Structure And Inheritance Of Genes Essay

1069 words - 5 pages

Explain the structure and inheritance of genes (15 marks)
A cell’s nucleus contains chromosomes, which are long, coiled packaged and organized chromatin (molecules of DNA).
Chromatin is a complex of molecules found in cells, consisting of DNA, protein and RNA. Proteins are polypeptides (a large sequence of amino acids). The chief protein component of chromatin are histones, which package and order the DNA into structural units called nucleosomes in order to fit in the cell, as well as reinforce the DNA molecule to allow mitosis to occur to prevent DNA damage. Another primary function of chromatin is to control gene expression and DNA replication. Chromosomes are made up of two homologous ...view middle of the document...

Monohybrid inheritance is where an individual inherits a characteristic which is determined by a single gene with two alleles. An individual can inherit an allele that is not expressed in the phenotype (a physical characteristic) but still be apparent in the genotype (ones genetic makeup) when combined with another allele. This is called a recessive allele. When combined with an allele that is expressed in the phenotype, the individual is heterozygous. It is however possible for recessive alleles to be expressed in the phenotype, however this only occurs if there are two of the same recessive allele, which is called being homozygous recessive. However alleles that are both expressed in the phenotype and genotype are inherited. These are called dominant alleles. It is possible to inherit two of these alleles causing the individual to be homozygous dominant.
Co-dominance however allows for both alleles to be expressed in the phenotype, for example, in the ABO blood group system, chemical modifications to a glycoprotein (the H antigen) on the surfaces of blood cells are controlled by three alleles, two of which are co-dominant to each other (IA, IB).
It is also possible for there to be more than two possible phenotypes through multiple alleles. Multiple alleles mean that there are more than two alleles that code for a certain characteristics. This essentially causes an inheritance pattern which involves more than two alleles that usually results in several phenotypes.
Monohybrid inheritance can be expressed via a monohybrid genetic cross, as can be seen below where the offspring between homozygous dominant and homozygous recessive individual is displayed:

(R = dominant allele) (r = recessive allele)
| r (homozygous recessive) | r (homozygous recessive) |
R (Homozygous dominant) | R r (heterozygous) | R r (heterozygous) |
R (Homozygous dominant) | R r (heterozygous) | R r (heterozygous) |

As you can see here the offspring have a 100% chance to be heterozygous, and will therefore express the dominant allele.

Sex inheritance allows for alleles to appear in the phenotype where usually would not be possible. There is a 50/50 chance of either producing male or female offspring, as seen below.
| (Male) X | (Male) Y |
(Female) X | X X (Female) | X Y (Male) |

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