1. Genetics & The Work of Mendel

2. Genetics The study of heredity What’s heredity?
The passing of traits from one generation to the next…
Serious study of heredity began in the mid 1800s

3. Ideas about Heredity in the 1800s…
Blending
Offspring was a BLEND between two parents
Like mixing different colors of paint.
Should have been easily dismissed as wrong because some pretty major traits are clearly NOT “blended”…

4. Experiments in heredity in the 1800s
POORLY designed
For example…
How is beauty passed from one generation to the next…?
Beauty is subjective
Humans are very poor experimental organisms for studies of heredity
Cannot control who they mate with
Takes too long to see how offspring turn out
Studies usually limited to just a few family groups

5. Gregor Mendel “The Father of Genetics”
Modern genetics began in the mid-1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in pea plants
He studied at the University of Vienna from 1851 to 1853 where he was influenced by a physicist who encouraged experimentation and the application of mathematics to science and a botanist who aroused Mendel’s interest in the causes of variation in plants. After the university, Mendel taught at the Brunn Modern School and lived in the local monastery.
The monks at this monastery had a long tradition of interest in the breeding of plants, including peas. Around 1857, Mendel began breeding garden peas to study inheritance.

6. What did Mendel do that was different?
Studied PEA PLANTS
EASY to grow
Small size allows you to grow lots in a relatively small space
Relatively quick generation time
A few weeks as opposed to years

7. Breeding Pea Plants allows GREAT CONTROL over the experiment
Why?
Because pea plants have BOTH reproductive parts
AND these are HIDDEN inside flower petals
This means that they NORMALLY SELF FERTILIZE
Pollen carrying sperm fertilizes eggs within the same plant
Plants do NOT generally get random pollen from other pea plants
If Mendel wanted to breed two plants, he could open petals and control what pollen reached the female parts of the plant

8. How Mendel did his crosses…
Got 2 pea plants that he wanted to use as parents… “mom” and “dad”
Made sure these parents were TRUE BREEDING
This means that no matter how many times a plant was allowed to self-fertilize, the types of offspring that resulted were ALWAYS the SAME
That is, a pink-flowered plant would ALWAYS have pink-flowered babies…
Why true breeding?
CONTROL VARIABLES!!!!

9. If Mendel wanted to breed two different plants together, here’s what he did:
Opened petals
Cut male parts off of each plant and used them to fertilize female parts of the other plant
Waited for seeds to form
Planted them to see what traits the next generation would have

10. Pea Plants were a good choice…
Lots of easily distinguishable traits that are either “on” or “off” and no inbetween
Either Tall or Short – no “medium” plants
Made it easier for Mendel to figure out his results
Mendel examined heredity of 7 different traits
Plant height
Flower color
Pea shape
Pea color
Etc…

11. Mendel collected data for 7 pea traits

12. What ELSE did Mendel do that was different?
He used true experimental method
NOT some very limited and subjective look at a human family…
Used QUANTITATIVE data collection and analysis
This means he COUNTED the pea plants that resulted from his crosses of parents
He ANALYZED the data using statistics
He crossed THOUSANDS of plants. Why??
Multiple trials!

13. A typical cross as done by Mendel:
Select 2 parents that differ in ONE trait only
Why?
LESS CONFUSION
Fewer variables
Called a MONOHYBRID cross
Mono = 1
Hybrid = difference
Example = Plant Height
One parent TALL
One parent SHORT
All other characteristics would be the same between the two parents

14. PHENOTYPE Term for the PHYSICAL APPEARANCE of a trait in an organism
The phenotype (physical appearance) of a tall plant is TALL
The phenotype (physical appearance) of a short plant is SHORT

15. A typical cross as done by Mendal:
The plants selected by Mendel would always be TRUE BREEDING
Tall parent always had tall offspring if left to self fertilize
Short parent always had short offspring if left to self fertilize

16. The cross was performed…
Mendel would cross the two parents
Called this generation the P (parent) generation
P = true breeding parents

17. Mendel would wait and observe offspring that resulted
First generation of offspring is called F1 generation
Mendel noticed the F1 generation always turned out TALL
How could this be? What happened to the short parent? He had to figure this out!

