A simpler way to do a Dihybrid Cross or beyond...

1
A simpler way to do a Dihybrid Cross or beyond...

• how to avoid making anything larger than a simple
  Punnett square for any genetics problem.

2

• Mendelian genetics reflect the laws of
  probability
• Mendel's laws are basically real-life
  applications of the rules of probability that
  apply to a coin toss, rolling a dice, or drawing
  from a deck of cards

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• Rule of multiplication Segregation of the
  alleles into gametes is like a coin toss (heads
  or tails equal probability). In the case of two
  CF carriers (Cc), the probability of the egg
  having the CF allele (c) is 1/2 and the sperm
  having the (c) allele is 1/2
• 1/2 x 1/2 1/4 chance of having child with CF
  (cc).
• (note that you didn't need to do a Punnitt square
  to figure this out)

4

• For a dihybrid cross - the chance that 2
  independent events will occur together is the
  product of their chances of occuring separately.

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• The chance of yellow (YY or Yy) seeds 3/4 (the
  dominant trait)The chance of round (RR or Rr)
  seeds  3/4 (the dominant trait)The chance of
  green (yy) seeds 1/4 (the recessive trait)The
  chance of wrinkled (rr) seeds 1/4 (the recessive
  trait) So... The chance of yellow and round 3/4
  x 3/4 9/16The chance of yellow and wrinkled
  3/4 x 1/4 3/16The chance of green and round
  1/4 x 3/4 3/16The chance of green and wrinkled
  1/4 x 1/4 1/16

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• Sound familiar? And no Punnett square needed
  (whew). Therefore, you can avoid doing a Punnett
  square if you can reduce the problem to a series
  of probability statements.

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With a tri-hybrid cross, you can avoid a huge
Punnett square with 64 boxes

• The chance of yellow (YY or Yy) seeds  3/4 (the
  dominant trait)The chance of round (RR or Rr)
  seeds 3/4 (the dominant trait)The chance of
  purple (PP or Pp) flowers 3/4 (the dominant
  trait)The chance of green (yy) seeds 1/4 (the
  recessive trait)The chance of wrinkled (rr)seeds
  1/4 (the recessive trait) The chance of white
  (pp) flowers 1/4 (the recessive trait)

8

• Try this problem
• You have freckles, dimples, and a widow's peak.
  Your S.O. has freckles and dimples, but a
  continuous hairline. In other words,
•  You  Your S.O. FfDdWw x FfDdWwQuestion What is
  the chance your darling child would have all
  three recessive phenotypes no freckles (ff), no
  dimples (dd) or a continuous hairline (ww)?
• Hint do three quick Punnett squares for each
  single trait. Take the proportions of the
  recessives and multiply away...!

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• But what if
•  You  Your S.O. FfDdWw x FfddwwQuestion What is
  the chance your darling child would have all
  three recessive phenotypes no freckles (ff), no
  dimples (dd) or a continuous hairline (ww)?