4.3: Theoretical genetics

• Genotype: the genes present or the set of alleles present, whether they produce an effect or not. 
• Phenotype: The appearance and functioning of an organism from the genes present and due to the effect of the environment. 
• Dominant allele: the form of a gene that will determine the resulting appearance in a heterozygous genotype. 
• Recessive allele: the form of a gene that is masked by the dominant allele in a heterozygous genotype. 
• Codominant alleles: the forms of a gene that have equal influence in a heterozygous genotype. 
• Locus: The location on a chromosome for a particular gene. 
• Homozygous: Both alleles for a particular gene are identical. 
• Heterozygous: the two alleles for a particular gene are different. 
• Carrier: A person heterozygous for a trait with a harmful recessive gene and not showing the characteristic though able to pass the allele to offspring.
• Test cross: Breeding sequence with a homozygous recessive to determine the genotype of an individual - whether the individual showing trait is homozygous or heterozygous.

Multiple alleles means there are more than two forms for a gene for a particular locus on a chromosome.

Multiple alleles for HLA genes means that each individual produces a specific range of proteins involved in the immune response. A successful organ transplant requires a suitable match between donor and recipient. Few HLA antigen matches in common between donor and recipient means there is a greater likelihood of organ rejection by the immune system of the recipient.

Humans have 46 chromosomes - 23 pairs. There are 22 pairs of autosomes and the 23rd pair of chromosomes are the sex chromosomes. Females are XX and males are XY.

The X chromosome is much larger than the Y chromosome and has more genes present. Most of the genes on the X chromosome are not concerned with sexual characteristics and most of these genes do not have a counterpart on the Y chromosome. The Y chromosome has the gene SRY (sex-determining region Y gene). The SRY gene causes male phenotype to develop and triggers testis development.

Sex linkage refers to genes on one of the sex chromosomes. Genes can be X-linked, e.g. red-green colour blindness or Y-linked, e.g. hairy ears.

Sex linkage refers to genes found on the sex chromosomes. When looking at problems associated with sex linkage, e.g. genetic diseases such as haemophilia, which occur more frequently in males as they have only one X chromosome, it is important to note the genes found on one of the sex chromosomes but not on the other. However, it is also important to look at the genes that are found on both sex chromosomes. The short sections at either end of the Y chromosome that are homologous with the corresponding parts of the X chromosome are very important as they allow the two sex chromosomes to act as an homologous pair, e.g. in meiosis in the testes.

Red-green colour blindness and haemophilia are X-linked traits. For a female to show the recessive form of these traits she needs both XX to carry the recessive gene, i.e. she needs to be homozygous. If she is heterozygous she is a carrier and does not show the trait. Males only have on e X and it is therefore more likely for a male to show the trait than the female.

• When constructing pedigree charts: 
• 1. Males are represented by squares and remales are represented by circles 
• 2. An individual with the charaterisitc under investigation, e.g. blue eyes, is shaded in. 
• 3. When drawing the family tree you always need to provide a key or legend to explain each structure 
• 4. The offspring from a couple are indicated from a vertical line.