1. Genetics #2: Mendel & Inheritance?

2. What has genes?
Genetics is the scientific study of heredity, or where we get our genes from and how.
Every living thing has genes, they are responsible for certain sets of characteristics which get passed from the parent(s) to the offspring.

3. Who discovered Genetics?
Gregor Mendel discovered genetics.
Austrian monk
Went to school for science and math
Taught high school
Was in charge of the monastery garden
He started experimenting with garden pea plants and discovered different modes of inheritance

4. What he knew
Part of each pea plant produces pollen which contains the male reproductive cells, sperm.
The female portion of the plant produces egg cells.
During sexual reproduction the male and female reproductive cells join, this is called fertilization.

5. The result of fertilization is a tiny embryo within a seed.
Pea flowers are normally self-pollinating which means that the pollen from the flower normally fertilizes the egg in the same flower
The seeds produced from a self-pollinating plant bear all the characteristics of their parent plant, therefore they have a single parent.

6. Here comes Mendel!
When Mendel took over the monastery garden there were several types of pea plants.
They produced different characteristics like tallness, shortness, green seeds and yellow seeds.
The plants were true-breeding, which means if they were allowed to self-pollinate they would produce offspring identical to themselves

8. Mendel wanted to produce seeds by joining male and female sperm and eggs from different pea plants.
Mendel started cross-pollinating.He started by cutting off the pollen-bearing male parts in a pea flower, then he dusted pollen from another plant onto the flower. This produces seeds with 2 different parents.

10. A trait is a characteristic like seed color or plant height that varies from one individual to another.
Mendel studied the effects of crossing 7 plants with different traits.
The original plants are called the P (parental) generation and the offspring are called F1 (first filal or “son”)
The offspring of crosses with different parents are called hybrids.

11. Results from the cross
Were the F1 generation plants a blended mix of both parents’ characteristics?
It turns out the F1 generation plants had either the characteristic of one parent OR the other, but not both!
NO!

12. Mendel’s 2 Conclusions
1. Biological inheritance is determined by factors that are passed from one generation to the next.
We call the chemical factors that determine traits, genes.
Each trait Mendel studied was controlled by one gene that occurred in 2 contrasting forms
The different forms of a gene are called alleles.

13. Mendel’s 2 Conclusions
The principle of dominance states that some alleles are dominant while others are recessive.
An organism with the dominant form of a trait will always exhibit that form of the trait. An organism with the recessive form of the trait will exhibit it only if the dominant trait is NOT present.

14. Segregation
Mendel wondered, “have the recessive alleles disappeared or are they still present in the F1 generation?”
He allowed F1 generation to self-pollinate and produce an F2 generation shown below:

15. What he discovered…
When the F1 generation was allowed to self-pollinate the recessive alleles reappeared in the F2 generation!
So why did the recessive alleles seem to disappear in the F1 generation? Let’s take a closer look.

16. Turns out…
The alleles segregate from each other during gamete formation when crossing-over occurs so that each gamete carries only a single copy of each gene.
Each F1 plant produces 2 types of gametes– those with the allele for tallness and those with the allele for shortness.

17. Crossing over - during meiosis, when homologous chromosomes are paired together, there are points along the chromosomes that make contact with the other pair.
These points of contact can allow the exchange of genetic information between chromosomes.

18. Crossing over
Crossing over occurs during prophase I and means that each daughter cell formed in meiosis has chromatids with a different combination of alleles.

19. Probability and Punnett Squares
Mendel discovered that the principles of probability could be used to explain results of genetic crosses
Example: He realized every time he crossed 2 plants hybrid for stem height (Tt), about ¾ of offspring were tall and ¼ were short!
The gene combinations that might result from a genetic cross can be determined by drawing a punnett square. Punnett squares can be used to predict and compare the genetic variations that will result from cross.

20. Punnett Square
The gametes produced by each F1 parent are along the top and left side of the square. The possible gene combinations for the F2 generation appear in the 4 boxes that make up the square.
The letters in the boxes represent alleles, T is the dominant allele for tallness and t is the recessive allele for shortness

21. remember
Organisms that have 2 identical alleles for a particular trait are called homozygous (TT or tt).
True-breeding
Organisms that have 2 different alleles for a particular trait are called heterozygous (Tt).
Hybrid

22. Break it down…
All of the tall plants have the same phenotype, or physical characteristics.
But, they do not have the same genotype, or genetic makeup.
This makes the ratio of tall to short plants 3:1

23. Independent Assortment
Mendel performed a two-factor cross
He first crossed true-breeding plants that produced only round yellow (RRYY) peas with plants that produced wrinkled green peas (rryy)
All F1 offspring are round yellow peas, showing those are the dominant alleles (RrYy)

24. What’s Next?
Mendel knew all F1 plants were heterozygous for both seed shape and color (RrYy)
F1s were crossed with each other to produce F2 generation
Independent allele segregation resulted

25. F2 generation results The F2 plants produced 566 seeds.
315 seeds were round and yellow
32 seeds were wrinkled and green
209 seeds had combinations of the phenotypes
Independent assortment occurred. The alleles for seed color and shape segregated independently.
The results were close to a 9:3:3:1 ratio

26. Independent Assortment
The principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes.
Independent assortment helps account for the many genetic variations observed in plants, animals and other organisms.

27. Summary of Mendel’s Principles
Inheritance of biological characteristics is determined by genes
In cases where 2 or more alleles of a gene for a single trait exist, some are dominant and some are recessive
Most parents have 2 copies of a single gene which segregate when gametes are formed
Alleles for different genes usually segregate independently of one another

28. It’s not black and white
Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes
Incomplete dominance- cases in which one allele is not completely dominant over another (blended phenotype)
Codominance- both alleles contribute to the phenotype (spotty or checkered phenotype)

29. Its not black & white
Multiple Alleles- more than 2 alleles which when they segregate can result in a wider variation of phenotypes (bunny fur color, blood type)
Polygenic traits- traits controlled by 2 or more genes (human skin color)

30. The characteristics of any organism are not determined solely by the genes it inherits.
The characteristics are determined by interaction between genes and the environment
Genes may affect a sunflower’s color and height, but the same factors are also influenced by climate, soil conditions and the availability of water