Biology (Year 12) - Genetics
There are four main modes of single-gene inheritance which can be observed in human populations, including;
Sex-linked recessive The two modes of inheritance in which we will focus on are autosomal dominant and autosomal recessive. This type of inheritance occurs on autosomal chromosomes, i.e., autosomes.
In autosomal dominant inheritance, a single dominant allele is responsible for the occurrence of a phenotype. Each affected person usually has an affected parent, and the phenotype occurs in every generation. A single copy of the affected allele is enough to cause the condition. A parent with a one copy of a dominant allele on an autosome, being heterozygous for the trait, will theoretically pass it on to 50% of the offspring. If the parent is homozygous dominant for the trait, then 100% of the offspring will be affected.
Case Study: Autosomal dominant pedigree
There are a few symbols/notations which you need to be aware of in order to understand pedigree charts.
Circle = Female
Square = Male
Line = Union of gametes
Shaded = Affected
Unshaded = Unaffected
In this example, let's say that the trait is for having a widow's peak (a v-shaped hairline which points to the centre of the head). We can use the notation Hh (as using the letter W is too similar in it's capitalised and non-capitalised forms; markers may not be able to read your workings!), where H is the dominant allele and codes for a widow's peak, and h is the recessive allele which codes for a straight hairline.
In the pedigree, we can see that the male parent has the trait, so he must either be homozygous dominant (HH) or heterozygous (Hh) for the trait, whilst the female parent does not have the trait, and so must be homozygous recessive (hh). If the male parent was homozygous dominant for the trait, then 100% of offspring would have the trait which is not the case indicated by the unaffected female; and so we can determine that he must be heterozygous for the trait.
Autosomal dominant inheritance occurs independently of biological sex (sex chromosomes).
Recessive inheritance of a disease means both genes in a pair must be recessive in order to cause the disease. People with only one defective allele in the pair of alleles are called carriers, and in most cases these people are not affected by the condition as a dominant allele will silence the effects of the recessive allele that causes the condition. They can pass the abnormal gene on to their children. The parents of an affected person are always at least carriers of the allele. Autosomal recessive inheritance will also often skip generations.
Case Study: Autosomal recessive patterns
In the pedigree above, let's say that the condition which is autosomal recessive is having attached ear lobes, and the dominant trait is having unattached ear lobes; we'll use the notation Aa, where A is the dominant trait and a is the recessive trait. The two parents are shown to be unaffected by the condition, meaning that they have at least one dominant allele. We can confirm whether they're heterozygous (Aa) or homozygous dominant (AA) by looking at the offspring; as there are affected offspring present, meaning that they are homozygous recessive (aa), both parents must be heterozygous for the trait as they have each had to pass one recessive allele onto their offspring.