Close relatives, screening and genetic counselling
Science Prep 3 Term 1, Unit 3: pp. 71–74
GoalExplain, calmly and respectfully, why close relatives share more genes, and what genetic screening and genetic counselling do.
New words
genetic screening · tests that look at many genes at once to find inherited conditions earlygenetic counselling · specialists study family medical history and test results and explain the chances to familiescommon ancestor · a grandparent or earlier relative from whom two people are both descended
Explanation
Relatives share some genes because they descend from common ancestors. This means that the chance that both partners carry the same rare recessive disease allele is higher than for two unrelated people. Health authorities therefore advise a medical check before marriage; the doctors say that it lowers the chance of passing on some diseases. This is a medical and community-health matter, stated here neutrally and with respect to every family. Comprehensive genetic screening examines thousands of genes at once and helps to find rare inherited diseases early. Genetic counselling means that specialists study the family’s medical history and the tests, and explain the chances, which helps families plan and can reduce the spread of inherited disease. A counsellor gives information, not orders. This lesson gives no advice for any real family: ask a doctor or a genetic counsellor.
common ancestors→relatives share some genes→a higher chance of carrying the same rare allele→a medical check gives information
Worked examples
An invented illustration: suppose 1 person in 50 carries a rare allele. For two unrelated people chosen at random, the chance that both carry it is 1/50 × 1/50 = 1/2500. For close relatives the chance is higher, because they may both have inherited the allele from the same ancestor. The numbers are invented for this example.
A family notices a rare disease in several cousins. A genetic counsellor studies the family’s medical history and the test results, explains in plain words what the chances are and answers questions. The family decides for itself.
Screening against testing one gene: screening can look at thousands of genes in one test, so it finds rare conditions early, while a single-gene test looks for one known change.
Class activity
Role cards for a calm talk, with a grown-up. Write on cards: a student, a doctor and a genetic counsellor. Practise a short conversation in which the student asks “what does a genetic counsellor do?” and the others answer in plain words. Avoid any blame, any talk of “bad genes” and any advice about a real family. Write three questions you would ask a doctor about inherited diseases, and one sentence on why information helps families.
Practice
1
Why do relatives share more genes than unrelated people?
They descend from common ancestors.
2
What do doctors say a medical check before marriage can do?
It lowers the chance of passing on some diseases.
3
What does comprehensive genetic screening do?
It examines thousands of genes at once, helping to find rare inherited diseases early.
4
What does a genetic counsellor do?
Studies family medical history and tests and explains the chances to the family.
5
In the invented example with 1 in 50 carriers, what is the chance that two unrelated people both carry the allele?
