1. Introduction to Mendelian Genetics Monohybrid Crosses
SBI3U

2. Learning goals
Understand the basic terminology of mendelian genetics, not limited to (phenotype, genotype, homozygous, heterozygous, P,F1,F2 generation(s), dominant, recessive, alleles, gene
Solve monohybrid problems using Punnett square method

3. Hippocrates (460 – 377 B.P.)
every part of the body was involved in the production of “seeds”
The seeds would fuse together to make a new person

4. ARISTOTLE (384- 322 B.C.E) Theory of Pangenesis
Egg and sperm consist of particles  pangenes from all parts of body
upon fertilization the pangenes developed into the parts of the body from which they were formed.

5. WILLIAM HARVEY (1578 – 1657)
New individual arose through the process of EPIGENESIS
 embryo grew in stages and was affected by factors both inside and outside of the mother

6. ANTON VAN LEEUWENHOEK (1632 – 1723)
Used a microscope to examine gametes
found “animalcules” in semen and decided they were preformed embryo’s
Female contribution was only the influence of the uterus during embryo development

7. BLENDING THEORY Characteristics of parents blended in the offspring
Blending was irreversible, thus original characteristics would not appear in future generations

8. FRANCIS GALTON (1870’s)
Disproved the theory of pangenesis (still influenced biology 100 years ago)
Genes formed in every organ of both male and female body and moved to genitals

9. CHARLES DARWIN (1809 – 1882)
Offspring have variations of their parents characteristics
Couldn’t explain why – suggested current science had not yet discovered the puzzle pieces yet

10. Mendel’s law of heredity
GREGOR MENDEL (1822 – 1884)
Developed the fundamental principles of modern genetics
Experimented on pea plants and proposed
Mendel’s law of heredity

11. 1677 Anton van Leewenhoek
First discovered living sperm (also designed first microscope)
Believed he saw a complete miniature person in the head of a sperm; homunculi

12. 19th Century British
From breeding ornamental plants, scientists believed in a blending theory of inheritance. 12

13. Gregor Mendel (1822-1884)‏ Austrian monk
Considered the father of genetics (now aka Mendelian Genetics)‏
Why the pea plant?
grows easily and quickly
has seven traits that are easily identified
it is easy to manipulate 13

14. Gregor Mendel ( )
Used pea plants to study genetics, heredity and variation.
Looked at 7 different traits that only had two variations.
From his meticulous work he came up with many “key terms” and, more importantly, two generalizations that later became known as Mendel’s Laws.

15. 7 traits Mendel looked at15

16. Mendel’s true breeding experiments
Pea plants reproduce sexually, but usually self fertilize
True breeding: organisms that exhibit the same traits generation after generation

17. Mendel's Pea Plant Experiment
Mendel selected true bred plants for different traits and cross-fertilized them to see what would happen.
True breeding: Individuals that only contain one variation of a trait and therefore can only pass this one variation on to future generations. We now call these individuals homozygous, or having two alleles that are the same.
For example:
Tall x dwarf
What do you think the offspring looked like? X

18. 100% of the offspring plants were tall
100% of the offspring plants were tall! (Instead of a blending – where all plants would be medium sized...) X 18

19. Mendel then let the F1 generation of offspring self-pollinate...
F1 generation – (filial (son or daughter) generation) offspring from the cross (reproduction) of the Parent or P generation.
this time most were tall, but some were short and ALWAYS 3:1 ratio x 19

20. Why?
Each plant in the F1 generation carried an allele from the P generation; a tall allele and a dwarf allele. They are what are called hybrids.
Hybrids: Individuals that contain more than one variation of a trait and therefore can pass on more than one variation to future generations. We now call these individuals heterozygous, or having two alleles that are different from each other.
Traits are dominant or recessive. If a dominant trait is inherited, it will be expressed.
What is the dominant trait for height in pea plants? 20

21. Mendel’s “Key Terms”
Phenotype: the appearance of traits in an organism (eg. short or tall)‏
Genotype: the specific genes that an organism has (which alleles, dominant or recessive)‏
Every individual has a phenotype and genotype for every trait.
The phenotype is easy to figure out, but occasionally the genotype is hard to determine, especially for dominant traits.

22. Alleles: Alternate forms of a gene

23. Genotype vs. Phenotype

24. Which Allele is expressed?

25. Can you tell the genotype of an individual by their phenotype?

27. Mendel's Laws and Principles
Law of Segregation: when any individual produces gametes, the copies of a gene separate so that each gamete receives only one copy of a gene and therefore only one allele for all possible traits. 27

28. Law of Segregation Principle of Segregation Demystified Segregation
Mendelian Genetics
9/21/2018
Law of Segregation
Principle of Segregation Demystified
Segregation
The principle of segregation is explained by the behavior of homologous chromosomes at meiosis.
G. Podgorski, Biol 1010 28

29. Law of Independent Assortment: alleles of different genes assort independently of one another during gamete formation, so different traits are inherited independently of each other
In plain English, this means that if you are able to be dominant for one trait, it will have no influence on whether you will be dominant for another trait.

31. Punnett Square

32. Punnett Square

33. Dominant traits This phenomenon can be explained by the
fact that some ________ are dominant. A
dominant allele is an allele that __________
its phenotypic effect when present in the
individual.

34. Recessive traits
9. A recessive allele is expressed only when _____ alleles are of the recessive form. The genotype of an individual showing a recessive phenotype must be ____________ for the recessive allele.

35. Punnett Square

36. Punnett Square

37. Genotypic proportions
11. The genotypic proportions of the offspring from the cross of two heterozygous parents (Bb) are:
BB __/4
Bb __/4
bb __/4

38. Phenotypic proportions
12. The phenotypic proportions of the offspring are:
Purple __/4 or __%
White __/4 or __%

39. Monohybrid cross problem
Curly hair is dominant over straight hair. If a
mother is homozygous dominant for curly
hair and the father has straight hair, what
type of hair will the children have?

40. Test Crosses
When geneticists want to know if an individual is heterozygous or homozygous
Why? For breeding purposes
Cross between unknown phenotype and homozygous recessive individual
(Pp or PP) x pp
Pp x pp = 50% purple, 50% white
PP x pp = 100% purple