CIE Categories Archives: 17. Inheritance

Monohybrid inheritance

17.5) 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.


  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



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.
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17.4) Meiosis

17.4) 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:

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17.3) Mitosis

17.3) 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.

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17.2) Chromosomes, genes and proteins

17.2) 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.

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17.1) Inheritance


Inheritance is the transmission of genetic information from generation to generation.

The Inheritance of such characteristics is called heredity and the branch of biology that studies how heredity works is called genetics.

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