Genetics 1C
Genotype and Phenotype

The genotype of the bee is derived from its complete assemblage of genes. Some of these genes will remain hidden and hence will not appear to be present. But they may appear in future generations. The genes are copied at the time of cell division or reproduction and are passed from one generation to the next. But each generation produces its own genotype.

The genome codes to produce the phenotype. This represents the bee's actual observable properties which for the most part can be measured: morphometry, colour, length of tongue, hygienic behaviour. etc.
The phenotype can be affected by the environment; such factors include diet, climate, illness and stress.

It is this interplay between the genotype and the phenotype of the bee which makes each bee unique.
Genetics - some basic comcepts

The Gene

Simply put a gene is a segment of the DNA situated on a specific site on a chromosome called the locus which codes for a molecular entity that has a function. That entity could be a protein or RNA is responsible for the physical and inheritable characteristics or phenotype of the bee. In the early days of genetics it was postulated that a single gene coded for a single protein. However, it is now know that a gene has a number of functions. Fundamentally it codes for an RNA molecule which may be an end in itself or it may go on to direct the synthesis of a protein. The gene can also be spiced in a number of ways so that it is coding for more than one protein. Before looking at how a gene encodes for a protein just to emphasise the difference between the genotype and phenotype.
genotype - phenotype relationship
Credit for the DNA sequence: Darryl Leja, NHGRI
When two chromosomes are joined together, each is called a chromatid. When the chromatids are identical as they are after DNA replication during interphase they are referred to as sister chromatids. When these are different they are called non-sister chromatids.

Alleles

An allele is an alternative form of a gene (one member of a pair) that is located at the locus on the chromosome. Alleles arise over time by mutation.

A chromosome homozygous for a gene (as in the left diagram) will have two copies of the same allele for that gene; e.g. AA, BB, and CC

A chromosome heterozygous for a gene (as in the right diagram) will have two different alleles; e.g. Aa, Bb, and Cc

Dominant and Recessive Genes

An allele can be dominant or recessive. A dominant allele is usually represented by a capital letter. i.e. in the above examples A, B, and C would be the dominant alleles; a, b, and c the recessive alleles
The Chromosome

The genome represents the total number of genes processed by the organism; these are stored on the chromosomes. A honey bee for example has 32 chromosomes arranged in 16 pairs. The honey bee as an example of a eukaryote must package its DNA in a condensed form such that it can be packed into a very small space in the cell nucleus. However, this packaging must ensure that during DNA replication and transcription the DNA helix must not only be unwound but also be acted on very rapidly. The basic relationships between the DNA helix and the structure of the chromosome is illustrated below.
chromosome characteristics
The diagram illustrates the structure of the chromosome. The coloured banding represents the genes. Each gene is positioned at a specific position on the chromosome called the locus.
The centromere is a region to which the spindle fibers attach to the chromosome during mitosis and meiosis. The centromere occupies a characteristic position that is constant for different types of chromosomes. Thus the centromere is important for studying and identifying chromosomes.
The end of the chromosome in eukaryotes is called a telomere.
This region is important because during DNA replication, the telomere does not always get duplicated properly and the chromosome shortens slightly. The telomere contains many repeating sections of DNA rather than regions of DNA that code for specific genes.
In the bee chromosome the repeat unit is (TTAGG)n.
The beginning of the condensation process starts with DNA being supercoiled around itself. Nucleosomes are next formed when the DNA is wrapped around an octomer of histone proteins which are crucial to the structure of the chromosome. The nucleosome are joined together by short strips of linker DNA forming chromatin rather like beads on a chain. The nucleosomes coupled together finally form chromatin fibre so found on the chromosome.
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