1. Meiosis and Mendel
Biology 1 Chapter 6

2. 6.1 Chromosomes and Meiosis
Cells can be divided into 2 major groups:
Somatic (body): most of body tissue and organs
Germ: found in reproductive organs, develop into gametes
Gametes are sex cells – ova (eggs) & spermatozoa (sperm)
The DNA in gametes is what is passed on to offspring
Each somatic cell in the human body contains 46 chromosomes which come in 23 pairs.

3. Cells within an organism are different from one another because different genes are expressed, not because they have different genes.
You receive 23 chromosomes from your mother and 23 from your father for a total of 46 in each of your cells.
Each pair of chromosomes in your cells come from a combination of 1 cell from your mother and 1 cell from your father.
Each pair of chromosomes is referred to as a homologous pair.
Homologous chromosomes have the same length, appearance, and genes.

4. Pairs of homologous chromosomes are ordered from largest to smallest and assigned numbers 1 – 23.
Pairs 1 – 22 are called autosomes and contain genes not related to the sex of the organism.
Pair 23 is composed of the sex chromosomes (X and Y).
XX – female
XY – male
The x chromosome is larger than the Y

5. The X chromosome contains several genes, many of which are not related to sex characteristics.
The Y chromosome contains genes the direct the development of the testes and male traits.
The Y chromosome is the smallest chromosome and carries the fewest genes.

6. Sexual reproduction involves the fusion of 2 gametes.
The fusion of a sperm and egg cells is called fertilization.
During fertilization, the nuclei of the sperm and egg fuse to form one nucleus.
In order for the new nucleus to have the correct number of chromosomes, the egg and sperm cells contain only half the usual number of chromosomes.

7. Body cells contain 2 copies of each chromosome
Body cells contain 2 copies of each chromosome. One copy from the mother and one copy from the father.
Any cell that contains 2 copies of each chromosome is called diploid (2n). The diploid number is 46.
Gametes only contain 1 copy of each chromosome.
Any cell that contains 1 copy of each chromosome is called haploid.

8. Each gamete (sperm or egg) has 22 autosomes and 1 sex chromosome.
In egg cells, the sex chromosome is always X
In sperm cells, the sex chromosome could be X or Y
Maintaining the correct number of chromosomes is important for survival.
Normally a change in the number of chromosomes is harmful.

9. Germ cells in the reproductive organs undergo meiosis in order to form gametes.
During meiosis, a diploid cell is divided into a haploid cell that contains half the number of chromosomes of the parent cell.

10. Differences between mitosis and meiosis:
Mitosis produces genetically identical cells. Meiosis produces unique cells.
Mitosis results in diploid cells (2n). Meiosis results in haploid cells (n).
Mitosis takes place throughout life. Meiosis takes place only at certain times.
Mitosis is involved in asexual reproduction. Meiosis is involved in sexual reproduction.

11. Mitosis
DNA is copied once and divided once.
Parent and daughter cells are diploid.
Development, growth, and repair
Meiosis
DNA is copied once but divides twice
Parent cells are diploid but daughter cells are haploid

12. 6.2 Process of Meiosis
During meiosis, 1 diploid cell creates 4 haploid cells.
There are 2 rounds of cell division in meiosis: I and II
Each round has 4 phases that are similar to mitosis.
Homologous chromosomes come from two different sources: one from the mother and one from the father.
Homologous chromosomes are very similar (same length and genes).
Homologous chromosomes are not copies of each other.

13. Each half of a homologous chromosome is called a chromatid.
The 2 chromatids that make up a chromosome are called sister chromatids.
Sister chromatids remain attached by a centromere.
Sister chromatids are exact copies of each other.
Homologous chromosomes are divided during meiosis I.
Sister chromatids are divided during meiosis II.

16. Meiosis I
DNA has already been copied
Divides homologous chromosomes producing 2 haploid (n) cells with duplicated chromosomes.
Described in terms of phases just like mitosis.

17. Meiosis I Phases
Prophase I
Nuclear membrane breaks down.
Centrosomes and centrioles move to opposite ends of the cell.
Spindle fibers assemble.
Duplicated chromosomes condense
Homologous chromosomes pair up

18. Metaphase I
Homologous chromosome pairs line up along the middle of the cell.
Of the 46 chromosomes, 23 are from the mom and 23 are from the dad.
In random order, 23 chromosomes line up on each side of the equator of the cell.
The random order allows for genetic diversity.

