The key to this problem is:
- The correct puzzle piece must be PRESENT in all AFFECTED individuals.
- The correct puzzle piece must be ABSENT from all UNAFFECTED individuals.
Only one puzzle piece fits these criteria: #36.
See for yourself:
All affected individuals (1, 3, 6,
11, and 12) must have the deafness puzzle piece.
Your first clue is color. Which color puzzle pieces do all affected
individuals share? RED. Here's another way to think about it:
since Individual #1 on the pedigree is affected and contains only
one color, RED, then you know
that the puzzle piece representing the mutant gene must be RED as well.
There are lots of red puzzle pieces, though. How do you know which
one contains the affected gene? To narrow your choices, find the
puzzles of the affected individuals' (1, 3, 6, 8, 10, 11, 12) that
have the fewest red pieces. Puzzles 6 and 12 each have 3 red pieces,
and puzzle 11 has 4.
Which red pieces do puzzles 6, 11 and 12 share? Only one piece
fits the bill: #36.
Now, check your work:
Do all AFFECTED individuals (1,
3, 6, 8, 10, 11, 12) have a red #36 puzzle
Do any UNAFFECTED individuals
(2, 4, 5, 7 and 9) have a red #36 puzzle
Congratulations! You discovered the deafness gene!
Answers to Bonus Questions
a recessive or dominant trait?
Some types of deafness are recessive and some are dominant traits.
Deafness is very likely inherited in a dominant manner if approximately
half of all the children born to a parent with deafness also are
Persons with a recessive deafness trait have two copies of a deafness
gene (one inherited from mom and one from dad). If the type of deafness
in this example is recessively inherited, the hearing individuals
who married into this family and had affected children (a, b, c, d, e, f, g, h, i) were ALL carriers of the
deafness gene (pretty unlikely).
Therefore, in this example, deafness is probably a dominant trait.
To be certain, though, we'd have to test all individuals in the
pedigree to confirm that only affected individuals have the deafness
Is deafness sex-linked?
Sex-linked genes are found either on the X or Y chromosome. Genes are
rarely found on the Y chromosome, though, so "sex-linked" usually
refers to genes on the X chromosome. Remember, females have two
X chromosomes (one from mom and one from dad), while males have
one X and one Y chromosome (X from mom, Y from dad).
If the deafness gene were sex-linked, then affected fathers would
pass the deafness gene to all of their daughters but none of their
sons. Affected mothers would pass the deafness gene equally to their
sons and daughters. This is not what the pedigree shows. Instead,
the pedigree shows that approximately equal numbers of boys and
girls are affected by deafness. Therefore, the gene is not sex-linked.
Since the gene is not sex-linked, it must be found on an "autosome,"
which is any chromosome other than X or Y.
A - D
What are the colors of the puzzle pieces that represent the DNA
of individuals A, B, C, and D (indicated in the same pedigree below)? What
fraction of the total number of puzzle pieces are represented by
Remember, each puzzle represents the total DNA of an individual.
Each puzzle piece represents a portion of that DNA.
A The colors of the puzzle pieces are: 1/2 red
Since they have the same parents, A and 3 must have the same colors of puzzle
3 has 1/2 red and 1/2 blue.
Therefore, A has 1/2
red and 1/2 blue.
B The colors of the puzzle
pieces are: all pink.
11's grandparents are B
11 inherited 1/4 of her genes from B,
1/4 from 8, and 1/2 from 9
pieces are pink, red, yellow and light blue.
8 and 9 have no pink puzzle pieces, so 11's pink puzzle pieces must have come
Consistent with this, 1/4 of 11's genes are pink.
Therefore, all of B's genes
C The colors of the puzzle
pieces are: all purple.
C and D.
4's grandparents (D's parents) are 2 and 3.
Therefore, 4 inherited 1/2 of his genes from C,
1/4 from 2, and 1/4 from 3.
4 has purple, red, green and blue puzzle
2 and 3 have no purple puzzle pieces, so 4's purple pieces must
have come from C.
1/2 of 4's puzzle pieces are
Therefore, C must have all purple
D The colors of the puzzle
pieces are: 1/2 green, 1/4 red, 1/4 blue.
D inherited 1/2 his puzzle
pieces from 2 and 1/2 from 3.
2 has only green puzzle pieces, so 1/2
of D's puzzle pieces are green.
3 has 1/2 red and 1/2 blue puzzle
pieces, which she passed on to her son D.
Therefore, D has 1/4
red and 1/4 blue puzzle pieces, inherited from 3.
Map a gene activity