Genetics 3
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.
genotype - phenotype relationship
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.
Credit for the DNA sequence: Darryl Leja, NHGRI
chromosome characteristics
A chromosome heterozygous for a gene will have two different alleles;
e.g. Aa, Bb, and Cc
The Chromosome

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.

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.
A chromosome homozygous for a gene will have two copies of the same allele for that gene; 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
Next page: Genetics 4.
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