GoalExplain carriers, why a recessive inherited disease can skip generations, and read the chances from an Rr × Rr cross.
New words
inherited disease · a disease caused by a change in a gene that can pass from parents to children; it is not catchingcarrier · a healthy person who has one normal and one disease allele and can pass the disease allele onaffected · having two disease alleles, so the disease shows
Explanation
Inherited diseases come from changes in genes, and they pass from parents to children. Inherited does not mean contagious: nobody can catch them. In many of them the disease allele is recessive. A person with one normal allele and one disease allele (Rr) is a carrier: healthy, with no symptoms, but able to pass the disease allele on. A child is affected only with two disease alleles (rr). When two carriers have children, each child has a 25 % chance of RR (healthy, not a carrier), a 50 % chance of Rr (a healthy carrier) and a 25 % chance of rr (affected), so there is a 75 % chance of being healthy. We say “25 % chance each time”, not “one in four children will be ill”. Two examples: thalassaemia affects the haemoglobin in the blood and causes anaemia; galactosaemia is when the body cannot break down galactose, a sugar in milk, and the skin and eyes can turn yellow (jaundice). A carrier is not ill and has no “bad genes”: we all carry some unusual alleles. For questions about a real family, ask a doctor or a genetic counsellor.
P: ♀ Rr × ♂ Rr
♀ \ ♂
R
r
R
RR
Rr
r
Rr
rr
Genotype: 1 RR : 2 Rr : 1 rr Phenotype: 3 : 1
Worked examples
A fictional plant disease, “leaf-curl”, is recessive. A carrier plant is Rr, and healthy plants look normal. Two carrier plants make seeds. The seeds are RR (25 %), Rr (50 %) and rr (25 %, curled leaves), so 75 % of the seeds grow healthy leaves. For 40 seeds we expect about 10 with curled leaves.
P: ♀ Rr × ♂ RR
♀ \ ♂
R
R
RR
r
Rr
Genotype: 1 RR : 1 Rr Phenotype: 2
A carrier (Rr) and a non-carrier (RR) have a child. The gametes are R, r and R, R. The child is RR or Rr, so 0 % chance of being affected (rr) in this cross. About half of the children may be carriers.
Why can a recessive disease skip a generation? A carrier is healthy, so the disease allele can pass quietly through carriers, and appear only when two disease alleles meet in one child.
Class activity
Carrier cards, with a grown-up. Make four paper cards for each parent: two marked R and two marked r (this is the fictional plant disease). Shuffle each pile. In each round draw one card from each parent’s pile, record RR, Rr or rr, and put the cards back. Do 40 rounds. About 10 should be rr (affected) but not exactly. Draw a pie of your tally and compare with 25 %, 50 %, 25 %.
Practice
1
A plant is Rr for the fictional leaf-curl disease. Is it affected, and can it pass r on?
It is not affected (it is healthy), but it can pass r on to its offspring.
2
Two carriers (Rr × Rr): what are the chances that a child is affected, a carrier or neither?
25 % affected (rr), 50 % healthy carrier (Rr), 25 % healthy and not a carrier (RR).
3
What percentage of children of two carriers are healthy?
25 % + 50 % = 75 %.
4
Is an inherited disease contagious? Explain.
No: it is caused by alleles passed from parents; nobody can catch it from another person.
5
Why should we avoid saying “bad genes”?
Everyone carries some unusual alleles; carriers are healthy and it is nobody’s fault.
