B5

• GENETIC KEY TERM:

  • A gamete is a sex cell.
  • DNA is a complex polymer, made up of two strands forming a double helix and determines the characteristics of a living organism.
  • Chromosomes are in the cell's nucleus. These are long threads of DNA, which are made up of many genes. Chromosomes are found in pairs. One chromosome is inherited from the mother and one is inherited from the father. The chromosomes in each pair carry the same gene in the same location.
  • A gene is a small section of DNA on a chromosome that codes for a particular sequence of amino acids, to make a specific protein.
• GENETIC KEY TERMS:

  • Alleles are genetic variants. For example, the gene for eye colour has an allele for blue eye colour and an allele for brown eye colour.
  • The genotype is the collection of alleles that determine characteristics and can be expressed as a phenotype.
• ALLELES:

  • A dominant allele: always expressed, even if only one copy is present. Represented by a capital letter. Two copies of the dominant allele will still produce a dominant characteristic.
  • A recessive allele: only expressed if the individual has two copies and does not have the dominant allele of that gene. Represented by a lower case letter.
  • A homozygous (recessive or dominant) individual has identical alleles for the same characteristic, for example AA or aa.
  • A heterozygous individual has two different alleles for the same characteristic, for example Aa.
• GENOME AND VARIATION:

  • The genome is the entire genetic material of an organism.
  • This means all of the genes encoded in all of the DNA in an organism.
  • The genome interacts with the environment to give some characteristics of the organism, for example, their weight.
  • The various alleles an organism inherits and the interaction of the organism's genome with the organism's environment leads to a large amount of variation between the individuals in a population.
• DISCONTINUATION VARIATION:

  Human blood group is an example of discontinuous variation. There are only four types of blood group. There are no other possibilities and there are no values in between. So this is discontinuous variation.

  A characteristic of any species with only a limited number of possible values shows discontinuous variation. Here are some examples:

  • sex (male or female)
  • blood group (A, B, AB or O)
  • eye colour (blue, brown, green)
• CONTINUOUS VARIATION:

  Height is an example of continuous variation. Height ranges from that of the shortest person in the world to that of the tallest person. Any height is possible between these values. So this is continuous variation.

  Examples of such characteristics are:

  • height
  • weight
  • foot length

  They usually show normal distribution
• VARIATION:

  • Genetic and environmental variation combine together to produce different phenotypes.
  • Discontinuous variation is usually caused by one gene and is not affected by the environment. Your eye colour is the same now as it was when you were younger.
  • Continuous variation, however, is often caused by a mixture of your genes and your environment. For example, you may inherit the genes to become really tall, but if you don't eat nutritious food then you won't be able to grow as tall as you might.
• MUTATION AND VARIATION:

  • Extensive genetic variation is contained within any species. This is clearly visible in the domestic dog species.
  • Variation within genes leads to different genotypes, and this can be seen by the individuals having different phenotypes. For example, the dogs above all have different fur colours and fur lengths.
  • All genetic variants arise from mutations and most have no effect on the phenotype.
• MUTATIONS:

  • A mutation is a random change in a gene or chromosome. Mutations arise spontaneously and happen continually.
  • A mutation rarely creates a new phenotype, but if the phenotype is suited to a particular environment, it can lead to rapid change in a species.
  • Generally, mutations will have no effect on the phenotype, and occasionally the mutation might have some influence on an organism's phenotype.
  • It is only in rare cases that a mutation will fully determine an organism's phenotype
• EFFECTS OF GENETIC VARIANTS ON THE PHENOTYPE:

  The structure of DNA is important for synthesising specific proteins that are needed in biological processes.
• MUTATIONS IN NON-CODING DNA:

  • Not all parts of DNA code for proteins.
  • As well as the coding parts of DNA that code for proteins, there are also non-coding parts of DNA. The non-coding parts of DNA can switch genes on and off.
  • When genes are switched off, the process of transcription stops. This means no mRNA is being made for that gene and therefore no protein can be made for that gene.
  • Therefore, a mutation in non-coding areas of DNA may affect gene expression, and whether the correct protein is synthesised or not.
• EFFECT OF GENETIC VARIANTS ON THE PHENOTYPE:

  MUTATIONS IN CODING DNA:

  • If you get a mutation in coding DNA, then the sequence of DNA could be changed.
  • Changing the sequence of DNA can change the amino acids and protein.
  • This could have consequences. Enzymes are made of proteins and have a specifically shaped active site to recognises a certain substrate.
  • If a random mutation happens in the coding DNA that codes for the enzyme, then the wrong amino acids could be used to make the enzyme. This could change the shape of the enzyme's active site and then the enzyme will not work.
• SEXUAL REPRODUCTION:

  Sexual reproduction involves two parents and the joining of gametes during fertilisation. The offspring inherit a mixture of genes from both parents, so are different to each other and their parents.

