Genotype and Phenotype

Section 1 of 7Genotype versus phenotype

Term	Meaning	Example
Genotype	The combination of alleles an organism has	Bb
Phenotype	The characteristic that results and can be observed	Brown eyes
Homozygous	Two identical alleles	BB or bb
Heterozygous	Two different alleles	Bb

Section 2 of 7What shapes a phenotype

Phenotype

Genotype

• Genotype The alleles inherited from both parents set the potential for a feature, such as eye colour.

Environment

• Environment Diet, climate, exercise and injury can alter features such as height, weight or skin colour.

Both together

• Both together Many characteristics, such as body mass, depend on genes and environment, while others such as blood group depend only on genes.

Section 3 of 7Inherited disorders

Polydactyly

• Polydactyly Extra fingers or toes caused by a dominant allele.
• A person needs only one copy to have the condition, so an affected parent can pass it on to children.

Cystic fibrosis

• Cystic fibrosis A disorder of cell membranes that produces thick mucus in the lungs and gut, caused by a recessive allele.
• Only people with two copies have the disease.

Carriers

• Carriers A heterozygous person with one recessive disorder allele shows no symptoms but can pass the allele to children.

Section 4 of 7Predicting cystic fibrosis risk

1

Let F be the normal allele and f the cystic fibrosis allele.

2

Two carrier parents each have the genotype Ff.

3

Cross them: possible children are FF, Ff, Ff and ff.

4

The chance of a child with cystic fibrosis (ff) is 1 in 4, or 25%.

5

The chance of a child being a carrier is 2 in 4, or 50%.

Section 5 of 7Screening and decisions

1Embryo or adult screening

• Embryo or adult screening Tests can show whether a person or embryo carries a disorder allele.

2Information gained

• Information gained Families learn their risk and can plan treatment or care.

3Ethical questions

• Ethical questions Issues include privacy, insurance, cost and whether it is right to select embryos.

Section 6 of 7Key terms

Term	Meaning
Genotype	The alleles an organism carries for a gene.
Phenotype	The observable characteristics of an organism.
Carrier	A person with one copy of a recessive disorder allele who shows no symptoms.
Polydactyly	An inherited condition with extra fingers or toes.
Cystic fibrosis	A recessive inherited disorder producing thick, sticky mucus.

Section 7 of 7Exam technique

1

Using genotype and phenotype as if they were the same thing.

2

Saying cystic fibrosis is caused by a dominant allele; it needs two recessive alleles.

3

Assuming every feature is fixed by genes alone, ignoring the environment.

4

Thinking carriers have the disease; they have one recessive allele and no symptoms.