1. Unit 6
Mendelian Genetics

2. Section 10.1 Summary – pages 253-262
Who is Mendel ?
Gregor Mendel - Mid-nineteenth century Austrian monk who carried out important studies of heredity.
First person to succeed in predicting how traits are transferred from one generation to the next.
“Father of Genetics”
Section 10.1 Summary – pages

3. Section 10.1 Summary – pages 253-262
What Did He Do?
Experimented with garden peas
reproduce sexually, which means that they produce gametes.
Section 10.1 Summary – pages

4. Section 10.1 Summary – pages 253-262
What Did He Do?
The male gamete forms in the pollen grain
The female gamete forms in the female reproductive organ.
Pollination
Fertilization
Zygote
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5. Section 10.1 Summary – pages 253-262
What Did He Do?
When he wanted to breed, or cross, one plant with another, Mendel opened the petals of a flower and removed the male organs.
Remove male parts
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6. Section 10.1 Summary – pages 253-262
What Did He Do?
He then dusted the female organ with pollen from the plant he wished to cross it with.
Pollen
grains
Transfer pollen
Female
part
Male
parts
Cross-pollination
Section 10.1 Summary – pages

7. Section 10.1 Summary – pages 253-262
What Did He Do?
This process is called cross-pollination.
By using this technique, Mendel could be sure of the parents in his cross.
Section 10.1 Summary – pages

8. Section 10.1 Summary – pages 253-262
Mendel’s Experiments
prefix
Mendel’s first experiments are called monohybrid crosses because mono means “one” and the two parent plants differed from each other by a single trait—height.
Section 10.1 Summary – pages

9. Section 10.1 Summary – pages 253-262
Mendel’s Experiments
He cross-pollinated this tall pea plant with pollen from a short pea plant.
All of the hybrid offspring grew to be as tall as the taller parent. P1
Short pea plant
Tall pea plant F1
Section 10.1 Summary – pages

10. Section 10.1 Summary – pages 253-262
Mendel’s Experiments
Mendel allowed the first generation to self-pollinate.
Three-fourths of the plants were as tall as the parent and first generations. P1
Short pea plant
Tall pea plant F1
All tall pea plants F2
3 tall: 1 short
Section 10.1 Summary – pages

11. Section 10.1 Summary – pages 253-262
Mendel’s Experiments
P1 generation = The original parents, the true-breeding plants
F1 generation = The offspring of the parent plants
F2 generation = cross two F1 plants with each other
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12. Section 10.1 Summary – pages 253-262
Mendel’s Experiments: The Conclusion
In every case, he found that one trait of a pair seemed to disappear in the F1 generation, only to reappear unchanged in one-fourth of the F2 plants.
Section 10.1 Summary – pages

13. Section 10.1 Summary – pages 253-262
Mendel’s Experiments: The Conclusion
The rule of unit factors
each organism has two factors that control each of its traits.
genes that are located on chromosomes.
Alleles: different forms of a gene
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14. Section 10.1 Summary – pages 253-262
Mendel’s Experiments: The Conclusion
The rule of unit factors
An organism’s two alleles are located on different copies of a chromosome—one inherited from the female parent and one from the male parent.
Section 10.1 Summary – pages

15. Section 10.1 Summary – pages 253-262
Mendel’s Experiments: The Conclusion
The rule of dominance
Dominant: the observed trait
Recessive: the trait that disappeared
Short plant
Tall plant T T t t
Mendel concluded that the allele for tall plants is dominant to the allele for short plants. T t F1
All tall plants T t
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16. Section 10.1 Summary – pages 253-262
Mendel’s Experiments: The Conclusion
The law of segregation
Every individual has two alleles of each gene and when gametes are produced, each gamete receives one of these alleles.
During fertilization, these gametes randomly pair to produce four combinations of alleles.
Section 10.1 Summary – pages

17. Section 10.1 Summary – pages 253-262
Phenotypes and Genotypes
Law of segregation
Tt ´ Tt cross
Two organisms can look alike but have different underlying allele combinations. F1
Tall plant
Tall plant T t T t F2
Tall
Tall
Tall
Short T T T t T t t t 3 1
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18. Section 10.1 Summary – pages 253-262
Phenotypes and Genotypes
Phenotype: the way an organism looks and behaves
Genotype: the allele combination an organism contains
An organism’s genotype can’t always be known by its phenotype.
Section 10.1 Summary – pages

19. Section 10.1 Summary – pages 253-262
Phenotypes and Genotypes
Homozygous: An organism that has two alleles for a trait that are the same.
The true-breeding tall plant that had two alleles for tallness (TT) would be homozygous for the trait of height.
Section 10.1 Summary – pages

