1. Introduction to Genetics
A quick review ….

2. What is genetics?
The scientific study of heredity

3. Gregor Mendel Born in 1822 in Czechoslovakia Became a monk in 1843
Taught biology & had interest in statistics
Also studied at the University of Vienna

4. Mendel continued While at the monastery, taught & worked in the garden
Between , he grew & tested over 28,000 pea plants

5. Mendel’s pea plants Easy to grow Easily identifiable traits
Can work with large numbers of samples

6. Mendel’s experiments
Mendel created “purebred” or true-breeding generations
Made sure certain pea plants were only able to self pollinate
Eliminated unwanted traits
He achieved this by cutting away the stamen, or male part, of each flower

7. Genes & dominance Trait: a characteristic
Mendel studied 7 traits
After Mendel ensured his true-breeding generation was pure, he then crossed plants showing contrasting traits
White X purple flowers
Round X wrinkled peas
He called the offspring the F1 generation, or 1st Filial

8. What will happen when you cross pure yellow peas with pure green peas?
All of the offspring were yellow
Hybrids = the offspring of crosses between parents with purebred contrasting traits

9. What did Mendel conclude?
Inheritance is determined by factors passed on from one generation to the next
Mendel knew nothing about chromosomes, genes, or DNA
These terms had not yet been discovered/defined

10. What were Mendel’s “factors”?
“Factors” are genes
Each gene has a different form called alleles
Mendel’s Law of Dominance
Stated that some alleles are dominant and some are recessive

11. Genotype The genetic makeup of an organism Symbolized with letters
Capitalized (T) or lower case (t)
Homozygous = when an organism possesses two identical alleles for the same trait
TT (homozygous dominant)
tt (homozygous recessive)
Heterozygous = when an organism possesses different alleles for the same trait Tt

12. Phenotype The physical appearance of the organism
What the organism looks like
Expression of the trait
Short, tall, yellow, smooth, et cetera

13. ? X = Mendel’s second cross
Mendel allowed the F1 generation to self-pollinate, thus producing the F2 generation
Did the recessive allele completely disappear?
What happened when he crossed two hybrid yellow (F1) pea plants? ? X =

14. Results ¾ of the peas were yellow, ¼ of peas were green
Mendel concluded that during the formation of the sex cells, or gametes, the alleles separated, or segregated, to different gametes (pollen or egg)
Mendel’s Law of Segregation

15. Probability The likelihood of a particular even occurring
Chance
Can be expressed as a fraction or percent
Example: coin flip

16. Probability & statistics
No one event has a greater chance of occurring than another
You cannot predict the precise outcome of an individual event
The more trials performed, the closer the actual results reflect the expected outcome

17. Punnett square Developed by Reginald Punnett
A diagram used to show the probability, or chances, of a certain trait being passed from one generation to the next

18. Reading Punnett Squares
Capital letters (Y) represent dominant alleles
Y = yellow pea color
Lower case letters (y) represent recessive alleles
y = green pea color
Gametes are placed above & to the left of the square
Offspring are placed in the square
Yy  Y and y gametes Y y YY Yy Y y Yy yy

19. Punnett Square review

21. Mendel’s Law of Independent Assortment
A cross involving two or more traits
Example: color of peas & shape of peas
Alleles separate independently of each other during the formation of gametes
Anaphase I

22. Monohybrid cross
Both parents are heterozygous (hybrid) for one trait

23. Dihybrid cross Both parents are heterozygous (hybrid) for two traits
Example: RrYy x RrYy
A specific type of two-trait problem
AaBb x AaBb is a dihybrid cross
Aabb x aaBb is not a dihybrid cross

24. Mendel’s death Mendel published his paper on heredity in 1866
The scientific community saw little, if any, importance in his work
Mendel died in 1884 with no recognition for his contributions to genetics

25. Some exceptions of Mendel’s principles
Some alleles are neither dominant nor recessive
Many traits are controlled by more than one gene (polygenic)

26. Incomplete dominance Neither allele is dominant
When both alleles are present, a “new” phenotype appears that is a blend of each allele
Alleles should be represented by capital letters
Use superscripts or prime marks to differentiate
Cr = red Cw = white or R’ = red R = white

27. Four-o-clock flowers Red flower genotype = CrCr (or R’R’)
White flower genotype = CwCw (or RR)
Pink flower genotype (the heterozygous state) = CrCw (or R’R)
Be careful:
Some worksheets use RR = red, WW = white, RW = pink … technically, this is WRONG RW = haploid = gamete

28. What happens when a red flower is crossed with a white flower?
CrCr X CwCw
All offspring are pink
CrCw
pink
CrCw
pink Cw
CrCw
pink
CrCw
pink Cw

29. Codominance
When the phenotype of both alleles is fully expressed in the heterozygote
CrCr = cattle with red hairs
CwCw = cattle with white hairs
CrCw = cattle with roan coat (mixture of red & white hairs)

30. Multiple alleles
When more than two alleles, for a single trait, are found in nature
Human blood types: A, B, O, and AB
A and B are codominant to each other
A and B are both dominant over O

31. Human blood types Type A Allele = IA
Blood cells have antigen A on membrane surface
Antigen = membrane protein

32. Human blood types Type B Allele = IB
Blood cells have antigen B on membrane surface

33. Human blood types Type AB Genotype = IAIB
Blood cells have both types of antigens on cell membrane
Known as the universal recipient

34. Human blood types Type O Allele = i
Blood cells have no antigens on cell membrane
Known as the universal donor

35. Human blood types Genotypes & phenotypes IAIA A blood IAi A blood
IBIB B blood
IBi B blood
IAIB AB blood
ii O blood

36. How common are the different blood types?
45 %
40 %
4 %
11 %

37. Try it! A sample problem
A man with type AB blood marries a woman with type B blood (whose father has type O blood).
What are the chances they have a child with type A blood?
A child with type AB blood?

38. Polygenic traits Traits controlled by two or more genes
Examples: human height, eye color, skin color, hair color, IQ

39. Rediscovery of Mendel’s work
Early 1900’s many scientists “rediscovered” Mendel’s work
Sutton
1908 – Garrod
Morgan

40. Thomas Hunt Morgan
Born in Kentucky, professor of Biology at Columbia University
Worked with fruit flies (Drosophila)
Nobel Prize in Medicine (1933)

41. Why do geneticists love the fruit fly?
Can work with large numbers of flies easily
Produce many offspring
Short reproductive cycle
Only 4 pairs of chromosomes

42. Gene linkage Morgan found that many genes are linked together
Morgan specifically noticed sex-linkage
It was also determined that chromosomes, not genes, assort independently during meiosis

43. Gene maps & crossing-over
First developed by Sturtevant in 1911
The farther apart two genes are, the more likely they will cross-over during meiosis

44. Any questions?