18. Mendel’s next step: Cross the F1 plants with….
THEMSELVES!
Actually allowed them to self-fertilize
Wait and observe the offspring that resulted
Second generation of offspring (grandchildren of original parents) is called F2 generation

19. How did the F2 generation turn out?
¾ were TALL
¼ were SHORT

20. SUMMARY of Mendel’s Results
In F1 generation, one trait APPEARED to be LOST
In the F2 generation, the “lost” trait always reappeared
In the F2 generation, the ratio of offspring ALWAYS was:
¾ possessed the “unlost” trait
¼ possessed the “lost” trait

21. Mendel EXPLAINED these strange result. He said…
Every trait is determined by 2 “factors” possed by the cells of the organism
Factor = GENE
Male parent contributes 1 factor to offspring
Female parent contributes 1 factor to offspring

22. Mendel’s explanation…
Factors (genes) may exist in alternate forms
Alternate forms called alleles
Example: the gene for pea plant height exists in two forms (ALLELES)
Tall allele
Short allele

23. Mendel’s explanation…
In the cases Mendel studied, one form (allele) seemed “stronger” than the other form
Term for an allele that can MASK another allele
DOMINANT
Term for an allele that is MASKED by another allele
RECESSIVE

24. How it all works… What’s dominant in pea plant height?
Let’s say that we have a cross as follows:
Male = Tall
Female = Short

25. Genetic notation rules:
Usually the first letter of the dominant trait is used to symbolize the gene being studied
Uppercase letter = dominant
Lowercase letter = recessive
Example:
T = tall
t = short

26. How it works…
Each parent possesses two alleles for the trait. Since Mendel started with TRUEBREEDING plants, he said their genetic make-ups were as follows:
Male = TT
Female = tt

27. GENOTYPE
The letter combinations showing an organisms genetic make-up with regard to a particular trait
for example: TT or tt

28. How it works… Each parent sends ONE of its alleles
The process that assures that only ONE allele from each parent will be sent to the offspring is…
MEIOSIS
Divides parent’s genetic material in HALF

29. How it works… At fertilization these alleles are COMBINED
Since the parents are true breeding, there is only one type of allele that each can send:
Male’s gametes (sperm) can only contain T allele for the height gene
Female’s gametes (egg) can only contain the t allele for the height gene
Offspring has NO CHOICE but to be Tt

30. Now we cross the F1 plants:
Cross of F1s is different because each F1 plant contains both T and t alleles!
Thus, each F1 plant can send EITHER a T or a t to the next generation
There is a 50% chance that T could be sent from one parent in an F1 cross
There is a 50% chance that t could be sent by the other parent in an F1 cross
There is NO CONTROL over which allele is sent by EITHER parent to the offspring. It is RANDOM.

31. Possible results:
You could get an offspring that gets a T from mom and a T from dad
This offspring would be TT
This offspring would be TALL

32. Possible results:
You could get an offspring that gets a T from Mom and a t from dad
This offspring would be Tt
This offspring would also be TALL because T is dominant over t

33. Possible results:
You could get an offspring that gets t from Mom and a T from dad
This offspring would also be Tt
This offspring would also be TALL because T is dominant to t

34. Possible results:
You could get an offspring that gets a t from mom and a t from dad
This offspring would be tt
This offspring would be SHORT

35. Possible results:
If you had LOTS of offspring, the LAWS of PROBABILITY (chance) say that:
1 out of 4 of them will be TT
2 out of 4 of them will be Tt
1 out of 4 of them will be tt

36. Possible Results
AND because of these combinations of alleles, we can also say that
¾ of them would appear TALL
¼ of them would appear SHORT
THIS is what Mendel observed
A 3:1 ratio
That is, for every 3 tall plants produced in the F2 generation, there was only one short plant produced.

37. Other Terms… HOMOZYGOUS HETEROZYGOUS
Describes an individual who has two alleles that are the SAME for a trait TT tt
HETEROZYGOUS
Describes an individual who has two alleles that are DIFFERENT for a trait Tt

38. Another Example:
In humans, tongue rolling is DOMINANT over non tongue rolling
Cross a male homozygous tongue roller with a female non tongue roller

39. What symbol to use?
R = roller
r = non roller

40. What are the genotypes?
Male = RR
Female = rr

41. What are the phenotypes?
Male = Roller
Female = Non roller

42. Intersecting blocks to get
Make a Punnet Square
Male genotype F E M A L
Combine letters of the
Intersecting blocks to get
Possible outcomes of
offspring

43. What will be the genotype fo the F1 generation?Rr

44. What will be the phenotype of the F1 generation?
Roller

45. Now cross the F1s

46. Define Phenotypic Ratio
Number of each phenotype you have
What is the phenotypic ratio of the F2s?
3:1

47. Define genotypic ratio
The number of each different gentoype you have
What is the genotypic ratio of F2s?
1:2:1

48. Any Questions??