1. Mendel and Meiosis
Unit 4
Chapter 10

2. Gregor Mendel Austrian monk
Studied patterns of heredity (passing on of characteristics from parent to offspring)
Used the common garden pea in experiments

3. Why did Mendel use peas?
Sexually reproducing: able to isolate both male and female gametes
Easy to identify traits (characteristics that are inherited)
Short life cycle: able to be grown quickly

4. Hybrid
Any offspring of parents with different traits (ex: tall plant x short plant)
Monohybrid cross: cross-pollination (breeding) between two parents with only one variation difference (ex: tall plant x short plant)
Dihybrid cross: cross-pollination (breeding) between two parents with two variation differences (ex: tall, green plant x short, yellow plant)

5. Pea cross-pollination experiments
PARENT GENERATION (P1)
Tall true breed x short true breed
FILIAL GENERATION (F1)
All tall hybrids
FILIAL GENERATION (F2)
75% tall hybrids, 25% short hybrids

6. Phenotypes from P1 to F2 Dihybrid Cross round yellow x wrinkled green
All round yellow F1 F2 9 3 3 1
Round yellow
Round green
Wrinkled yellow
Wrinkled green

7. What did Mendel observe?
When a true-breeding tall plant is crossed with a true-breeding short plant in the P generation, the F1 height trait is always predictable. 100% are tall plants.
P generation F F2

8. What happens when the F1 tall plants are crossed together?
Mendel observed that the F2 generation, the offspring of F1 plants, are always in a fixed ratio of 3:1 tall:short.
Why?
P generation F F2

9. Pea traits that Mendel identified
Through multiple crosses, Mendel determined that all these traits displayed a mathematical predictability for inheritance.
Seed Shape
Seed Color
Seed Coat
Color
Pod
Shape
Pod Color
Flower Position
Plant Height
Round
Yellow
Gray
Smooth
Green
Axial
Tall
Wrinkled
Green
White
Constricted
Yellow
Terminal
Short
Round
Yellow
Gray
Smooth
Green
Axial
Tall

10. Mendel’s conclusions
There must be two variations for every trait, where each variation is called an allele.
Each offspring inherits only one allele from each parent.
The alleles are either dominant or recessive.
To show the recessive trait, two recessive alleles must be inherited.

11. Dominant and recessive traits
The traits that seem to mask other traits when present are called dominant traits.
The traits that seem to be hidden in the presence of dominant traits are called recessive traits.

12. Dominant and recessive traits
Seed
shape
Seed
color
Flower
color
Flower
position
Pod
color
Pod
shape
Plant
height
Dominant
trait
axial
(side)
round
yellow
purple
green
inflated
tall
Recessive
trait
terminal
(tips)
wrinkled
green
white
yellow
constricted
short

13. Homozygous vs. Heterozygous
Homozygous: inherits two similar alleles from the parents for a particular gene
Ex: tall allele and tall allele, written as TT
Ex: short allele and short allele written as tt
Heterozygous: inherits two different alleles from the parents for a particular gene
Ex: tall allele and short allele, written as Tt

14. Law of Segregation
Mendel concluded only one allele is passed from parent to offspring for each trait.
F1 plants must be heterozygous because the P generation only passed on one tall allele and one short allele.
The F1 plant will then pass on to its offspring either a tall or a short allele, never both.

15. A a A a Using a Punnett square AA x aa  100% Aa
Each of the four squares represents 25% chance of inheritance for one offspring. A a
A a

16. Phenotype vs. Genotype Phenotype: physical appearance of the trait
Ex: purple flowers
Genotype: homozygous or heterozygous inheritance
Ex: PP, Pp, pp

17. Law of independent assortment
Because organisms are made up of more than one trait, Mendel concluded that the inheritance of one trait does not influence the inheritance of a second trait.
Example: Height of the pea plant does not influence the color of the peas
Height is independently assorted from color.

18. Using dihybrid crosses to show independent assortment
A smooth, yellow pea (RrYy) can pass on these combinations of genes to its offspring: RY, Ry, rY, or ry.

