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IGCSE BIOLOGY TOPIC 17 INHERITANCE

Summary notes

Key words:
● Inheritance – the transmission of genetic information from parents to offspring.
● Chromosome – a thread-like structure of DNA that carries genetic information in the
form of genes.
● Gene – a length of DNA that codes for a particular protein.
● Allele – one of a number of different versions of a gene.
● Genotype – the genetic makeup of an organism, consisting of all the alleles present.
● Phenotype – the observable features of an organism as a result of the expression of
particular alleles of the gene.
● Homozygous – an organism containing two identical alleles of a particular gene.
● Heterozygous – an organism containing two different alleles of a particular gene.
● Diploid nucleus – nuclei which contain a full set of chromosomes (23 pairs).
● Haploid nucleus – nuclei which only contain half the number of chromosomes. These
cells are egg and sperm cells, which fuse during fertilization to produce a diploid cell.
There are 23 pairs of chromosomes in the nucleus of all diploid human cells. One of these pairs
determine gender. These chromosomes are therefore called the sex chromosomes. For
females these chromosomes are XX and for males the chromosomes are XY. All female egg cells
contain only an X chromosome, whereas male sperm cells may contain an X or a Y, thus the
gender of the baby depends on which sperm cell fertilizes the egg cell.
Protein synthesis:
DNA controls the function of the cell by regulating protein synthesis, antibodies and receptors
for neurotransmitters. Protein synthesis is important to maintain cell structure, as well as for
producing enzymes to catalyze metabolic reactions:

  1. The gene which codes for the protein is used to make an mRNA copy in the nucleus.
    mRNA is similar to a single strand of DNA, although contains the base U rather than T.
  2. The mRNA copy leaves the nucleus and travels through the cytoplasm to a ribosome.
  3. The ribosome uses the mRNA strand to produce a chain of amino acids which make up
    the protein. The order of the amino acid chain is determined by the order of bases on
    the mRNA. This order is specific to each protein made.
    As each type of cell has a different function, not every type of protein is synthesized in every
    cell. The genes to code for each protein are present in every cell, however. These genes are
    therefore switched ‘on’ or ‘off’. When the gene is switched on, it is expressed, and the protein
    associated with the gene is synthesized. When the protein is not required, the gene is switched
    off.

    Mitosis:
    Mitosis is a form of cell division. During mitosis, nuclear division of a parent cell occurs,
    producing two genetically identical daughter cells. Mitosis is used to create new cells in the
    body to repair and replace old and damaged tissues, as well as allowing growth of the
    organism and playing a role in asexual reproduction. Mitosis involves the splitting of
    chromosomes into their two halves, each of which are known as a chromatid:
  4. Before replication can occur, the chromatids in the parent cell must replicate to
    produce identical copies of themselves which pair as chromosomes.
  5. The chromosomes line-up along the nucleus.
  6. The chromosomes separate so that each identical chromatid is pulled to opposite poles
    of the cell.
  7. The cell membrane constricts in the middle and the nucleus is separated as the cell
    splits into two new daughter cells. Each of these cells will contain a set of chromatids,
    which then replicate themselves again to produce new chromosomes.
    Stem cells:
    Stem cells are found in embryos or in the bone marrow. These cells are unspecialized and
    divide by mitosis to produce daughter cells which then specialize to have a variety of functions.
    Cells, once specialized, cannot produce unspecialized cells. For example, a cell which makes up
    the heart tissue cannot divide to make a cell which can act as skin tissue as the cell produced
    will already be specialized as a heart cell.
    Meiosis:
    Meiosis is used to make four genetically unique daughter cells and is used in the production of
    gametes. During meiosis, the chromosome number is halved, and a diploid cell divides to
    produce four haploid cells. As each gamete produced is genetically unique, each of the
    offspring will also be unique. This is beneficial for a species as it produces genetic variation.
    Monohybrid inheritance:
    Offspring inherit characteristics from both their mother and father and two sets of genes are
    inherited, one from each. If the mother and father pass down the same allele for a particular
    trait, e.g. they both pass down the allele for blue eyes, the offspring will have two identical
    alleles for this trait, which is referred to as homozygous. If two separate alleles are passed
    down, e.g. the mother has blue eyes and the father has brown eyes, the offspring will have two
    different alleles for the gene, which is called heterozygous.

