1. Objective
SWBAT complete Punnett squares for monohybrid crosses.

2. Segregation of gametes

3. Genes are on chromosomes

4. Alleles

5. Probability & Punnett Squares
VOCABULARY:
Probability
Punnett Square
Homozygous
Heterozygous
Phenotype
Genotype
KEY CONCEPTS:
How do geneticists use the principles of probability?

6. A little background info…
Every time Mendel performed a cross with his pea plants, he carefully counted the offspring
Out of the over 20,000 plants he counted he noticed there was a pattern
Whenever he crossed 2 plants that were hybrid for stem height (Tt), about ¾ of the resulting plants were Tall and ¼ were short (3:1 ratio)
He realized that the principles of probability could be used to explain his results!

9. The work of Gregor Mendel
worked with pea plants…
…he called them his children!
Why pea plants???
There was a long-standing tradition of breeding pea plants at the monastery where Mendel lived and worked

10. So…they were readily available and they come in lots of varieties!
…there were plants with different flower colors, seed color, flower position etc..

11. Mendel’ Pea Plants
Mendel based his laws on his studies of garden pea plants. Mendel was able to observe differences in multiple traits over many generations because pea plants reproduce rapidly, and have many visible traits such as:
Pod color
Seed Color
Plant Height
Green
Yellow
Green
Yellow
Seed Shape
Short
Pod Shape
Tall
Wrinkled
Round
Smooth
Pinched

12. Probability
Definition: the likelihood that a particular event will occur
Ex: flip a coin: ½ or 50% chance it will come up heads
If you flip a coin 3 times in a row, what is the probability it will come up heads 3 times in a row?
Each event is an independent therefore it is:
½ x ½ x ½ = 1/8
Past outcomes do not influence future outcomes
The principles of probability can be used to predict the outcomes of genetic crosses

13. Punnett Squares
The gene combinations that MIGHT result from a genetic cross can be determined by drawing a Punnett Square
The parents go on the outside of the square along the side and top
The possible outcomes of the cross are on the inside of the square
The letters in the Punnett Square represent alleles

14. Alleles in a Punnett Square
“T” represents the Dominant allele: TALL
“t” represents the recessive allele: short
Organisms with 2 identical alleles (TT or tt) for a trait are said to be HOMOZYGOUS
Mendel called these: true-breeding
Organisms with 2 different alleles for a particular trait are said to be HETEROZYGOUS
Mendel called these hybrids

15. Punnett Squares
A capital letter represents the dominant allele for tall.
A lowercase letter represents the recessive allele for short.
In this example,
T = tall
t = short
The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants.

16. MORE VOCABULARY FUN!
Phenotype: the physical characteristics of the organism (what the organism looks like)
Ex: Tall; short
Genotype: the actual genetic makeup of the organism (the 2 alleles the organism inherited)
Ex: TT; Tt: tt

17. Punnett Squares
The plants have different genotypes (TT and Tt), but they have the same phenotype (tall).
Although these plants have different genotypes (TT and Tt), they have the same phenotype (tall). TT
Homozygous Tt
Heterozygous

18. Probabilities Predict Averages
Probabilities predict the outcomes of a LARGE number of events
Probabilities cannot predict precise outcomes of an event
Ex: flip a coin twice you may get 100% heads
You need to flip the coin many, many times to get close to 50%

19. Probability and Segregation
One fourth (1/4) of the F2 plants have two alleles for tallness (TT).
2/4 or 1/2 have one allele for tall (T), and one for short (t).
One fourth (1/4) of the F2 have two alleles for short (tt).
The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants.

20. Because the allele for tallness (T) is dominant over the allele for shortness (t), 3/4 of the F2 plants should be tall.
The ratio of tall plants (TT or Tt) to short (tt) plants is 3:1.
The predicted ratio showed up in Mendel’s experiments indicating that segregation did occur.

21. What is the expected ratio for this cross? Tt x tt

22. Punnett Square Rules
1. Choose a letter to represent the dominant allele and capitalize it (choose a letter that is easy to distinguish between upper-case and lower-case).
2. Use the same letter but use lower case to represent the recessive allele.
3. Put the male on the left of the square and the female on the top.
4. Write "genotype" and "phenotype" below the square.
5. Use "x" to indicate a cross.
6. Write the dominant allele first if you are writing out a hybrid.

23. Question 1 Probability can be used to predict:
A. average outcome of many events.
B. precise outcome of any event.
C. how many offspring a cross will produce.
D. which organisms will mate with each other.

24. Question 2 Compared to 4 flips of a coin, 400 flips of the coin is
A. less likely to produce about 50% heads and 50% tails.
B. guaranteed to produce exactly 50% heads and 50% tails.
C. more likely to produce about 50% heads and 50% tails.
D. equally likely to produce about 50% heads and 50% tails.

25. Question 3
Organisms that have two different alleles for a particular trait are said to be
A. hybrid.
B. heterozygous.
C. homozygous.
D. recessive.

26. Question 4 The Punnett square allows you to predict
A. only the phenotypes of the offspring from a cross.
B. only the genotypes of the offspring from a cross.
C. both the genotypes and the phenotypes from a cross.
D. neither the genotypes nor the phenotypes from a cross.

27. Question 5
Two F1 plants that are homozygous for shortness are crossed. What percentage of the offspring will be tall?
A. 100%
B. 50%
C. 0%
D. 25%