This document discusses key concepts in probability and genetics. It defines important terms like genotype, phenotype, and codominance. It explains that probability can be used to predict the results of genetic crosses using tools like Punnett squares. Punnett squares show all the possible allele combinations in offspring from a genetic cross based on the independent assortment and transmission of alleles from each parent. The document contrasts phenotypes, which are visible traits, from genotypes, which are an organism's genetic makeup in terms of its allele combinations.

1. Key Concepts
•What is probability and how does it help
explain the results of genetic crosses?
•Whatis meant by genotype and
phenotype?
•What is codominance?

2. Key Terms
 Probability
 Punnett square
 Phenotype
 Genotype
 Homozygous
 Heterozygous
 codominance

3. 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%

4. Percentage
• Percentage – a number compared to 100
• If 3 out of 5 coins land with heads up. The
percentage is calculated as follows:
l 3 out of 5 = 3/5
3/5 x 100% = 60%
• Practice:
Suppose 3 out of 12 coins landed with tails up.
How can you express this as a percentage?

5. Percentage
 3 out of 12 = 3/12 or ¼
 ¼ x 100% = 25%
1

6. 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?

7. Independence of Events
 The probability is still 1 out of 2 or 50%

8. 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

9. Punnett Squares
 A chart that shows all the possible
combinations of alleles that can result
from a genetic cross

10. 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

11. Punnett Square
3. Copy the female parent’s alleles into the
boxes to their right
R r
R R R R
r
r r
4. Copy the male parent’s alleles into the boxes
beneath them
R r
R RR Rr
r Rr rr

12. 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

13. 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

14. 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

15. Phenotypes and Genotypes
 Homozygous- an organism that has
two identical alleles for a trait
 Ex. TT or tt
 Heterozygous – an organism that has
two different alleles for a trait. Also
called hybrid
 Ex.Tt

16. Codominance
 Not all traits
have alleles
that are
dominant or
recessive.
 Codominance –
both alleles are
expressed in
the offspring

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

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