Single gene inheritance notes

Single gene inheritance:

Genotype – set of genes that individual has, the alleles of all genes; genome contains ~30,000 genes
and different

Phenotype – detectable inherited traits/characteristics that are the outward expression of an
organism

Alleles - different variants of genes; changes in the genes gives rise to different phenotypes Variant
alleles – have changes in the DNA sequence

- Genes are contained on chromosomes
- All somatic cells have a paternal and a maternal copy – 2 copies of each gene
- The two different copies of the same gene are known as alleles
- Two copies of each chromosome means two copies of every gene
- Two different copies of the same gene are known as alleles, paternal and maternal allele of
each gene ( often identical but variance may occur leading to change in phenotype)

Variant allele occurs - Change in a DNA sequence leads to change in the encoded protein which
leads to change in phenotype

The most common allele is often known as the wild type. A wild type allele usually encodes a
functional protein. Thus wild type alleles will normally act dominantly over a variant allele that
produces an inactive form of the protein.

Variant or mutant alleles occur when there is a change or mutation in the allele DNA sequence.

A variant allele can change the function of the protein it encodes.

This change can cause a loss of function, there is no functional protein or a gain of function, the
protein gains additional activities. Both of these changes can create new phenotypes or be the
cause of genetic disease.

Allele and phenotype
new phenotypes (due to mutations) result from changes in functional activity of the cellular
product specified by that gene.

–Mutation may change overall enzyme shape and thus reduce affinity for substrate.

•Loss-of-function allele - mutation may cause a complete loss of function, e.gelimination of
substrate binding.

•Null allele - Some mutations may enhance allelic function.

Gain-of-function allele Usually increases quantity of gene product by affecting regulation of
transcription of the gene

Neutral mutation - Some mutations do not show any change in functio

, Wild type alleles = normal allele.

Recessive allele = allele whose phenotype can be masked. Recessive alleles are often loss-of-
function or null alleles, i.e. non-functioning versions

Dominant allele = allele whose phenotype can mask a recessive allele.

Wild type alleles generally act as dominant when in combination with a null allele, i.ethe normal
wild type allele dominates over a loss of function allele.



Dominant alleles
- Dominant alleles can also be gain-of-function alleles that enhance the genes activity
- This can occur as a result of an increase in activity of the gene
- Alternatively a dominant allele can have a new activity that can mask the activity of a
wild type allele.
- E.g. oncogenes often alleles with increased activity and drive increased cell division

Semidominantallele
- Incomplete or semidominantallele = allele that creates an intermediate phenotype when
heterozygous. E.g. a/a = recessive phenotype, A/a = intermediate phenotype, A/A =
dominant phenotype.
- Allele is not fully dominant, a single copy of the semidominantallele does not completely
mask (or compensate for) the recessive allele.
- One copy of the semidominantallele causes a phenotypic affect but it is not the same as
when the dominant allele is homozygous, also known as co-dominance.
- In some cases the wild type allele can be semi-dominant if one copy is insufficient to
provide full activity.
- e.g. petal colour in Antirrhinum

Co-dominant allele
- Co-dominant allele = allele that can be distinguished in the phenotype in the presence of
another allele.
- Co-dominant alleles produce different gene products both of which are detectable in the
phenotype
- E.g. blood groups A and B alleles are co-dominant since when both are present they can
both be detected.
- Both A and B allele are dominant over O allele but co-dominant with each other.
- Different alleles affect a gene coding for a glycosyltransferase
- Different alleles lead to addition of alternative sugar to glycolipid
- O = null allele which fails to add sugar