B6 - Inheritance, Variation and Evolution

• Sexual reproduction involves the joining (fusion) of male and female gametes:

  • sperm and egg cells in animals

  • pollen and egg cells in flowering plants.
• Asexual reproduction involves only one parent and no fusion of gametes. There is no mixing of genetic information. This leads to genetically identical offspring (clones). Only mitosis is involved.
• Cells in reproductive organs divide by meiosis to form gametes. When a cell divides to form gametes:

  • copies of the genetic information are made

  • the cell divides twice to form four gametes, each with a single set of chromosomes

  • all gametes are genetically different from each other
• Gametes join at fertilisation to restore the normal number of chromosomes. The new cell divides by mitosis. The number of cells increases. As the embryo develops cells differentiate.
• The genetic material in the nucleus of a cell is composed of a chemical called DNA. DNA is a polymer made up of two strands forming a double helix. The DNA is contained in structures called chromosomes.
• A gene is a small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids, to make a specific protein.
• The genome of an organism is the entire genetic material of that organism. The whole human genome has now been studied and this will have great importance for medicine in the future.
• Understanding the human genome is highly important, as knowing it allows for:

  • search for genes linked to different types of disease

  • understanding and treatment of inherited disorders

  • use in tracing human migration patterns from the past.
• Some characteristics are controlled by a single gene, such as: fur colour in mice; and red-green colour blindness in humans. Each gene may have different forms called alleles.
• The alleles present, or genotype, operate at a molecular level to develop characteristics that can be expressed as a phenotype.
• A dominant allele is always expressed, even if only one copy is present. A recessive allele is only expressed if two copies are present (therefore no dominant allele present).
• If the two alleles present are the same the organism is homozygous for that trait, but if the alleles are different they are heterozygous.
• Some disorders are inherited. These disorders are caused by the inheritance of certain alleles.

  • Polydactyly (having extra fingers or toes) is caused by a dominant allele.

  • Cystic fibrosis (a disorder of cell membranes) is caused by a recessive allele.
• Ordinary human body cells contain 23 pairs of chromosomes. 22 pairs control characteristics only, but one of the pairs carries the genes that determine sex.

  • In females the sex chromosomes are the same (XX).

  • In males the chromosomes are different (XY).
• Sometimes, a mutation can occur in the chromosomes of gamete, meaning that the faulty gene will be present in every cell of the body, and in some rare cases this causes a genetic disorder.
• Genetic screening is the analysis of a person's DNA to see if they are carrying any alleles that cause genetic disorders, it can be done at any point in a person's life, but most commonly happens before they are born.
• The 3 types of genetic screening include:

  • Antenatal: Analysing a person's DNA/Chromosomes before they are born
  • Neonatal: Analysing a blood sample from a newborn baby, it detects genetic disorders in order to treat them early
  • Pre-implantation Genetic Diagnosis (PGD): Otherwise known as embryo screening, it used on embryos before they are implanted.
• There are some problems regarding genetic screening, 2 of the most common being:

  • False positives, individual could believe something is wrong when it is actually fine.
  • False negatives, arguably more of an issue since it can have a major impact on the parent and child's life
• Besides technical problems, there are some ethical problems regarding genetic screening, these include:

  • The belief it is wrong to tamper with the natural process of reproduction
  • The belief that it is considered murder to destroy embryos in the screening process
• Differences in the characteristics of individuals in a population is called variation and may be due to differences in:

  • the genes they have inherited (genetic causes) [sex, blood type]

  • the conditions in which they have developed (environmental causes) [tattoos, scars]

  • a combination of genes and the environment. [weight, height, hair colour]
• Mutations occur continuously. Very rarely a mutation will lead to a new phenotype. If the new phenotype is suited to an environmental change it can lead to a relatively rapid change in the species.
• All variants rise as a result of mutations or meiosis, most have no effect on the phenotype, although some influence it, but very few determine it.
• The theory of evolution by natural selection states that all species of living things have evolved from simple life forms that first developed more than three billion years ago.
• If two populations of one species become so different in phenotype that they can no longer interbreed to produce fertile offspring they have formed two new species.
• Selective breeding (artificial selection) is the process by which humans breed plants and animals for particular genetic characteristics.
• Humans have been using selective breeding for thousands of years since they first bred food crops from wild plants and domesticated animals.
• Selective breeding involves choosing parents with the desired characteristic from a mixed population.

  • They are bred together.
  • From the offspring those with the desired characteristic are bred together.
  • This continues over many generations until all the offspring show the desired characteristic.
• The characteristic can be chosen for usefulness or appearance:

  • Disease resistance in food crops.

  • Animals which produce more meat or milk.

  • Domestic dogs with a gentle nature.

  • Large or unusual flowers.
• Selective breeding can lead to ‘inbreeding’ where some breeds are particularly prone to disease or inherited defects.
• Plant crops have been genetically engineered to be resistant to diseases or to produce bigger better fruits.
• Bacterial cells have been genetically engineered to produce useful substances such as human insulin to treat diabetes.
• In genetic engineering, genes from the chromosomes of humans and other organisms can be ‘cut out’ and transferred to cells of other organisms
• Crops that have had their genes modified in this way are called genetically modified (GM) crops. GM crops include ones that are resistant to insect attack or to herbicides. GM crops generally show increased yields.
• Concerns about GM crops include the effect on populations of wild flowers and insects. Some people feel the effects of eating GM crops on human health have not been fully explored.
• Modern medical research is exploring the possibility of genetic modification to overcome some inherited disorders.
• In Genetic Engineering:

  • Enzymes used to isolate required gene
  • Gene is inserted into a vector (bacterial plasmid or virus)
  • Vector used to insert the gene into required cells
  • Genes transferred to cells of animals, plants or microorganism at an early stage
  • Allows them to develop with desired characteristics
• The theory of evolution by natural selection is now widely accepted. Evidence for Darwin’s theory is now available as it has been shown that characteristics are passed on to offspring in genes. There is further evidence in the fossil record and the knowledge of how resistance to antibiotics evolves in bacteria.
• Fossils are the ‘remains’ of organisms from millions of years ago, which are found in rocks.
• Fossils may be formed:

  • from parts of organisms that have not decayed because one or more of the conditions needed for decay are absent

  • when parts of the organism are replaced by minerals as they decay

  • as preserved traces of organisms, such as footprints, burrows and rootlet traces.
• Scientists cannot be certain about how life began on earth for these reasons:

  • Many early life forms were soft bodied, meaning that they have very few traces left behind
  • Geological activity may have destroyed much of the traces left from those life forms