B11: Inheritance

B11.1: Chromosomes and genes

Terms

  • Inheritance: the transmission of genetic information across generations
  • Chromosomes: a thread-like structure of DNA, carrying genetic information in the form of genes
  • Gene: a length of DNA that codes for a protein
  • Allele: different forms of a gene (eg. black, blond, brown hair colour...)
  • Haploid Nucleus: a nucleus containing one set of unpaired chromosomes (like in gametees)
  • Diploid Nucleus: a nucleus containing two sets of paired chromosomes (like in body cells)
  • In diploid cells, chromosomes are arranged in 23 pairs

Inheritance of Gender

  • The gender is determined by an entire chromosome pair inherited from the mother and father
  • Females have sex chromosomes XX and Males have sex chromosomes XY
  • These chromosomes are from the egg and sperm cells of the parents
  • Egg cells always contain an X chromosome
  • However, half of sperm cells carry an X chromosome while the other half carries a Y sex chromosome
  • If a sperm cell with the Y chromosome fertilises the egg cell, the child will be a male (as it will have an X and Y sex chromosome pair)
  • If a sperm cell with the X chromosome fertilises the egg cell, the child will be a girl (as it will have a sex chromosome pair of two X sex chromosomes)


    • B11.2: Cell Division

    Mitosis

    • Mitosis is a type of cell division in which nuclear division giving rise to genetically identical cells
    • Mitosis is used in the body for growth, tissue repair, cell replacement and asexual reproduction
    • Cells in the body (except for gametes) originate from the division of the zygote through mitosis
    • Steps of Mitosis
      • The chromosomes duplicate themselves in the nucleus (to form two copies in the same cell)
      • The chromosomes are then split into two separate nucleus that have been split from the original one
      • The membrane of the cell folds to create two identical daughter cells, with both having one copy of the chromosomes


    Meiosis

    • Meiosis is a reduction division in which the chromosome number is halved from diploid to haploid resulting in genetically different cells
    • This means that the daughter cells in meisosis will have half the chromosomes of the original cell
    • Meiosis is the process used to produce gametes, and produces genetic variation (because of the different combinations of chromosomes that can be made)


    B11.3: Monohybrid Inheritance

    Terms

    • Genotype: the genetic make-up of an organism in terms of the alleles present
    • Phenotype: the observable features of an organism
    • Homozygous: having two identical alleles of a particular gene
    • Heterozygous: having two different alleles of a particular gene
    • Dominant: An allele is said to be dominant if it will be expressed even with the presence of other alleles (eg. dark hair is expressed if the allele for dark hair is present, regardless of a blond or red hair allele that may be present)
    • Dominant alleles are normally written with a capital letter
    • Recessive: An allele is said to be recessive if it can only be expressed when there is no dominant allele (eg. Blue eyes only are expressed if there are two copies of the blue eye alleles
    • Recessive alleles are normally written in a lower case letter


    Pure Breeding

    • Pure breeding happens when two parents with a pair of identical alleles mate
    • This will result into offspring that have the same particular phenotype as the parents
    • However, when a parent does not have two identical alleles, it will not perform pure-breeding, and will instead create variation


    Genetic Diagrams

    • Genetic Diagrams can help us work out the possible combinations of the alleles and the probability of certain phenotypes being displayed
    • Note that the recessive allele can only be displayed when the dominant allele is not present
    • Also note that any combination with the dominant allele will cause that allele to be displayed
    • A diagram that can help us with this is the Punnet Square


    Here are the steps to creating a Punnet Square

    1. Identify the two alleles that can be carried by the gametes from each of the parents (eg. a parent with a Gg genotype will have gametes with G and g alleles)
    2. Create a table like the following image (this is called a Punnet Square)
    3. Write the different combinations of genotypes of the offspring in the grids
    4. Identify the phenotype displayed in each outcome combination (you can work out the chance of a phenotype being displayed with this information)


    B11.4: Monohybrid Inheritance

    Terms

    • Variation is defined as the differences between individuals of the same species
    • Phenotypic variation is variation in the physical features that can be seen
    • Genetic variation is the variation of genotypes, which may not be able to be observed (eg. blood groups)
    • Phenotypic variation can be affected by their environment as well (eg. a tree without the right conditions will not grow tall despite having a gene that allows it to have the potential to grow tall)
    • Continuous variation is a type of variation that is not definite and is normally measured through a range (eg. height)
    • Discontinuous variation is the type of variation where the results are put in defined groups (eg, A, B, AB, O blood groups)
    • Discontinuous variation normally resulsts from only genetic factors
    • Mutation is the change in a gene or chromosome (this is how new alleles are formed, and are the source of genetic variation)
    • Mutations can be both harmful and beneficial
    • The rate of mutation increases with ionising radiation and chemicals
    • Mutation can occur by itself, or be caused by ionisingradiation or certain chemicals


    Natural Selection

    • Individuals with the features best adapted to the environment have the highest chances of survival and reproduction
    • Variation caused by the genes cause these differences in the ability to survive and reproduce
    • There is often more offspring produced than the environment can sustain, causing competition for resources
    • As organisms with better suited alleles have a higher chance to reproduce, the next generation also therefore tends to have better suited characteristics
    • Natural selection suggested this theory by Charles Darwin (this is known as the 'survival of the fittest')
    • Over different generations, adaptive features change, which is the process of adaptation
    • An example of adaptation is the development of bacteria resistant to antibiotics

    Selective Breeding

    • Selective Breeding is the selection of individuals with desirable features
    • The offspring will be more likely to have these desirable features
    • This process is repeated to produce generations of offspring with selected characteristics
    • There are two types of selective breding: natural and artificial
    • Natural Selection
      • Happens naturally and results in offspring having better suited features
      • Usually takes a long time
    • Artificial Selection
      • Happens with artificial means (involving human intervention)
      • Artificial selection produces offspring that aims to be useful to the general population, but not aimed towards survival
      • Takes less time
      • The selective breeding of plants in order for desired traits (eg. resistance, more produce, better taste...)
      • For example, wild brassica is selectively bred to produce plants like cababge, brocolli, brussel sprouts, and cauliflower


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