Key Concepts

•What is probability and how does it help

explain the results of genetic crosses?

•What is meant by genotype and phenotype?

•What is codominance?

Key Terms

Probability Punnett square Phenotype Genotype Homozygous Heterozygous codominance

Principles of Probability

Probability – a number that describes how likely it is that an event will occur

Mathematics of probability – In a coin toss there are 2 possible outcomes. The coin can land heads up or tails up. Each result has an equal chance of occurring – 1 out of 2.

Probability can be expressed as a ratio, fraction orpercent – 1:2, ½, or 50%

Percentage• Percentage – a number compared to 100

• If 3 out of 5 coins land with heads up. The percentage is calculated as follows:1. 3 out of 5 = 3/5 2. 3/5 x 100% = 60%

• Practice: Suppose 3 out of 12 coins landed with tails up. How can you express this as a percentage?

Percentage

3 out of 12 = 3/12 or ¼ ¼ x 100% = 25%

1

Independence of Events

The results of one coin toss do not affect the results of subsequent coin tosses

Each event occurs independently Ex. If you toss a coin 5 times and it

lands heads up each time. What is the probability that it will land heads up on the next toss?

Independence of Events

The probability is still 1 out of 2 or 50%

Probability and Genetics

• Mendel was the first to recognize that the principles of probability could be used to predict the results of genetic crosses

• When he crossed the two plants that were hybrids for stem height, ¾ had tall stems and ¼ had short stems

• The probability of such a cross producing tall stems was 3 in 4. The probability of producing short stems was 1 in 4

Punnett Squares

A chart that shows all the possible combinations of alleles that can result from a genetic cross

To make a Punnett square• 1. draw a box and divide it into 4 boxes

• 2. Write male parent’s alleles along the top and female parent’s alleles along the side

R r

R

r

Punnett Square3. Copy the female parent’s alleles into the

boxes to their right

R r

R R

r

4. Copy the male parent’s alleles into the boxes beneath them

R r

R

r

R R

r r

RR Rr

Rr rr

Using a Punnett Square

The Punnett square show all the possible allele combinations in the offspring

In a genetic cross the

allele that a parent

passes on to an

offspring is based on

probability

Predicting Probabilities

Punnett squares can be used to predict probabilities.

If a purebred pink flamingo is crossed with a purebred blue flamingo. There is a 100% probability that the offspring will be pink

Phenotypes and Genotypes Phenotype – an organism’s physical

appearance, or visible traits (what you see)

Genotype – genetic makeup, or allele combination

The genotypes Tt and TT have the same phenotype – tall

The genotype tt has the phenotype short

Phenotypes and Genotypes Homozygous- an organism that has

two identical alleles for a traitEx. TT or tt

Heterozygous – an organism that has two different alleles for a trait. Also called hybridEx.Tt

Codominance

Not all traits have alleles that are dominant or recessive.

Codominance – both alleles are expressed in the offspring

Examples of codominance ABO blood types Fur color in mammals Spotted flowers

http://i.ytimg.com/vi/EhaobvCp_Oc/2.jpg http://www.suite101.com/content/incomplete-dominance-and-

codominance-a173718 http://www.explorelearning.com/index.cfm?

method=cResource.dspDetail&ResourceID=453 http://www.world-builders.org/lessons/less/les4/casino/cas1ck.html http://evolutionlist.blogspot.com/2009_03_01_archive.html