19. Anaphase I
The paired homologous chromosomes separate from each other
The chromosomes move to opposite ends of the cell.
The sister chromatids remain attached.

20. Telophase I
Nuclear membrane forms again
Spindle fibers disassemble
The cell undergoes cytokinesis
The end result is 2 cells that have a unique combination of 23 chromosomes coming from both parents.

22. Meiosis II
Division of sister chromatids.
Results in undoubled chromosomes.
Both new cells from meiosis I will undergo meiosis II.
DNA is not duplicated again.

23. Phases of Meiosis II
Prophase II
Nuclear membrane breaks down
Centrosomes and centrioles move to opposite sides of the cell.
Spindle fibers form.

24. Metaphase II
The 23 chromosomes line up along the cell’s equator.
Each chromosome still has 2 sister chromatids.
Anaphase II
Sister chromatids are pulled apart and move to opposite ends of the cell.

25. Telophase II
Nuclear membrane form around each set of chromosomes.
Spindle fibers break apart.
Cell undergoes cytokinesis.
End result is 4 haploid cells with a combination of chromosomes from the mother and father.

27. Key differences between mitosis and meiosis.
Mitosis has 1 division, meiosis has 2
In mitosis homologous chromosomes never pair up. In meiosis, they do.
In anaphase of mitosis, sister chromatids separate. In meiosis, they remain together.
Mitosis results in diploid cells (2n). Meiosis results in haploid cells (n).

28. Gametogenesis is the process where the haploid cells produced by meiosis become mature gametes.
The sperm cell is much smaller than the egg cell.
The sperm cell’s main contribution is DNA.

29. During gametogenesis of sperm cells:
A round cell becomes a streamlined cell that can move rapidly.
DNA is tightly packed in the head and a lot of the cytoplasm is lost.
A flagellum tail allows for movement.
The neck contains mitochondria to produce energy.

30. The formation of egg cells begins before birth in the female embryo and continues for many years.
The egg cell contributes DNA, organelles, and molecular building blocks.
Only one of the 4 cells produced by meiosis will become an egg. It is the one that receives the most materials.
The other 3 cells will become polar bodies that eventually break down.

32. 6.3 Mendel and Heredity Traits are characteristics that are inherited.
Traits are hereditary meaning they are passed on from one generation to the next.
Genetics is the study of inheritance.
Most of our understanding was established by Gregor Mendel.
Mendel used plants to predict the results of meiosis long before chromosomes were discovered.

33. Mendel recognized that traits are inherited as separate units from the parents.
He recognized that organisms inherit 2 copies of each unit.
These units are what we now call genes.
He described how traits are passed between generations.

34. Mendel’s experiments had 4 variables:
He used all pea plants
Control over breeding by removing male parts of plants.
Use of purebred plants (genetically the same)
Used either-or traits that only appeared in 2 forms.

35. Mendel chose 7 traits:
Pea shape
Pea color
Pod shape
Pod color
Plant height
Flower color
Flower position

36. In genetics, the mating of 2 organisms is called a cross.
In Mendel’s experiments, the first plants that he crossed were called the parental (P) generation.
The offspring of this cross was called the first filial (F1) generation.

37. The F1 generation all had the same form of the trait.
When the F1 generation was allowed to fertilize, an F2 generation was produced.
The F2 generation showed both forms of the trait which means that the trait did not disappear but was hidden.

38. Mendel’s Experiment

39. Mendel crossed many plants and was able to observe patterns.
Each cross produced the same ratio in the F2 generation.
¾ of the plants had one form of the trait
¼ of the plants had the other form
The produced a 3:1 ratio

40. Mendel’s conclusions:
Traits are inherited as separate units
The law of segregation (Mendel’s 1st law)
Organisms inherit 2 copies of each gene, one from each parent.
Organisms only donate one copy of each gene. The 2 copies of each gene segregate (separate) when gametes form.

41. 6.4 Traits, Genes, and Alleles
A gene is a section of DNA that gives instructions for how to make proteins.
Each gene has a locus which is a specific position on a chromosome.
An allele is a different form of a gene.
Your cells have 2 alleles for each gene, one on each homologous chromosome.
Each parent gives one allele for each gene.

