1. Mendel and the Laws of Inheritance
Biology
Dobson High School
Hope Finzer

2. Who was Gregor Mendel? Austrian monk and scientist
His work marks the start of modern genetics
Used pea plants to study the passing of traits from generation to generation
First person to succeed in predicting how traits are transferred from one generation to the next

3. Why did Mendel choose pea plants?
Pea plants have both male and female gametes in each plant
They reproduce sexually by the transfer of pollen grains from the male gamete to the female ovule
The plants reproductive structures are easily identified
Short 90 day life cycle Distinct traits: seed shape, seed color, pod shape, pod cover, and height; are some examples

4. Pea Traits

5. How did Mendel control his experiment?
By manually transferring pollen from one plant to another plant with traits that he wanted to test

6. What is a cross?
Manually transferring pollen from a designated plant with the desired trait to another plant with another desired trait

7. What is pollination?
The transfer of the male pollen grains to the female organ of flower, resulting in a fertilized mature seed

8. Mendel’s Investigations
His goal was to explain the patterns of inheritance at the time, no explanation could accurately explain heredity

9. What is a hybrid?
An organism that receives different genetic information for a trait from each parent

10. Mendel’s Experiment
Pure-bred – organisms that when allowed to self-fertilize, only produce offspring the parental trait(s)
Mendel started by crossing pure-bred tall pea plants with pure-bred short pea plant

11. What sort of plants did Mendel get in his first generation?
He called the first pair the (P) or parental group and the first generation the first filial group or the F1 generation
The F1 generation were all tall but each offspring had both the tall and short genes
He then allowed the F1 group self-fertilize and they produced a F2 generation of 3/4th tall plants and 1/4th short plants

12. Parent, F1 and F2 generations

13. How did this happen? What did Mendel determine?
That one trait must be stronger than the other

14. What are the types of traits?
Dominant – the trait that shows in a hybrid
Recessive – trait that does not show in a hybrid

15. Dominant & Recessive traits

16. Mendel’s Hypothesis
Each trait is controlled by something he called a “factor” within the organism
We now call this factor a “gene”
Gene – genetic factor that controls a trait Since 2 parents contribute to the offspring, there must be 2 genes that separate when gametes form
Since traits can disappear for a generation and return unchanged to the next, they must be separate and distinct

17. What is an allele?
Gene form for each variation of a trait of an organism

18. Mendel’s pea plants
The pure-bred plants have 2 of the same alleles; example Mendel’s tall pea plants had 2 alleles for tallness
The hybrid crosses have alleles of 2 different traits
Example Mendel’s F1 generation had 1 allele for tallness and 1 allele for shortness

19. Laws of Segregation
The 2 alleles for each trait must separate when gametes form
A parent passes on at random only 1 allele for each trait to each offspring

20. What is a phenotype?
The way an organism looks and behaves – the outward appearance regardless of what is in the genetic code
Example Mendel’s F1 generation looked tall but had the gene for shortness

21. What is a genotype?
The gene combination an organism contains; example even though Mendel’s F1 generation was all tall plants, they all had for both tallness and shortness
If the organism has the same alleles or traits; example having both tall alleles, usually written as TT; then the organism is considered homozygous

22. What is a genotype?
If the organism has different alleles or traits; example having one tall allele and one short allele, usually written as Tt; then the organism is considered heterozygous

23. What is a Dihybrid cross?
A cross involving two different traits

24. What happens in the F1 generation?
Mendel took a pea plant with round, yellow seeds (these are the dominant traits) and crossed this plant with a pea plant with wrinkled, green seeds (these are the recessive traits)

25. What happens in the F1 generation?
The genotypes of these are:
RRYY and rryy
The entire F1 generation were genotype – RrYy
The entire F1 generation had the phenotype - round, yellow seeds

26. What happens in the F2 generation?
Mendel allowed the plants in the F1 generation to self-pollinate and Mendel found that there were some plants with round, yellow seeds and some wrinkled, green seeds

27. What happens in the F2 generation?
He also found that some plants had round, green seeds and some wrinkled, yellow seed
He found that the traits appeared in a fixed ratio of:
9 round, yellow
3 round, green
3 wrinkled, yellow
1 wrinkled, green

28. What is Mendel’s Law of Independent Assortment?
Genes for different traits are inherited independently of each other

29. What is a Punnett Square?
Short hand way of finding the expected proportions of possible genotype in offspring

30. Monohybrid crosses in the Punnett Square
The Punnett Square is 2 boxes tall and 2 boxes wide.
One parents’ genotype is separated and placed on the top of the Punnett Square and the other parents genotype is separated and placed along the left side of the boxes

31. Monohybrid crosses in the Punnett Square
The alleles are then written into the squares that are underneath or beside them and then all possible offspring can be determined

32. Examples:

33. Exceptions to Mendel’s Laws
Incomplete dominance
Codominance
Sex-linked traits

34. Incomplete Dominance Shows the appearance of a third phenotype
Example: crossing a red flowering snapdragon with a white flowering snapdragon produces pink flowering snapdragons in the F1 generation

35. Incomplete Dominance

36. Codominance Both alleles are expressed in the F1 generation
Example: Cross a black feathered chicken with a white feathered chicken
The F1 generation will show both black and white feathers in a checkerboard pattern

37. Codominance

38. Sex Linked Traits
The expression of traits from genes found on the sex chromosomes – X or Y
Example eye color in fruit flies

39. Examples: Incomplete Dominance

40. Examples: Codominance

41. Examples: Sex-linked traits