20. Section 10.1 Summary – pages 253-262
Phenotypes and Genotypes
Heterozygous: An organism that has two alleles for a trait that differ from each other.
Therefore, the tall plant that had one allele for tallness and one allele for shortness (Tt) is heterozygous for the trait of height.
Section 10.1 Summary – pages

21. Section 13.1 Summary – pages 337 - 340
Determining Genotypes
The genotype of an organism that is homozygous recessive for a trait is obvious to an observer because the recessive trait is expressed.
However, organisms that are either homozygous dominant or heterozygous for a trait controlled by Mendelian inheritance have the same phenotype.
Section 13.1 Summary – pages

22. Section 10.1 Summary – pages 253-262
Mendel’s Experiments: The Conclusion
The law of independent assortment
Mendel’s second law states that genes for different traits—for example, seed shape and seed color—are inherited independently of each other.
This conclusion is known as the law of independent assortment.
Section 10.1 Summary – pages

23. Section 10.1 Summary – pages 253-262
Can we predict outcomes of offspring??
Yes Punnett Squares
In 1905, Reginald Punnett, an English biologist, devised a shorthand way of finding the expected proportions of possible genotypes in the offspring of a cross.
This method is called a Punnett square.
Section 10.1 Summary – pages

24. Section 10.1 Summary – pages 253-262
Punnett Squares
If you know the genotypes of the parents, you can use a Punnett square to predict the possible genotypes of their offspring.
Section 10.1 Summary – pages

25. Section 10.1 Summary – pages 253-262
Monohybrid crosses
A Punnett square for this cross is two boxes tall and two boxes wide because each parent can produce two kinds of gametes for this trait.
Heterozygous
tall parent T t T t T t T T t t T t
Heterozygous
tall parent
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26. Section 10.1 Summary – pages 253-262
Monohybrid crosses
The two kinds of gametes from one parent are listed on top of the square, and the two kinds of gametes from the other parent are listed on the left side.
Heterozygous
tall parent T t T t T t T T t t T t
Heterozygous
tall parent
Section 10.1 Summary – pages

27. Section 10.1 Summary – pages 253-262
Monohybrid crosses
It doesn’t matter which set of gametes is on top and which is on the side.
Each box is filled in with the gametes above and to the left side of that box. You can see that each box then contains two alleles—one possible genotype.
After the genotypes have been determined, you can determine the phenotypes.
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28. Section 10.1 Summary – pages 253-262
Punnett Square of Dihybrid Cross
Dihybrid crosses
Gametes from RrYy parent RY Ry rY ry
RRYY
RRYy
RrYY
RrYy
A Punnett square for a dihybrid cross will need to be four boxes on each side for a total of 16 boxes. RY
RRYy
RRYy
RrYy
Rryy Ry
Gametes from RrYy parent
RrYY
RrYy
rrYY
rrYy rY
RrYy
Rryy
rrYy
rryy ry
Section 10.1 Summary – pages

29. Section 10.1 Summary – pages 253-262
Punnett Square of Dihybrid Cross
Dihybrid crosses
Gametes from RrYy parent RY Ry rY ry
RRYY
RRYy
RrYY
RrYy RY
F1 cross: RrYy ´ RrYy
RRYy
RRYy
RrYy
Rryy Ry
round
yellow
Gametes from RrYy parent
RrYY
RrYy
rrYY
rrYy
round
green rY
wrinkled
yellow
RrYy
Rryy
rrYy
rryy ry
wrinkled
green
Section 10.1 Summary – pages

30. Section 10.1 Summary – pages 253-262
Probability
In reality you don’t get the exact ratio of results shown in the square.
That’s because, in some ways, genetics is like flipping a coin—it follows the rules of chance.
A Punnett square can be used to determine the probability of getting a result
Section 10.1 Summary – pages

31. Genetics
Mendel and Meiosis
Meiosis
Unit Overview – pages

32. Section 10.2 Summary – pages 263-273
Genes, Chromosomes, and Numbers
Genes do not exist free in the nucleus of a cell; they are lined up on chromosomes.
Typically, a chromosome can contain a thousand or more genes along its length.
Section 10.2 Summary – pages

33. Section 10.2 Summary – pages 263-273
Diploid and haploid cells
In the body cells of animals and most plants, chromosomes occur in pairs.
Diploid : A cell with two of each kind of chromosome (2n)
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34. Section 10.2 Summary – pages 263-273
Diploid and haploid cells
This pairing supports Mendel’s conclusion that organisms have two factors—alleles—for each trait.
Organisms produce gametes that contain one of each kind of chromosome.
Haploid: a cell containing one of each kind of chromosome (n)
Section 10.2 Summary – pages