19. 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

20. Dihybrid crosses Punnett Square of Dihybrid Cross
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

21. Modernizing Mendelian genetics
DNA is the basis for inheritance.
DNA are coiled into chromosomes.
Parts of the DNA that code for a trait are called genes.
Some genes have only two alleles and other have more.
Gene for hairline Allele: A
Genotype: Aa
Gene for hairline Allele: a

22. How do these pictures compare?

23. Variations on Mendel
Incomplete dominance: the heterozygous genotype shows a blend of the two parents and not the dominant allele

24. Variations on Mendel
Codominance: the heterozygous genotype shows both inherited alleles
Example of roan horse coat: AA (dark red) x aa (white)  Aa (dark red and white)

25. Variations on Mendel
Multiple alleles: when there are more than two alleles that code for a trait
Example: ABO blood type
A type = AA or Ao
B type = BB or Bo
O type = oo
AB type = AB

26. Blood typing

27. Variations on Mendel
Polygenic trait: when more than one gene codes for a particular trait
Example: fur color, human height, human skin color, eye color

28. Variations on Mendel
Linked genes: Mendel concluded that traits are assorted independently, but some traits are linked.
This means that two genes are almost always inherited together (ex: red hair, green eyes).

29. Cells and chromosomes
A cell with two of each kind of chromosome is called a diploid cell and has diploid, or 2n, number of chromosomes.
Organisms produce gametes that contain one of each kind of chromosome

30. Homologous chromosomes
The two chromosomes of each pair in a diploid cell are called homologous chromosomes.

31. Homologous chromosomes
On homologous chromosomes, the same types of genes are arranged in the same order.
Because there are different possible alleles for the same gene, the two chromosomes in a homologous pair are not always identical to each other.

32. Making haploid cells
Meiosis is the process of producing haploid gametes with a ½ the amount of DNA as the parent cell.
A cell with one of each kind of chromosome is called a haploid cell and has a haploid, or n, number of chromosomes.
Meiosis enables sexual reproduction to occur.

33. Mitosis and Development
Sexual reproduction
Haploid gametes
(n=23)
Sperm Cell
Meiosis
Meiosis
Egg Cell
Fertilization
Diploid zygote
(2n=46)
Mitosis and Development
Multicellular
diploid adults
(2n=46)

34. Interphase During interphase, the cell replicates its chromosomes.
After replication, each chromosome consists of two identical sister chromatids, held together by a centromere.

35. Prophase I The chromosomes coil up and a spindle forms.
Homologous chromosomes line up with each other gene by gene along their length, to form a four-part structure called a tetrad.

36. Prophase I – crossing over
Chromatids are wrapped so tightly the chromosomes can actually break and exchange genetic material in a process known as crossing over.
Crossing over results in new combinations of alleles on a chromosome.

37. Metaphase I
The centromere of each chromosome attaches to a spindle fiber.
The spindle fibers pull the tetrads into the middle, or equator, of the spindle.

38. Anaphase I
Homologous chromosomes separate and move to opposite ends of the cell.
This critical step ensures that each new cell will receive only one chromosome from each homologous pair.

39. Telophase I
The spindle is broken down, the chromosomes uncoil, and the cytoplasm divides to yield two new cells.
Each cell has half the DNA as the original cell because it has only one chromosome from each homologous pair.

40. Prophase II
A spindle forms in each of the two new cells and the spindle fibers attach to the chromosomes.

41. Metaphase II.
The chromosomes, still made up of sister chromatids, are pulled to the center of the cell and line up randomly at the equator.

42. Anaphase II The centromere of each chromosome splits.
The sister chromatids to separate and move to opposite poles.

43. Telophase II
Finally nuclei reform, the spindles breakdown, and the cytoplasm divides.
Four haploid cells have been formed from one diploid cell

44. Why meiosis is important
Forms gametes for sexual reproduction
Crossing over during meiosis which rearranges allele combinations so that the offspring generations are genetically different than the parents.

45. Nondisjunction leading to trisomy
This can lead to gamete formations having too many or too few chromosomes.
Ex: A gamete with 2 copies of #21 chromosome fertilizes a gamete with 1 copy of #21. The result is an embryo with trisomy 21. This causes Down Syndrome in humans.

46. Trisomy leading to monosomy
A gamete with one copy of the X chromosome fertilizes a gamete missing a copy of the X chromosome.
The result is monosomy X, which in humans causes Turner Syndrome.
Affects 1 in every 2,500 girls.
Most girls with Turner Syndrome are infertile.

47. Nondisjunction leading to polyploidy
When a gamete with an extra set of chromosomes is fertilized by a normal haploid gamete, the offspring has three sets of chromosomes and is triploid.
The fusion of two gametes, each with an extra set of chromosomes, produces offspring with four sets of chromosomes and is a tetraploid.
This occurs often in flowering plants, leading to larger fruit production.