  The advantages of sexual reproduction:

  • produces genetic variation in the offspring
  • species adapt to new environments due to variation, giving them an advantage
  • a disease is less likely to affect the whole population

  The disadvantages of sexual reproduction:

  • time and energy are needed to find a mate
  • it is not possible for an isolated individual to reproduce
• ASEXUAL REPRODUCTION:

  In asexual reproduction there is only one parent. The offspring are clones of the parent and each other.

  The advantages of asexual reproduction include:

  • the population can increase rapidly in good conditions
  • only one parent is needed
  • time and energy efficient (no mate)
  • faster than sexual reproduction

  The disadvantages of asexual reproduction include:

  • it does not lead to genetic variation in a population
  • the species may only be suited to one habitat
  • disease may affect all the individuals in a population
• BACTERIAL REPRODUCTION:

  • Bacteria, such as E. coli, reproduce asexually.
  • An advantage of this is that they can produce many bacteria very quickly.
  • A disadvantage is that all of the bacteria are genetically identical. If an antibiotic was used on the bacteria, then all of them would die. The population would be wiped out. The only way for variation to be introduced into the population is by random mutation.
• RABBIT REPRODUCTION:

  • Most animals reproduce sexually
  • The process of sexual reproduction introduces variation into the species because the alleles are mixed together in the offspring.
  • A disadvantage is that sexual reproduction takes a long time. A mate must be found, the egg must be fertilised by sperm.
  • The benefit of introducing genetic variation into the species is that if a disease were to hit the rabbit population, then not all of the rabbits would be affected because of the variation in the population.
  • This means that some individuals would survive and be able to reproduce.
• ASEXUAL AND SEXUAL REPRODUCTION OF FUNGI:

  • Fungi are able to reproduce sexually and asexually.
  • Fungi reproduce using spores they release into the environment.
  • A new fungi will grow from the spore. Fungi can produce spores by sexual reproduction, creating variation in the species.
  • This is advantageous in a changing environment as it increases the probability of a species dealing with change. 
  • Spores can be produced asexually quickly and in large quantities.
  • A disadvantage of these spores is that they generate offspring that are unlikely to be resistant due being genetically identical.
• SEXUAL REPRODUCTION:

  • Two parents are needed in sexual reproduction.
  • During this process the nucleus of the male and female gametes are fused creating a zygote- fertilisation.
  • Gametes contain half the number of chromosomes of all other cells in the organism (haploid).
  • When the male and female gametes combine in fertilisation they create an embryo with the full complement of chromosomes (diploid).
  • Cells which are diploid have two sets of chromosomes - for most organisms this means the cells have one set of chromosomes from their mother and one set from their father.
• GAMETES:

  The gametes in animals are sperm (male) and eggs (female).

  The gametes in flowering plants are pollen (male) and ovules (female).

  The offspring produced in sexual reproduction are genetically different to each other and to their parents. This process results in variation within a population because it involves the mixing of genetic information.
• MEIOSIS:

  Sexual reproduction uses the process of meiosis, which creates gametes. The process of meiosis happens in the male and female reproductive organs.

