Your genotype is fixed for life. Your phenotype is not. This page covers how the environment reaches in and changes gene expression, why two healthy parents can have a child with a genetic condition, and what happens when a gene has more than two alleles.
📚 What you need to know
Phenotypic plasticity: the genotype stays the same, but the way genes are expressed — and so the phenotype — can change during life.
Light, temperature, diet, drugs and hormones all change how much of a gene product is made.
An autosomal recessive condition needs two copies of the faulty allele. Two heterozygous carriers have a 1 in 4 chance per child.
Carriers show no symptoms — one working allele makes enough working protein.
Alleles differ by only a few bases; those exact positions are single nucleotide polymorphisms (SNPs).
A gene can have many alleles in a population, but any diploid individual still carries only two. ABO blood group has three: IA, IB and i.
The environment changes gene expression
Genes are not switched on all the time at the same level. Cells control how much of each gene is transcribed, and that control responds to what is happening outside the cell.
Two examples worth learning:
Sunlight and skin. Ultraviolet light activates the enzymes that make melanin, so more pigment is produced and the skin darkens. The alleles for skin colour have not changed at all.
Temperature and fur colour. In the Himalayan rabbit, the enzyme that makes dark pigment only works in cooler conditions. The animal’s ears, feet, tail and nose lose heat fastest, so those parts are dark and the warm body stays pale.
Chain to remember. Environmental signal → change in regulatory proteins or transcription factors → change in how much protein is made → change in phenotype. The DNA sequence never changes. Say that chain and you have the marks.
Watch the wording. Plasticity is about expression, not mutation. If your answer says the environment “changed the genes”, you have described a mutation instead and lost the mark.
Inheriting a recessive condition
Many genetic conditions are caused by a recessive allele on an autosome — any chromosome other than the sex chromosomes. Phenylketonuria (PKU) and cystic fibrosis are both like this.
In PKU the faulty allele codes for a non-working version of the enzyme that breaks down the amino acid phenylalanine. Phenylalanine builds up in the blood and, without treatment, damages the developing brain. Newborns in many countries are screened for it with a heel-prick blood test in the first few days of life.
The key genetics point: a heterozygote is healthy. One working allele makes enough working enzyme. That person is a carrier, and can pass the faulty allele on without ever knowing they have it. So a child can be born with the condition to two parents who show no symptoms at all.
Two of the four boxes are carriers, so 2 in 4 children are expected to be carriers — twice as likely as being affected.
Gametes
A
a
A
AA — unaffected
Aa — carrier
a
Aa — carrier
aa — affected
Alleles differ by only a few bases
Two alleles of the same gene sit at the same locus and are nearly identical. They may differ at a single base position. That is enough: one changed base can change one amino acid, which can change the shape of the protein and stop it working.
The exact positions where alleles differ are called single nucleotide polymorphisms, or SNPs. Since the human genome was sequenced, SNPs can be read directly and used to tell alleles apart, to track inheritance in families, and to look for links between alleles and disease risk.
Multiple alleles: the ABO blood group
Most of the examples so far had two alleles. Plenty of genes have more — a population can carry many versions. The individual is still limited to two, because you only have two copies of the chromosome.
The ABO gene has three common alleles:
IA — makes antigen A on the red blood cell surface
IB — makes antigen B on the red blood cell surface
i — makes no antigen at all, and is recessive to both of the others
IA and IB are codominant with each other, so someone carrying both makes both antigens and has blood group AB. Getting this wrong in a transfusion is dangerous: the immune system attacks red cells carrying an unfamiliar antigen and the blood clots.
Blood group
Possible genotypes
Antigens on red cells
A
IAIA or IAi
A only
B
IBIB or IBi
B only
AB
IAIB
A and B
O
ii
none
Group O is the only blood group where the genotype is certain from the phenotype — it has to be ii. For groups A and B there are always two possible genotypes, so never write just one unless the question gives you extra information.
Worked examples
WORKED EXAMPLE 1
Both parents are carriers of PKU. They already have one child with PKU. Calculate the probability that their next child also has PKU, and explain your answer.
Step 1: genotypes
Carrier means heterozygous: Pp × PpStep 2: gametes and grid
Each parent makes P or p → PP, Pp, Pp, ppStep 3: which box is affected?
Only pp, which is 1 box out of 4
Probability = 1/4 (25%)The first child makes no difference. Fertilisation is a fresh random event each time, so the odds start again at 1 in 4.
WORKED EXAMPLE 2
A woman of blood group A and a man of blood group B have a child of blood group O. Deduce the genotypes of both parents.
Step 1: start with the child
Group O has only one genotype: iiStep 2: trace the alleles
The child got one i from the mother and one i from the father
Step 3: fit that to the parents’ groups
Mother shows A, so she must also carry IA. Father shows B, so he must also carry IBMother IAi, father IBiTheir four possible children are AB, A, B and O — one of each, in a 1 : 1 : 1 : 1 ratio.
Gametes
IB
i
IA
IAIB — group AB
IAi — group A
i
IBi — group B
ii — group O
💡 Exam tip
“Carrier” means heterozygous and healthy. Use the word only for recessive conditions.
Each pregnancy is independent. Any question that mentions earlier children is testing this — the probability does not change.
For blood group crosses, write I with the superscript every time. Writing A and B alone loses the codominance.
For plasticity, name the environmental factor and the gene product it affects. Vague answers about “the environment” score nothing.
Read whether the question wants a fraction, percentage or ratio — 1/4, 25% and 1 in 4 are the same, but examiners sometimes ask for one form.
⚠ Common mix-up
Saying carriers have mild symptoms. They have none — the working allele covers the job completely.
Thinking a diploid individual can carry three alleles. The population has three; you get two.
Claiming the environment “changes the DNA”. It changes expression, not sequence.
Forgetting that i is recessive to both IA and IB. Codominance only applies between IA and IB.
Writing “25% of their children will have PKU” as if it were guaranteed. Say “expected” or “probability”.
Mixing autosomal with sex-linked. Autosomal conditions affect males and females equally often.
Up next: Sex Determination — how the X and Y chromosomes decide sex, and why sex-linked conditions turn up far more often in males.
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