1. Genetics
Biology Honors 2015

2. Genetics and Heredity
Heredity is how traits or characteristics are passed from parent to offspring.
Genetics is the scientific study of how traits are inherited.

3. How Heredity Happens
Chromosomes carry genes which are sections of DNA.
Genes code for traits like hair color.
Parents pass genes to offspring during fertilization or pollination, thus passing on traits.

4. Gregor Mendel https://s-media-cache-ak0.pinimg.com
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Gregor_Mendel.jpg

5. Gregor Mendel Considered “The Father of Genetics”
Austrian monk who tended the gardens of the monastery.
Used pea plants to study the passing of “factors” from parent to offspring.

6. Gregor Mendel
Success due to meticulous record keeping and observations.
Mendel didn’t know about DNA, genes or chromosomes, but he figured out how traits are passed from parent to offspring.
AMAZING!!!

7. Peas – Good Genetics Organisms
True breeding (self-pollinating)
One gene for one trait
Produce generations quickly (not like humans)
Easy to grow and observe
Easy to cross pollinate
Produce many offspring

8. Pea Flowers – Self-Pollinating

9. Pea Traits Observed by Mendel

10. The Beginning of Genetics
Mendel noticed that peas are self-pollinating or “true breeding.” All their genes they get from one parent.
Tall plants produced tall offspring. Short plants produced short offspring.
To cross the two plants with different traits, Mendel collected pollen from a tall plant and pollinated a short plant with it. This created a “hybrid.”

11. Cross Pollination forms a Hybrid
Not this kind of Hybrid!

12. A Closer Look at Cross-Pollination

13. Traits and Alleles
Traits are carried in genes. Genes make up chromosomes. Genes are sections of DNA.
Genes that have different forms are called alleles (pronounced al-leels).
An example is the allele for tall plants and the allele for short plants. Both are alleles for the trait of height.

14. Genes

15. DNA, chromosomes and genes

16. Sexual Reproduction Gametes are sex cells.
Animals produce offspring through sexual reproduction. Their gametes (sperm and egg) unite causing fertilization forming a zygote.
Each gamete carries an allele for each trait. (A or a) Aa a A x =
Sperm
Egg
Zygote

17. Sexual Reproduction
The offspring get 1 allele from Mom and 1 from Dad, giving the offspring a total of two alleles for each trait. (AA, Aa, aa)
The fertilized egg or zygote will have two letters representing each trait. Aa a A x =
Sperm
Egg
Zygote

18. Pollination
When plants reproduce sexually, their pollen and ovule unite through pollination to form the zygote or seed.
x o = AA A A
Pollen grain
Ovule
Seed

19. Pollination
Mendel saw that the peas were self-pollinating, making their offspring exactly the same as the parents.
This is called true breeding.

20. Genotype and Phenotype
There are two ways to describe a genetic organism:
Genotype – the actual genes that make up the organism like TT or tt or Tt
Phenotype – the physical traits or what the organism looks like, tall or short organisms

21. Gregor Mendel https://s-media-cache-ak0.pinimg.com
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36562bac2f jpg
Gregor_Mendel.jpg

22. Pea Flowers and Cross-Pollination
pollination_of_pea_plants/ eng-AU/cross_pollination_of_pea_plants_oversize.jpg

23. Hybrid Peas
A hybrid is a cross between 2 pure plants that have different characteristics for one trait.
(An example: a purple flowering plant crossed with a white flowering plant–two different characteristics for the trait of flower color)
This is a monohybrid because it is a cross for one trait.

24. Creating a Hybrid
If we use T to represent tall and t to represent short, a true breeding tall plant would have the genotype of TT and a short plant would be tt.
TT x tt = Tt
Tt = Hybrid

25. Generation Designations
The first organisms crossed are the parents, or the parental generation. (P)
The offspring of the parental cross is called the first filial or F1 generation.
When two F1 individuals are crossed, they produce the second filial or F2 generation.

26. Generation Designations
Parental (P)
First Filial (F1)
F1 x F1
Second Filial (F2)

27. Mendel’s Hybrids

28. Principle of Dominance
Some traits will be exhibited if they are present. These are called dominant traits and this is the Principle of Dominance.
Dominant traits are represented by a capital letter, for example: T or A.
Only 1 dominant allele needs to be present for the trait to be expressed.

