Polydactyly and cystic fibrosis, carriers, chances for a couple's children, and the arguments for and against screening embryos.
What you will learn
- Explain how polydactyly and cystic fibrosis are inherited
- Work out the chance a child inherits a disorder
- Evaluate embryo screening using ethical and economic arguments
Two inherited disorders
Polydactyly (extra fingers or toes) is caused by a dominant allele, P.
One copy is enough, so there are no healthy carriers: Pp and PP both have it. A person with polydactyly whose parent does not have it must be Pp, because that parent could only give p.
Cystic fibrosis is a disorder of cell membranes caused by a recessive allele, f. Only ff has it. Ff people are carriers: they do not have the disorder but can pass on f.
- Two carriers: Ff × Ff
- Boxes: FF, Ff, Ff, ff
- Cystic fibrosis (ff): 1 of 4 = 25%
- Carrier (Ff): 2 of 4 = 50%
Across a population, multiply the chances. If 1 in 25 people is a carrier, two unrelated people are both carriers with a chance of 1/25 × 1/25 = 1/625, and then each child has a 1/4 chance: 1/625 × 1/4 = 1 in 2500.
A carrier does not have the disorder. When asked for the chance of a carrier, count only the Ff boxes.
The rest of this lesson and its 10 practice questions are locked.
In this lesson
- Two inherited disorders
- Screening embryos
10 practice questions, marked as you go; about 20 minutes in all.
Key terms
Polydactyly, Dominant, Cystic fibrosis, Recessive, Embryo screening, Evaluate
In this unit: Inheritance, variation and evolution
Meiosis and DNA, Punnett squares and family trees, inherited disorders and embryo screening, natural selection, selective breeding, genetic engineering and cloning, and classification.