different with sex-linked genes due to their location on sex chromosomes.
        
        An inheritance pattern where the ratios are different in males and females is called sex linkage.
        
        First example of sex linkage was discovered by Thomas Morgan in the fruit flu, Drosophila. Geneticists had observed that the inheritance of genes and of chromosomes showed clear parallels and so genes were likely to be located on chromosomes.
      - Sex linkage are all due to genes located on the X chromosome, as there are very few genes on the Y chromosome. 2 examples of sex-linked conditions due to genes on the X chromosomes are described here: red-green colour-blindness and hemophilia.
        
        Red-green colour-blindness is caused by a recessive allele of a gene for one of the photoreceptor proteins. These proteins are made by cone cells in the retina of the eye and detect specific wavelength ranges of visible light.
  - - - Inheritance of cystic fibrosis and Huntington's disease.Cystic fibrosis is the commonest genetic disease in parts of Europe. It is due to a recessive allele of the CFTR gene. This gene is located on chromosome 7 and the gene product is a chloride ion channel that is involved in secretion of sweat, mucus and digestive juices. The recessive alleles of this gene result in chloride channels begin produced that do not function properly. Sweat containing excessive amounts of sodium chloride is produced, but digestive juices and mucus are secreted with insufficient sodium chloride. As a result not enough eater moves by osmosis into the secretions making them very viscous. Sticky mucus builds up in the lungs causing infections and the pancreatic duct is usually blocked so digestive enzymes secreted by the pancreas do not reach the small intestine.
        
        About one in 10,000 people have a copy of the Huntington's allele, so it is very unlikely for 2 parents both to have a copy. A person can nonetheless develop the disease if only one of their parents has the allele because it is dominant.
    - - Radiation increases the mutation rate if it has enough energy to cause chemical changes in DNA. Gamma rays and alpha particles from radioactive isotopes, short-wave ultraviolet radiation and X-rays are all mutagenic.
      - Some chemical substances cause chemical changes in DNA and so are mutagenic. Ex are benzopyrene and nitrosamines found in tobacco smoke and mustard gas used as a chemical weapon in the WWI.
      - Mutations in body cells, including those that cause cancer, are eliminated when the individual dies, but mutations in cells that develop into gametes can be passed on to offspring.
      - Consequences of nuclear bombing and accidents at nuclear power stations
        
        -4km2 of pine forest downwind of the reactor turned ginger brown and died.
        -Horses and cattle near the plant died from damage to their thyroid glands.
        -Lynx, eagle owl, wild boar and other wildlife subsequently started to thrive in a zone around Chernobyl from which humans were excluded.
        -Bioaccumulation caused high levels of radioactive caesium in fish as far away as Scandinavia and Germany and consumption of lamb contaminated with radioactive caesium was banned for some time as far away as Wales.
        -Concentrations of radioactive iodine in the environment rose and resulted in drinking water and milk with unacceptably high levels.
        -More than 6,000 cases of thyroid cancer have been reported that can be attributed to radioactive iodine released during the accident.
        -According to the report ''Chernobyl's Legacy Health, Environmental and Socio-Economic Impacts'', produced by the Chernobyl Forum, there is no clearly demonstrated increase in solid cancers or leukemia due to radiation is the most affected populations.
- - - - DNA is heated to 95 °C to separate the two strands.
        
        The temperature is reduced to 53 °C, which allows primers to bind to both strands of the DNA next to the sequence that is to be copied.
        
        The temperature is increased to 73 °C, which encourages Taq DNA polymerase to replicate both strands, starting at the primer, producing two double-stranded copies of the original DNA.
  - - - Forensic investigations
      - Paternity investigations
  - - - Potential benefits of maize
        
        Higher crop yields and thus more food for humans, due to less pest damage.
        
        Less land needed for crop production, so some could become areas for wildlife conservation.
        
        Less use of insecticide sprays, which are expensive and can be harmful to farm workers and to wildlife.
      - Possible harmful effects of maize
        
        Insects that are not pests could be killed. It will damage the ecosystem
        
        The transferred gene might spread to populations of wild plants by cross-pollination, making them also toxic to insects feeding on them.
        
        The insects pests of corn may develop resistance to the Bt toxin.
  - - - It is much more difficult to clone an adult animal with known characteristics, but methods have been developed. One method is somatic-cell nuclear transfer
- - - - Sickle cell anemia is caused by base substitution mutation, change to the base sequence of mRNA transcribed from it and a change to the sequence of a polypeptide in hemoglobin.
        
        Most humans have the allele Hb4. If a base substitution mutation converts the sixth codon of the gene from GAG to GTG, a new allele is formed, called Hb5.The mutation is only inherited by offspring if it occurs in a cell of the ovary or testis that develops into an egg or sperm. When the Hb5 allele is transcribed, the mRNA produced has GUG as its sixth codon instead of GAG, and when this mRNA is transcribed, the sixth amino acid in the polypeptide is valine instead of glutamic acid. This change causes hemoglobin molecules to stick together in tissues with low oxygen concentrations. The bundles of hemoglobin molecules that are formed are rigid enough to distort the red blood cells into a sickle shape.
        
        These cause damage to tissues by becoming trapped in blood capillaries. Both the hemoglobin and the plasma membrane are damaged and the life of a red blood cell can be shortened to as little as 4 days. The body cannot replace red blood cells at a rapid enough rate and anemia therefore develops.
  - - - Comparing numbers of genes. Different number of genes are needed to construct living organism. For example, Populus trichocarpa consist of 46,000 genes, and Homo sapiens consist of 23,000 genes.
- - - - Homologous chromosomes are pulled to opposite poles. This halves the chromosome number.
      - The cell membrane around the equator will soon be pulled inwards to divide the cell
    - - New spindle microtubules grow from the poles to the equator.
    - - Spindle microtubules attach to different chromosomes in each pair.
      - The pairs of chromosomes line up on the equator
    - - The centromeres have divided making the chromatids into separate chromosomes which move to opposite poles.
      - The cell membrane is pulled inwards again to divide the cells.
    - - Chromosomes pair up. The chromosomes in each pair are homologous.
      - Spindle microtubules grow from each pole to the equator.
      - Nuclear membrane will soon break down
    - - Nuclear membranes reformed
      - Each nucleus now has half as many chromosomes as the nucleus of the parent cell, now there are 4 haploid cells
  - - - amniocentesis :
        a sample of amniotic fluid is removed from the amniotic sac around the fetus.
      - chorionic villus sampling (CVS):
        cells are removed from fetal tissues in the placenta called chorionic villi.