Inheritance, pedigrees and genetic screening

What you’ll learn

• Use allele notation for dominant, recessive, codominant and X-linked inheritance.
• Calculate monohybrid probabilities and interpret pedigree evidence.
• Distinguish carrier screening, embryo testing and prenatal testing, including ethical considerations.

Understand the idea

Alleles are alternative versions of a gene. A genotype records the alleles, while phenotype is the observable outcome in a particular environment. Homozygotes have two matching alleles at a locus; heterozygotes have different ones. A dominant allele affects the heterozygote phenotype, a recessive phenotype requires the relevant absence of a dominant allele, and codominance means both alleles are expressed in a heterozygote. Dominant does not mean common, beneficial or stronger.

Gametes carry one allele at each locus, so a monohybrid cross lists the gametes and combines them systematically. In a simple autosomal recessive disorder, two unaffected parents producing an affected child are consistent with both being carriers. An X-linked recessive allele can be expressed in a person with one X chromosome because there is no corresponding dominant allele on the Y chromosome. Red-green colour blindness is a familiar example. Calculate risks for each pregnancy afresh and distinguish a risk among sons from a risk among all children.

Carrier screening can identify a recessive allele in someone who is unaffected. Pre-implantation genetic testing examines embryos produced through IVF before transfer; prenatal diagnostic sampling includes chorionic villus sampling and amniocentesis. Tests differ in timing, the material sampled, what they detect and procedure risks. Decisions involve informed consent, privacy, uncertainty, access, views about embryos and pregnancy, and the perspectives of people living with the condition. A balanced ethical answer explains different positions without treating one family’s decision as compulsory for everyone.

Key formulae

$$Aa\times Aa\quad\Rightarrow\quad\tfrac14AA:\tfrac12Aa:\tfrac14aa$$

$$P(\text{independent outcomes})=P(\text{first})\times P(\text{second})$$

Worked example

For a recessive condition, two carriers have genotypes Aa. Calculate the probability that a child is affected and the conditional probability that an unaffected child is a carrier.

1. Each parent produces A and a gametes with equal probability.
2. The four equally likely combinations are AA, Aa, aA and aa; one is affected, giving probability 1/4.
3. Among the three unaffected combinations, two are heterozygous, so the conditional carrier probability is 2/3.

Answer: Affected: $1/4$; carrier given unaffected: $2/3$.