Inheritance And Probability printable pdf download

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NHERITANCE AND

ROBABILITY

In genetics, when we are trying to figure out how traits may be passed from parents to offspring,

probability

is an important consideration. If we know a little bit about the trait we are studying,

we can use

probability

predict

how the gene in question will be transferred from parents to

offspring. As an example, let’s toss a coin:

If you examine a coin, you’ll see that it has 2 sides—heads and tails—and so there are 2 possible

outcomes when the coin is tossed: it will land with the “heads” side up, or with the “tails” side up.

When you toss the coin, the

probability

of getting a “heads” is ½, or 50%. This represents the

number of desired outcomes (heads = 1) divided by the number of possible outcomes (heads +

tails = 2). There is also a probability of ½ that we will get a “tails” if we toss the coin. If we toss

our coin only two times, we might get 2 “heads”, 2 “tails”, or 1 “heads” and 1 “tails.” But the

more times we toss our coin, the more likely we are to get closer and closer to our

expected

probability

of 50% “heads” and 50% “tails.” Based upon what we know about coins, we form an

hypothesis

that the probability of getting a “heads” when we toss the coin is ½ or 50%. Then we

test

our hypothesis by tossing the coin and recording the results.

Eye color in mice

Now that we’ve explored how probability is important in genetics, let’s investigate the

inheritance of a genetic trait in mice that is determined by a

single

gene. Mice may have either

black eyes or red eyes, and from the work of other mouse researchers we know that pure black-

eyed mice bred to pure red-eyed mice always produce offspring with black eyes. Thus, we say

that the gene for black eyes is

dominant

and the gene for red eyes is recessive, because the gene

for black eyes “dominates” the gene for red eyes.

In genetics, it is often useful to represent genetic crosses like the one described above using a

Punnet

square:

We’ll represent the mouse eye color gene by the letter B. The dominant gene for black eyes will

be designated by “B”; the recessive gene for red eyes by “b”. In the cross described above,

because Dad is a “pure” black-eyed mouse, he’s designated as BB. Mom is a “pure” red-eyed

mouse, designated by bb.

Dad’s genes

pure black eyes (BB)

Dad can produce gametes

that have only “B” genes.

Mom can produce gametes

Offspring:

that have only “b” genes.

all

All offspring will be

and

black eyes

have black eyes.

(Bb)

genetics & probablility

Savannah River Ecology Laboratory [

Inheritance And Probability

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