Ask A Scientist Molecular Biology Archive

name         Mary  B.
status       educator
age          30s

Question -   For my 3rd grader's science fair project, we want to use
a family tree to prove that 1 out of 4 will have the recessive gene.  We
read this a few years ago in an older son's science book, but we have not
been able to locate the book or find the same information.  Is this a
valid theory?  (We have looked in the library, encyclopedias and other
on-line places)
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A family tree is a great idea, but the science is based on probability.  The
probability that a recessive trait will show up in a family is 25% if both
parents are carriers.  In order for a recessive trait to show up in a person,
he/she must inherit a copy of the recessive trait from both parents.  If one
parent is recessive, let's say "rr" and the other parent is a carrier, say
"Rr", the probability is 50%.  If both parents are recessive, the probability
is 100% (rr x rr).  If neither parent carries the trait the probability is 0%
(RR x RR).  BUT, this only tells you what MIGHT happen.  We all know that the
probability of having a boy is 50:50, but we all know families of all boys.
So it would be interesting to see if the probability works out in your
family.  If you do a search on the web for "genetics" or "probability and 
genetics" you will get plenty of hits.
vanhoeck
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Mary, you have part of it right. It is not correct to say that 1 in 4 will
always have the recessive gene. The key is, what alleles (=versions of the
gene) do the parents have? Remember, each parent starts with two alleles, and
puts one or the other randomly into a sperm or egg.

The idea is that if you have two parents, each with a normal (+) and a mutant
(-) copy of the gene (i.e., each parent is +/-), and those parents produce
offspring, then there is a 1 in 4 chance of an offspring getting BOTH mutant
copies, i.e., -/-. The other possibilities are +/+, +/-, and -/+. If the
minus sign represents a recessive mutation, then only the -/- child will
display the mutant trait. Therein lies the 1 in 4 chance.

The 1 in 4 odds only apply to this case though. You could have, say, a +/+
parent mated to a -/- parent. Here, all children would be +/-. They would all
carry the mutant version, but if it was recessive, none would visibly show
the mutant trait.


You will find examples of this if you get a high school or better biology
textbook and look up the term "Punnett square", which is an easy way to
visualize the outcomes of various mating possibilities.

Paul Mahoney, PhD
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I think you mean that in a simple (or oversimplified case) case of
inheritance, if both parents carry a recessive gene for a certain trait and
both also carry the dominant form ( this would be called heterozygous...a
mix of both dominant and recessive) there is a 25 % chance that any one of
the children will show the recessive phenotype (appearance).  The classic
example is eye color where both parents have brown eyes but each parent
carries a blue eyed gene (which is masked by the dominant brown).  The
Punnett square shows the probability with the symbol for brown eye gene
upper case B since it is dominant and the symbol for the blue eyes gene
lower case b since it is recessive.  BB means the child has  both brown eyed
genes and therefore is brown eyed in appearance (phenotype).  Bb is still
brown eyed in appearance since brown dominates blue.  The only chance for a
blue eyed child is when both parents give the offspring a blue eye gene bb .
Which the chart shows is 1 out of 4 or 25%.  Of course, eye color is far
more complex...how about green and very dark brown etc...which has to do
with the thickness of the iris...but we will be like the physics chaps who
will often assume for simplicity such things as, the Earth is a perfect
sphere, friction is non-existent, and the reaction occurs at standard
temperature and pressure, etc.



         B      b    (mother)

B     BB    Bb

b      Bb    bb

f
a
t
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Peter Faletra Ph.D.
Assistant Director
Science Education
Office of Science
Department of Energy
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The wording of your question is confusing. Are you asking whether 1 in 4
people have a trait or condition in their bodies (their phenotype) or
whether they carry one recessive gene for a trait along with a dominant gene
that masks it (their genotype)? Our genes are present in our cells in pairs.
We say a person is a heterozygote if he or she has two different alleles, or
"flavors," or versions, of a particular gene. If so, one of the alleles may
be recessive to the other. In such a case the recessive gene will not show
its presence in the person, unless a carrier test is available to detect its
presence.

You often cannot prove if the members of a family have a recessive gene,
unless it is a particular condition for which carrier testing exists.

If two parents are carriers of the same recessive gene, then the chances of
the recessive condition showing up in their children is 1 in 4 for each
pregnancy. If they have a large enough family (many children) or if we
combine the data for many similar sets of parents then, on average, about
one quarter of the children may be expected to show the trait.

It is not typical to find a family tree in which one quarter of the members,
from various generations and branches of the family, have a recessive
condition. On the contrary, since the parents are unaffected carriers, it is
more likely that the condition will show up in one or more of their children
as a sudden unexpected occurrence. Consanguinity, or marrying among
relatives, increases the chances that a child could have a recessively
inherited condition. This is because cousins carry a greater chance or
carrying the same recessive gene as each other, inherited from a common
ancestor.

Please note: There is a big difference between gene frequencies in a family
and in the population.  A rare gene may exist in a population for a
condition that is either dominantly or recessively inherited. For example,
the gene achondroplasia is dominantly inherited, so that a person who has
the gene does have the condition, and has a 50% chance of passing it on with
each pregnancy. Yet the incidence of this condition is very low, certainly
way less than 1 in 2 or 1 in 4. A recessive condition may occur in 1 out of
4 of the children in a family, but only 1 out of 3,600 babies born to the
population. In such a case, about 1 in 30 people in this population will be
carriers of the recessive gene.

Sarina M Kopinsky, MSc, CGC, HED
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