1. Biology Unit Four B – Mendelian Genetics
Mrs. Pearson’s First Semester Biology
*For those watching by recording, watch for questions on the slides throughout
the presentation. me the answers to the questions within two days of the
lesson being presented in order to receive credit for watching the recording.

2. Deal or No Deal?

3. Deal or No Deal?
What are the chances that you have the million dollar case?
One out of…
1/26
We can also write that 1:25
You pick three cases and none of them are the million dollar case. Now what are your chances that you have the million dollar case?
1/23 (1:22)

4. Deal or No Deal?
You are feeling lucky so you pick three more cases and none of them is the million dollar case. What are the chance that yours is the million dollar case now?
One out of…
1/20 (1:19)
What percent is that?
1 divided by 20 = .05
.05 x 100% = 5%

5. Deal or No Deal?
You are on a roll so you pick five more cases and none of them is the million dollar case. What are the chances that your case is the million dollar case now?
One out of…
1/15 (1:14)
What percent is that?
1 divided by 15 = .067
.067 x 100% = 7%

6. What are the chances?
In this unit, you will be working on probability of genetic traits – remember your fractions as you determine how likely it is that an organism will have a certain trait!

7. Lesson Objectives
Apply Mendel’s First and Second Laws of Inheritance to genetic crosses.

8. KEY CONCEPT The inheritance of traits follows the rules of probability.

9. Pearson’s Perfectly Pleasant Prefixes and Parts
Homo-
Hetero-
Mono-
Di-

10. Punnett squares illustrate genetic crosses.
The Punnett square is a grid system for predicting all possible genotypes resulting from a cross.
The axes represent the possible gametes of each parent.
The boxes show the possible genotypes of the offspring.
If “A” is the allele for Amber flowers and “a” is the allele for blue flowers, what is the genotype of parent 1? The phenotype?

11. Punnett squares illustrate genetic crosses.
The Punnett square yields the ratio of possible genotypes and phenotypes.
What is the ratio of genotypes in this cross? What is the ratio of phenotypes?

12. A monohybrid cross involves one trait.
Monohybrid crosses examine the inheritance of only one specific trait.
homozygous dominant-homozygous recessive: all heterozygous, all dominant
What is the ratio of genotypes in this cross? The ratio of phenotypes?
**Those watching by recording, include in your the definitions for homozygous and heterozygous

13. heterozygous-heterozygous—1:2:1 homozygous dominant: heterozygous:homozygous recessive; 3:1 dominant:recessive
What is the ratio of genotypes in this cross? The ratio of phenotypes?

14. heterozygous-homozygous recessive—2:2 heterozygous:homozygous recessive; 2:2 dominant:recessive
A testcross is a cross between an organism with an unknown genotype and an organism with the recessive phenotype. Why?
What is the ratio of genotypes? Phenotypes?

15. A dihybrid cross involves two traits.
Mendel’s dihybrid crosses with heterozygous plants yielded a 9:3:3:1 phenotypic ratio.
Mendel’s dihybrid crosses led to his second law, the law of independent assortment.
The law of independent assortment states that allele pairs separate independently of each other during meiosis.

16. Heredity patterns can be calculated with probability.
Probability is the likelihood that something will happen.
Probability predicts an average number of occurrences, not an exact number of occurrences.
Probability =
number of ways a specific event can occur
number of total possible outcomes
Probability applies to random events such as meiosis and fertilization.
**Those watching by recording, include in your the definition for probability.

17. Practicing the Punnett
Let’s say you are working with a plant whose genetics for flower color show that the dominant condition yields purple flowers and the recessive condition yields blue flowers.
You are crossing a plant that is homozygous recessive with a plant that is homozygous dominant. What would your Punnett Square look like?
Let’s use P to represent the dominant gene – remember we only use one letter when we are working with one trait so the letter to show the recessive gene will be p. DON’T switch letters – you would NOT use P for purple and B for blue.

18. Practicing the Punnett
Remember that every organism has two copies of every gene.
What would the parent that is homozygous recessive have for its two genes? PP Pp pp
What would the parent that is homozygous dominant have for its two genes?
So the cross will be PP x pp

19. Practicing the Punnett

20. p P

21. p P

22. p P

23. p P Pp

24. p P Pp

25. p P Pp
So, what are the chances of having offspring that are homozygous recessive?
Zero out of four (0:4) = 0/4 = zero percent

26. p P Pp
So, what are the chances of having offspring that are homozygous dominant?
Zero out of four (0:4) = 0/4 = zero percent

27. p P Pp What are the chances of having offspring that are heterozygous?
Four out of four (4:0) = 4/4 = 100 percent

28. Practicing the Punnett
Let’s try one more example…
This time, let’s cross two heterozygous plants.
So our cross will be Pp x Pp

30. P p

31. P p

32. P p

33. P p PP Pp

34. P p PP Pp pp

35. Practicing the PunnettPP Pp pp
So, what are the chances of having offspring that are homozygous recessive?
One out of four (1:3) = 1/4 = .25 = 25%

36. Practicing the PunnettPP Pp pp
So, what are the chances of having offspring that are homozygous dominant?
One out of four (1:3) = 1/4 = .25 = 25%

37. Practicing the PunnettPP Pp pp
So, what are the chances of having offspring that are heterozygous?
Two out of four (2:2) = 2/4 = .50 = 50%

38. Practicing the PunnettPP Pp pp
If we wanted to compare all three, we could use ratios and say 1:1:2 homozygous recessive to homozygous dominant to heterozygous.

39. P p PP Pp pp
What are the chances of having blue flowers? Homozygous recessive is the only combination that could give you blue flowers so…
1:3, one out of four, ¼, or 25%

40. P p PP Pp pp
What are the chances of having purple flowers? Homozygous dominant and heterozygous will both give you purple flowers so…
2 heterozygous + 1 homozygous dominant = 3
3:1, three out of four, ¾, or 75%

41. Practicing the Punnett
REMEMBER when we are talking about the type of genes, that is genotype. When we want to know what characteristic is seen, that is phenotype.

42. KEY CONCEPT Independent assortment and crossing over during meiosis result in genetic diversity.

43. Sexual reproduction creates unique combinations of genes.
Sexual reproduction creates unique combination of genes. Why?
independent assortment of chromosomes in meiosis
random fertilization of gametes
Unique phenotypes may give a reproductive advantage to some organisms. How?

44. Crossing over during meiosis increases genetic diversity.
Crossing over is the exchange of chromosome segments between homologous chromosomes.
occurs during prophase I of meiosis I
results in new combinations of genes

45. Chromosomes contain many genes.
The farther apart two genes are located on a chromosome, the more likely they are to be separated by crossing over. Why?
Genes located close together on a chromosome tend to be inherited together, which is called genetic linkage.
Genetic linkage allows the distance between two genes to be calculated.

46. Review
Punnett squares illustrate simple genetic crosses.
A monohybrid cross involves one trait.
A dihybrid cross involves two traits.
Heredity patterns can be calculated using probability.
Sexual reproduction creates unique combinations of genes.
Crossing over during meiosis increases genetic diversity.

47. Questions?