Inheriting Alleles (HL IB Biology)

• Phenotypic plasticity is the idea that although genotype remains fixed throughout an organism's lifetime, the way that the phenotype is expressed can vary during this time
• An organism’s internal or external environment can influence gene expression patterns, and therefore phenotype
• The levels of regulatory proteins or transcription factors can be affected in response to environmental stimuli such as light, and chemicals including drugs and hormones
• For example, enzymes are activated in response to ultraviolet radiation and increase the expression and production of melanin, leading to skin pigmentation
• Temperature can also influence gene expression as demonstrated by organisms
  • The Himalayan rabbit (Oryctolagus cuniculus L.) possesses a gene for the development of pigmentation in its fur
    • The gene is inactive above 35°C but active between 15°C and 25°C
    • In the parts of the body that are cooler such as ears, feet and nose the gene becomes active making these areas black

• Phenylketonuria (PKU) is an inherited condition caused by a recessive allele on an autosome
• It is a condition that can lead to symptoms such as mental disorders and seizures
• It is caused by a build-up of the amino acid phenylalanine in the body 
  • Phenylalanine comes from broken down protein from diet and our cells
  • The enzyme phenylalanine hydroxylase breaks down phenylalanine
  • This enzyme is coded for by the PAH gene 
  • PKU is caused by a mutation to the PAH gene that results in a non-functional enzyme so that the phenylalanine does not get broken down
• In the UK around 1 in 10,000 people are born with PKU

• In order for a child to have PKU, they must first inherit two recessive alleles from each of their parents
• Because it is caused by a recessive allele it means that two non-PKU sufferers could have a child with PKU if both parents are heterozygous carriers of the mutated PAH gene
• An example genetic cross is shown below:

A genetic cross between two PKU carrier parents diagram

Diagram showing the parental phenotypes, genotypes, gametes and a Punnett square predicting the possible genotypes of their offspring. Both parents are PKU carriers and their offspring have a 25% chance of inheriting the disorder.

• The genetic cross shown on the Punnett square above demonstrates that the offspring of the PKU carrier parents have a 75% chance of not having PKU and a 25% chance of inheriting 2 PKU alleles and therefore having the condition
• This pattern of inheritance is the same with any autosomal recessive condition, for example cystic fibrosis
• Every baby born in the UK and in many other countries around the world are tested for several genetic conditions including PKU
  • The babies have a small prick of blood taken from the sole of their foot a few days after being born in order to be screened for the condition

• Many genes have more than two alleles
• However, a diploid individual will still only inherit two of the possible alleles
• Alleles differ from each other by one or only a few bases
• Even a very small change in base sequence can bring about a large effect in gene function, with a large knock-on effect on the phenotype
• Even though different alleles of a gene have slightly different base sequences, they still occupy the same locus on the chromosome
• Since the Human Genome Project, sophisticated techniques can analyse different alleles
• The exact positions where bases differ between alleles are called SNPs or snips (Single Nucleotide Polymorphisms)
  • An allele can have several SNPs but still only differ by a few bases from its other allele

• Inheritance of blood group is an example of co-dominance with multiple alleles
• This is of critical importance when deciding to give blood transfusions following injury or illness
• Use of the wrong blood group can cause an immune response that coagulates (solidifies) blood, leading to clots and serious illness/death
• There are three alleles of the gene controlling a person's blood group instead of the usual two
  • I represents the gene
  • Superscripts A and B represent the codominant alleles, I^A for example
  • Lowercase i with no superscript represents the recessive allele
• I^A results in the production of antigen A on the surface of red blood cells
• I^B results in the production of antigen B on the surface of red blood cells
• i results in no antigens being produced on the surface of red blood cells
• These three possible alleles can give us the following genotypes and phenotypes

Blood Genotype & Phenotype Table

• We can use genetic diagrams to predict the outcome of crosses that involve the codominant alleles controlling blood groups

Punnett square of the inheritance of blood group

Punnett square showing the inheritance of blood group with two heterozygous parents, type A and type B