42. Alleles may be the same or different.
Homozygous means that the alleles are the same.
Heterozygous means that the alleles are different.
A genome is all of an organism’s genetic material.
The human genome contains 3 billion nucleotide pairs.
With the exception of identical twins, all humans have a unique genome.

43. Genotype is a genetics term that refers to genetic makeup.
Phenotype is a genetics term that refers to the physical characteristics or traits.
When referring to heterozygous traits, the terms dominant and recessive are used.
A dominant allele is the trait that will be expressed.
A recessive allele will not be expressed if the dominant allele is present.
It is only expressed when 2 copies are present.

44. The terms dominant and recessive do not necessarily mean that one is better or stronger.
Alleles are usually represented with letters.
A genotype is usually shown with 2 letters, one per allele received from each parent.
Upper case letters are used for dominant alleles and lower case letters are used for recessive alleles.

45. A genotype can be:
Homozygous dominant
Homozygous recessive
heterozygous
When the genotype is homozygous dominant or heterozygous, the dominant trait will be expressed.
The only time when the recessive trait will be expressed is when both of the alleles are recessive (homozygous recessive).
Which allele is dominant depends on the proteins made by the organism.

46. In general, inheritance is much more complex.
Genes can be codominant instead of dominant or recessive.
Many traits are influenced by several genes.
The environment can also affect gene expression.

47. 6.5 Traits and Probability
A Punnett square is a grid system that predicts all possible genotypes that will result from cross.
A Punnett square lists the genotypes for the parents and all of the possible genotypes of the offspring.
Each combination of alleles is as likely to be produced as the others.
By looking at the results of the cross, we can find the ratios of genotypes or phenotypes.

49. Monohybrid Crosses homozygous – homozygous heterozygous - heterozygous
heterozygous – homozygous

50. A dihybrid cross examines the inheritance of 2 different traits.
In Mendel’s experiments he always started with purebred plants.
In each dihybrid cross experiment, the F1 generation were all heterozygous and all looked the same.
When the F1 generation was allowed to self pollinate he always obtained the same results in the F2 generation - 9:3:3:1

52. Mendel always obtained the same results in each trial regardless of the combination of traits.
Based on these results he realized that the presence of one trait did not affect the presence of another trait.
This allowed him to come up with his second law of genetics called the law of independent assortment.
This law states that allele pairs separate independently of each other when gametes form.
In other words, different traits are inherited separately.

54. Probability is the likelihood that an event will happen.
It predicts the average number of occurrences, not the exact number.
Formula for probability:
Number of ways an event can occur / number of total possible outcomes.

55. If a germ cell from one plant that is heterozygous for a trait undergoes meiosis, the probability that the gamete will get the dominant allele is ½ and the probability that the gamete will get the recessive trait is ½.

56. If 2 plants that are heterozygous fertilize each other, the probability that both the sperm and egg have the dominant allele is ¼.
This is because when calculating the probability of 2 events happening together, you multiply the probability of the 2 individual events: ½ x ½ = ¼

58. 6.6 Meiosis and Genetic Variation
The major advantage of sexual reproduction is that there is a lot of genetic variation. This is due to:
Independent assortment
Mixing of alleles during fertilization
The pairing of homologous chromosomes is completely random so its completely a matter of chance which chromosome ends up on a side of the dividing cell.
In humans that gives 8 million combinations.

59. Based on these numbers, if a sperm and egg cell are randomly combined then the total number of possible combinations would be 70 trillion.
Sexual reproduction creates unique combinations of genes which results in unique phenotypes.
The offspring of sexual reproduction have a mixture of both parents’ traits.

60. A process called crossing over helps create even more genetic variation.
Crossing over is the exchange of chromosome segments between homologous chromosomes.
It occurs during prophase 1 of meiosis 1.
When homologous chromosomes are close together, parts of the chromatids from each chromosome can break off and reattach to the other chromosome.

61. Meiosis Crossing Over

62. Recombination refers to mixing of alleles forming new combinations of genes.
Some genes on the same chromosome are close together and some are far apart.
The farther apart 2 genes are, the more likely they are to be separated when crossing over happens.
Genes that are close together tend to be inherited together. This is called genetic linkage.