Two offspring from the same parents often look different as they have different phenotypes. Phenotypes are the visual or biochemical characteristics of the organism, and they are the result of a combination of gene expression and environmental factors. 7

Within the human population everyone has the same genes, 9 a gene is a section of DNA that contains coded information, it is located at a particular position on the DNA molecule called a locus. A gene contains a specific sequence of nucleotide bases, 11 that come in threes called a codon along the DNA molecule that contain the information for making up the sequence of amino acids in polypeptides, one codon codes for one amino acid. 13 The sequence of amino acids and their peptide bonds dictate the primary structure of a protein, the secondary structure is controlled by weak hydrogen bonds and the tertiary structure of a protein is controlled by many bonds such as; hydrogen, covalent, ionic and disulphide bridges. It is the structure of a protein that determines its properties and biological function.

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On occasions a mutation occurs, this is a change to the sequence of nucleotide bases in the DNA molecule, there are many different types of gene mutation.
The first type is substitution of bases where a nucleotide in a section of DNA is replaced by another nucleotide with a different base, depending on which base is substituted there are three different outcomes; A stop codon is formed thus ending the polypeptide chain prematurely, or a codon for a different amino acid is formed. Both of these change the shape of the polypeptide chain, the bonds and therefore the tertiary structure if the protein affecting its properties and function. If the proteins no longer work as they are supposed to it can change how the genes are expressed in a person and therefore their phenotype. This change in proteins and gene expression can change the visual characteristics of a person and could be one of the reasons why two siblings sharing the same parents look different. The third outcome of substitution of bases is a different codon being formed but it produces the same amino acid, as most have more than one codon. This means there is no change to the protein and the mutation has no effect.
The second type is deletion of bases this is when one nucleotide is lost, this means that each three nucleotide bases are shifted to the left. This has a major impact and is called a frame shift as the genes are now read in the wrong three base groups. This causes a change in codons, the amino acids produced, polypeptide chains and the overall protein produced is different and normally not functional, this can have a dramatic effect on phenotype as the entire correct and functional protein is completley changed. This affect on phenotype can cause a huge impact on the visual characteristics of a person and can be a mjor factor in why two siblings from the same parents can look entirely differnet.

There are alternate forms of each gene, these are called alleles. 8 Normally two forms can exist but occasionally there are more this is called multiple alleles however as humans inherit only one allele from each parent they will only have two alleles for each gene.10 why homolgous chromosomes? Mutations, lost or replaced homo hetero dom recess co dom etc..

The nucleus of a eukaryotic cell is contains Chromatin, Chromatin when condensed is what forms Chromosomes which can only be seen during cell division, it is made of nucleosomes. 5 Nucleosomes are made up of negatively charged DNA wrapped around positively charged proteins called histones. Histones are proteins that help it condense into chromatin, the DNA and histones form a DNA-Histone complex, it is known are as a nucleosome. 3 Effect of histones on gene expression

Humans have 23 pairs of chromosomes, during reproduction there are two gametes, one paternal and one maternal. Both gametes are haploids and contain 23 chromosomes each. They fuse together to form a zygote which is a diploid which has 23 pairs of chromosomes.4 random fertilisation of gametes