  As a cell divides to form gametes:

  • A copy of all of the genetic information is made.
  • The cell divides twice to form four gametes, each with a single set of chromosomes (haploid). This means the chromosome number has halved.
  • All gametes are genetically different from each other.
• FERTILISATION:

  • Gametes have half the total number of chromosomes that the organism needs to develop and are referred to as haploid.
  • Humans need 46 chromosomes to develop, therefore a human gamete has 23 chromosomes. Fertilisation is the fusion of the nucleus of a male gamete with the nucleus of a female gamete.
  • When the two gametes combine, they merge the two sets of chromosome to create a cell with the total number of chromosomes needed to develop, known as a diploid cell.
• FERTILISATION:

  • In humans when the haploid sperm and egg cell join in fertilisation the resulting zygote has a total of 46 chromosomes the correct number to develop.
  • By having gametes which are haploid, when the gametes combine, diploid cells are maintained. Also, the mixing of chromosomes in fertilisation is a source of genetic variation.
  • Fertilisation produces a zygote, which will mature into an embryo. The number of cells increases by mitosis, and as the embryo develops, the cells begin to differentiate (or specialise).
• GENETIC CROSSES:

  Genetic crosses of single gene combinations (monohybrid inheritance) can be shown and examined using Punnett squares. This shows which possible offspring combinations could be produced, and the probablity of these combinations occurring.
• DOMINANT AND RECESSIVE:

  There are dominant and recessive alleles in all organisms and dominant alleles are represented and capital letters whilst recessive are lower case letters.
• HOMOZYGOUS RECESSIVE: 2 recessive genes (hh)

  HOMOZYGOUSE DOMINANT: 2 dominant genes (HH)

  HETEROZYGOUS: 1 dominant and 1 recessive gene (Hh)
• SEX DETERMINATION:

  Human body cells have twenty-three pairs of chromosomes in the nucleus. Twenty-two pairs are known as autosomes, and control characteristics, but one pair carries genes that determine sex - whether offspring are male or female:

  • males have two different sex chromosomes, X Y
  • females have two X chromosomes, XX
• A genetic cross, like a Punnett square, shows how the alleles inherited from the parents combine in a zygote.

  Mothers/female alleles - XX and the fathers/male alleles - XY

  The two possible combinations are:

  • an X chromosome from the mother and an X chromosome from the father - producing a girl (female phenotype from the XX genotype)
  • an X chromosome from the mother and a Y chromosome from the father - producing a boy (male phenotype from the XY phenotype)

  The ratio of female to male offspring is 50% either way
• MENDEL'S OBSERVATIONS:

  • In the mid-19th century a monk called Gregor Mendel (1822-1884) studied the inheritance of different characteristics in pea plants. He also carried out work with flowers.
  • Mendel discovered that when he bred red-flowered plants with white-flowered plants, all of the offspring had red flowers.
  • If he bred these red flowering plants with each other, most of the offspring had red flowers, but some had white flowers.
  • We now know that the reason behind this observation is because the allele for red flowers is dominant, and the allele for white flowers is recessive.
• MEDEL'S OBSERVATIONS:

  The genetic diagram shows the outcome of Mendel's first cross of red and white flowers. All of the offspring have red flowers (100%), even though they are heterozygous and carry the recessive allele for white flowers (Ff).

  When Mendel crossed two of these offspring together, he obtained the results shown in the genetic diagram below.

  Three-quarters (75%) of the offspring have red flowers (FF and Ff) and a quarter (25%) have white flowers (ff).
• UNDERSTANDING GENETICS:

  Mendel's work expanded our knowledge of genetic inheritance before DNA had even been discovered.

  Mendel's work, however, was not accepted by most scientists when he was alive for three main reasons:

  1. When he presented his work to other scientists he did not communicate it well so they did not really understand it.
  2. It was published in a scientific journal that was not well known so not many people read it.
  3. He could not explain the science behind why characteristics were inherited.
• MENDEL:

  The idea that genes were located on chromosomes emerged in the late 19th century when better microscopes and staining techniques allowed scientists to visualise the behaviour of chromosomes during cell division.

  • In the early 20th century, it was observed that chromosomes and Mendel's 'units' behaved in similar ways.
  • This led to the theory that the 'units', now called genes, were located on chromosomes.

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• WATSON & CRICK:

  • James Watson and Francis Crick worked out the structure of DNA.
  • Rosalind Franklin and Maurice Wilkins produced data and Watson and Crick built a model of DNA.
  • Bases occur in pairs and there are two chains wound into a double helix.
  • In the early 21st century, the entire human genome was sequenced. Scientists are now working out the functions of our different genes.
  • Many years of work from different scientists focusing on DNA, chromosomes and genes have led us to the possibility of treating genetic conditions by changing our genes. This is called gene therapy.