    If two identical homozygous individuals are bred together, it is referred to as pure-breeding,
    and the offspring will have the same characteristics as the parents. Breeding heterozygous
    parents is not pure-breeding, as there are a number of different alleles that the offspring could
    display in their phenotype.
    Alleles can be dominant and recessive. A dominant allele is always expressed if present,
    whereas the recessive allele is only expressed in the absence of the dominant allele. For
    example, the allele for brown eyes is dominant and the allele for blue eyes is recessive. This
    means that if two parents with blue and brown eyes were bred, the offspring would have
    brown eyes as this allele is dominant.
    Monohybrid crosses:
    Monohybrid crosses are used to predict the ratios of inherited characteristics in a population.
    There are always four outcomes. The dominant allele is written as a capital letter and the
    recessive as the lowercase of that letter.
    E.g.1) Crossing two heterozygous green and yellow pea plants (both parents contain the alleles
    for both traits):
    G= green (dominant allele)
    g= yellow (recessive allele)
    Parent one →
    Parent two ↓
    G g
    G GG Gg
    g Gg gg
    The outcomes are GG, Gg, Gg and gg. As G is dominant, there is a 75% chance that the offspring
    will display this allele in the phenotype and be green. There is a 25% chance that the offspring
    will be yellow therefore the ratio is 3:1.
    E.g.2) Crossing a homozygous recessive (yellow) pea plant with a heterozygous pea plant:
    G= green (dominant allele)
    g= yellow (recessive allele)
    Parent one (homozygous) →
    Parent two (heterozygous) ↓
    g g
    G Gg Gg

    g gg gg
    The outcomes are Gg, Gg, gg and gg. There is a 50% chance of the offspring being green or
    yellow, therefore the ratio is 1:1.
    Co-dominance:
    Some alleles are co-dominant, meaning that neither is recessive, and they are both displayed
    in the phenotype. An example of this is blood groups. The three possible alleles for blood
    groups are A, B and O. The A and B alleles are co-dominant, which leads to the AB blood group.
    O is recessive, and thus is only displayed in the phenotype if both parents have O blood groups.
    When writing codominant alleles, a capital letter is used to show the gene, and a superscript
    letter is used to denote the allele, e.g. Cw
    Sex-linked characteristics:
    Some genes are located on the sex chromosomes. A characteristic which comes from one of
    these genes is referred to as a sex-linked characteristic. A result of this is that some traits are
    more common to one gender, for example any gene located on the Y chromosome can only be
    present in males as females do not have this chromosome. An example of a sex-linked
    characteristic is colour blindness, which is a recessive characteristic found on the X
    chromosome.

Chromosomes, genes and proteins

Chromosome is a thread of DNA, made up of a string of genes.

Genes is a length of DNA that codes for a protein.

Allele is a version of a gene.

A human body (somatic) cell nucleus contains 23 pairs of chromosomes.

These are difficult to distinguish when packed inside the nucleus, so scientists separate them and arranged them according to size and appearance. The outcome is called karyotype.

One of these pairs controls the inheritance of biological gender – whether offspring are male or female:

  • males have two different sex chromosomes, X and Y
  • females have two X chromosomes, XX

The ratio of female to male offspring is 1:1 – on average, half of the offspring will be girls and half will be boys

The genetic code:

  • Each nucleotide carries one of four bases (A, T, C or G). a string of nucleotides therefore holds a sequence of bases. This sequence forms a code, which instructs the cell to make particular proteins.
  • Proteins are made from amino acids linked together. The type and sequence of the amino acids joined together will determine the kind of protein formed.
  • Its is the sequence of bases in the DNA molecule that decides which amino acids are used and in which order they are joined. Each group of three bases stands for one amino acid.
  • A gene, then, is a sequence of triplets of the four bases, which specifies an entire protein.
  • The chemical reactions that take place in a cell determine what sort of a cell it is and what its functions are. These chemical reactions are, in turn, controlled by enzymes.
  • Enzymes are proteins. It follows, therefore, that the genetic code of DNA, in determining which proteins, particularly enzymes, are produced in a cell, also determines the cell’s structure and function. In this way, the genes also determine the structure and function of the whole organism.
  • Other proteins coded for in DNA include antibodies and the receptors for neurotransmitters.

The manufacture of proteins in cells:

  • DNA molecules remain in the nucleus, but the proteins they carry the codes for are needed elsewhere in the cell. A molecule called messenger RNA (mRNA) is used to transfer the information from the nucleus.
  • mRNA is much smaller than a DNA molecule and is made up of only one strand. Also it contains slightly different bases (A,C,G and U). Base U is uracil.
  • To pass on the protein code, the double helix of DNA unwinds to expose the chains of bases.
  • One strand acts as template. A messenger RNA molecule is formed along part of this strand, made up of a chain of nucleotides with complementary bases to a section of the DNA strand.
  • The mRNA molecule carrying the protein code then passes out of the nucleus, through a nuclear pore in the membrane. Once in the cytoplasm it attaches itself to a ribosome.
  • Ribosomes make proteins. The mRNA molecule instructs the ribosomes to put together a chain of amino acids in a specific sequence, thus making a protein.