29. Recessive Traits
Recessive traits are over shadowed by dominant traits. These traits will only be exhibited when there is no dominant trait present, leaving 2 lower case letters present.
Recessive traits are represented by 2 lower case letters such as: tt or aa

30. Dominant and Recessive
If an pea plant has the genotype TT or Tt, it”s phenotype will be that of a tall plant.
If a pea plant has the genotype tt, it’s phenotype will be that of a short plant.

31. Homozygous and Heterozygous traits
If an organism has two alleles that are the same, it is said to be “pure” or “homozygous.”
Examples would be “AA or tt”
If an organism has two different alleles for the same trait, it is called a “hybrid” or “heterozygous.”
An example would be “Aa or Tt”

32. The Principle of Segregation
When sex cells are produced, the two alleles for a trait in the parent must separate (segregate) from each other leaving only 1 allele in the gamete.
If a parent has Aa for alleles, when the gametes are formed, one gamete will have “A” and another will have “a.” A a Aa

33. Punnett Squares
To help predict what the probable outcome of a cross will be, we use Punnett Squares. We can predict the likelihood of a certain trait appearing using math and the Punnett Squares.

34. How to use a Punnett square
Cross a homozygous dominant tall plant with a recessive short plant
T T Tt t t

35. Probability
Probability can be expressed either in a ratio or a percentage. For example, the chance of a penny landing on heads is 1 out of 2 or 50%.
Using the Punnett Squares, we can predict the genotypes (letters) and the phenotypes (physical appearance) for the offspring of a cross.

36. Dihybrids
When two pure parents SSTT and sstt are crossed for two different traits like pea seed color and height, they will produce a dihybrid, a hybrid for 2 traits.
SSTT x sstt
SsTt (a dihybrid)

37. Dihybrids
When Mendel cross pollinated plants for 2 traits like height and flower color, he found:
dominant phenotype of one parent (56%)
recessive phenotype of one parent (6%)
38% had a phenotype that was neither dominant or recessive.
For example: 56% - tall with purple flowers
38% - either tall w/ white flowers or short w/ purple flowers
6% - short with white flowers

38. Principle of Independent Assortment
Mendel decided that the alleles for the different traits separated and randomly paired or independently assorted.
To find out what a dihybrid would have in the gametes, you need to pair the alleles as many different ways as possible. For example:
SsTt x SsTt would result in the following combinations: ST, St, sT, st for each of the parents.

39. How to find the alleles combinations
Take each parent separately.
SsTt becomes: Ss Tt = ST
Ss Tt = St
Ss Tt = sT
Ss Tt = st

40. Putting the gamete combinations on a Punnett Square
ST St sT st ST St sT st

41. What the dihybrid cross looks like
ST St sT st
SSTT
SSTt
SsTT
SsTt
SStt
Sstt
ssTT
ssTt
sstt ST St sT st

42. Interpreting Dihybrid Crosses
In a dihybrid cross, the dominant traits will show up in 9 boxes, the recessive traits will show up in 1 box and the boxes that are a mix of dominant and recessive traits will show up in 3 boxes each.
Let the allele S represent smooth pea pods verses lumpy ones and T represent tall pea plants verses short pea plants.

43. What the dihybrid cross looks like
ST St sT st
SSTT
SSTt
SsTT
SsTt
SStt
Sstt
ssTT
ssTt
sstt ST St sT st

44. Interpreting Dihybrid Crosses
The traits in the dihybrid Punnett square:
the most dominant traits are, smooth pea pods with tall plants. (S_T_) IN RED
Traits in between dominant and recessive are,
Smooth pea pods with short plants (S_tt) IN BLUE and lumpy pea pods with tall plants (ssT_) IN PURPLE
The most recessive traits are, lumpy pea pods with short plants (sstt) IN GREEN

45. How many are there of each phenotype?
ST St sT st
SSTT
SSTt
SsTT
SsTt
SStt
Sstt
ssTT
ssTt
sstt ST St sT st

46. Dihybrid Ratio 9 smooth pods and tall plants
3 smooth pods and short plants
3 lumpy pods and tall plants
1 lumpy pods and short plants
It is always a 9:3:3:1 ratio for a dihybrid cross.