35. Section 10.2 Summary – pages 263-273
Homologous chromosomes
Homologous chromosomes: the two chromosomes of each pair in a diploid cell
Each pair of homologous chromosomes has genes for the same traits.
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36. Section 10.2 Summary – pages 263-273
Homologous chromosomes
On homologous chromosomes, these genes are arranged in the same order, but because there are different possible alleles for the same gene, the two chromosomes in a homologous pair are not always identical to each other.
Homologous Chromosome 4 a A
Terminal
Axial
Inflated D d
Constricted T t
Short
Tall
Section 10.2 Summary – pages

37. Section 10.2 Summary – pages 263-273
Why meiosis?
When cells divide by mitosis, the new cells have exactly the same number and kind of chromosomes as the original cells.
Each pea plant parent, which has 14 chromosomes, would produce gametes that contained a complete set of 14 chromosomes…. And each mitosis cycle would continue to double the chromosomes…….
Section 10.2 Summary – pages

38. Section 10.2 Summary – pages 263-273
Why meiosis?
There must be another form of cell division that allows offspring to have the same number of chromosomes as their parents.
Meiosis: a kind of cell division, which produces gametes containing half the number of chromosomes as a parent’s body cell
Section 10.2 Summary – pages

39. Section 10.2 Summary – pages 263-273
Why meiosis?
Meiosis consists of two separate divisions, known as meiosis I and meiosis II.
The eight phases of meiosis
prophase I prophase II
metaphase I metaphase II
anaphase I anaphase II
telophase I telophase II.
Section 10.2 Summary – pages

40. Section 10.2 Summary – pages 263-273
Why meiosis?
Meiosis I begins with one diploid (2n) cell.
By the end of meiosis II, there are four haploid (n) cells.
Sperm: male gametes
Eggs: Female gametes
( sperm fertilizes an egg, the resulting zygote is diploid)
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41. Section 10.2 Summary – pages 263-273
Why meiosis?
Haploid gametes
(n=23)
Sexual reproduction: reproduction involving the production and fusion of haploid sex cells
Sperm Cell
Meiosis
Egg Cell
Fertilization
Diploid zygote
(2n=46)
Multicellular
diploid adults
(2n=46)
Mitosis and Development
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42. Section 10.2 Summary – pages 263-273
The Phases of Meiosis
During meiosis, a spindle forms and the cytoplasm divides in the same ways they do during mitosis.
However, what happens to the chromosomes in meiosis is very different.
Section 10.2 Summary – pages

43. Section 10.2 Summary – pages 263-273
Prophase I
chromatin coils up into visible chromosomes
The spindles form
Synapsis: the homologous chromosomes line up forming a four part structure called a tetrad
Crossing over may occur
exchange genetic material
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44. Section 10.2 Summary – pages 263-273
Metaphase I
Chromosomes become attached to the spindle fibers by their centromeres and tetrads line up on the midline of the cell
Anaphase I
Homologous pairs separate, sister chromatids remain attached
Telophase I
Chromosomes unwind, spindles break down, cytoplasm divides
Two new diploid cells are formed
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45. Section 10.2 Summary – pages 263-273
Prophase II
The spindles form
Metaphase II
Chromosomes become attached to the spindle fibers by their centromeres and chromosomes line up on the midline of the cell
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46. Section 10.2 Summary – pages 263-273
Anaphase II
The centromere of each chromosome splits and sister chromatids separate and move to opposite poles
Telophase II
Chromosomes unwind, spindles break down, cytoplasm divides, nuclei re-form
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47. Section 10.2 Summary – pages 263-273
MEIOSIS I
MEIOSIS II
Possible gametes
Possible gametes
Chromosome b
Chromosome A
Chromosome B
Chromosome a
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48. Section 10.2 Summary – pages 263-273
Meiosis Provides for Genetic Variation
Cells that are formed by mitosis are identical to each other and to the parent cell.
**Crossing over during meiosis, increases the genetic variability due to allele combinations.
Genetic recombination is a major source of variation among organisms
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49. Section 10.2 Summary – pages 263-273
Genetic recombination
It is a major source of variation among organisms.
MEIOSIS I
MEIOSIS II
Possible gametes
Possible gametes
Chromosome A
Chromosome B
Chromosome a
Chromosome b
Section 10.2 Summary – pages