Gene expression:

  • Body cells do not all have the same requirements for proteins. For example, the function of some cells in the stomach is to make the protein pepsin. Bone marrow cells make the protein haemoglobin, but do not need digestive enzymes.
  • Specialised cells all contain the same genes in their nuclei, but only the genes needed to code for the specific proteins are switched on (expressed). This enables the cell to make only the proteins it needs to fulfil its function.

Number of chromosomes:

Haploid nucleus: is a nucleus containing a single set of unpaired chromosomes present, for example, in sperm and egg cells.

Diploid nucleus: is a nucleus containing two sets of chromosomes present, for example, in body cells.

In a diploid cell, there is a pair of each type of chromosome and in a human diploid cell there are 23 pairs.

Mitosis

Mitosis: is nuclear division giving rise to genetically identical cells.

  • Cells have a finite life: they wear out or become damaged, so they need to be replaced constantly.
  • The processes of growth, repair and replacement of cells all rely on mitosis.
  • Organisms that reproduce asexually also use mitosis to create more cells.

The process of mitosis:

  • Each chromosome duplicates itself and is seen to be made up of two parallel strands, called chromatids.
  • When the nucleus divides into two, one chromatid from each chromosomes and later they will make copies of themselves ready for the next cell division.
  • The process of copying is called replication because each chromosome makes a replica of itself.
  • Mitosis produces two genetically identical cells in which the number of chromosomes is the same as in the original cell.

Stem cells are those cells in the body that have retained their power of division. Examples are the basal cells of the skin, which keep dividing to make new skin cells, and cells in the red bone marrow, which constantly divide to produce the whole range of blood cells.

Meiosis

Meiosis: is nuclear division, which gives rise to cells that are genetically different.

  • Meiosis takes place in the gonads of animals (eg. the testes and ovaries of mammals)
  • The cells formed are gametes (sperm and egg cells in mammals). Gametes are different from other cells because they have half the normal number of chromosomes (they are haploid).
  • Meiosis produces four genetically different haploid cells. Unlike mitosis, meiosis is a reduction division – the chromosome number is halved from diploid
  • As a result of meiosis and fertilisation, the maternal and paternal chromosomes meet in different combinations in the zygotes. Consequently, the offspring will differ from their parents and from each other in a variety of ways.

Mitosis and meiosis compared:

Monohybrid inheritance

Allele: is a version of a gene.

Genotype: is the genetic makeup of an organism in terms of the alleles present.

Phenotype: is the observable features of an organism.

Homozygous: is having two identical alleles of a particular gene. Two identical homozygous individuals that breed together will be pure-breeding.

Heterozygous: is having two different alleles of a particular gene. A heterozygous individual will not be pure-breeding.

Dominant: is an allele that is expressed if it is present.

Recessive: is an allele that is only expressed when there is no dominant allele of the gene present.

Pedigree diagrams and inheritance:

Pedigree diagrams are similar to family trees and can be used to demonstrate how genetic diseases can be inherited.

They include symbols to indicate whether individuals are male or female and what their genotype is for a particular genetic characteristic.

Testcross:

  1. the organism with the dominant trait is always crossed with an organism with the recessive trait
  2. if ANY offspring show the recessive trait, the unknown genotype is heterozygous
  3. if ALL the offspring have the dominant trait, the unknown genotype is homozygous dominant
  4. large numbers of offspring are needed for reliable results

Co-dominance:

If both genes of an allelomorphic pair produce their effects in an individual (ie. neither allele is dominant to the other) the alleles are said to be co-dominant.

The inheritance of the human ABO blood groups provides an example of codominance.

The gene controlling human ABO blood groups has three alleles, not just two:

  • I^A and I^B are not dominant over one another
  • both are dominant over I^O

The table shows the possible genotypes (alleles present) and phenotypes (blood group).

Since the alleles for groups A and B are dominant to that for group O, a group A person could have the genotype I^AI^A or I^AI^O. Similarly for group B. There are no alternative genotypes for groups AB and O.

Sex linkage:

Sex-linked characteristic is one in which the gene responsible is located on a sex chromosome, which makes it more common in one sex than the other.

Colour blindness is an example:

  • In the following case, the mother is a carrier of colorblindness (X^CX^c). This means she shows no symptoms of colour blindness, but the recessive allele causing color blindness is present on one of her X chromosomes.
  • The father has normal colour vision (X^CY).
  • If the gene responsible for a particular condition is present only on the Y chromosome, only males can suffer from the condition because females do not possess the Y chromosome.
  • F1 genotypes: X^CX^C X^CX^c  X^CY  X^cY
  • F1 phenotypes: 2 females with normal vision; 2 males, one with normal vision, one with colour blindness.