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Meiosis and
Mendel
CHAPTER 6 PAGE 156
A. Chromosomes and Meiosis
1.
Body cells and gametes
a. Many types of specialized cells
b. Somatic cells (body cells)
1) Make up tissues and organs
2) Spleen, kidney, eyeballs
c. Germ cells
1) In reproductive organs (ovaries and testes)
2) Develop into gametes
a) Females – eggs (ova)
b) Males – sperm (spermatozoa)
3) Have DNA that can be passed to offspring
2. Autosomes and sex chromosomes
a. Humans have 23 pairs of chromosomes
1) Homologous pair – same structure
2) One from each pair comes from each parent
3) Have same genes but possibly different traits
b. More chromosomes does NOT mean more
complex
1) Fruit flies – 8 chromosomes
2) Fern – 1200+ chromosomes
c. Autosomes – pairs 1 – 22
d. Sex chromosomes
1) X and Y
a) Not homologous
b) X – larger and more genes
2) Pair 23
3) Determines gender
4) XX = female
5) XY = Male
3. Body cells = diploid; gametes = haploid
a. Sexual reproduction – fusion of 2 gametes
b. Fertilization – actual fusion of egg and sperm
c. Diploid (2n)
1) 2 copies of each chromosome
2) Half from each parent
d. Haploid (n)
1) 1 copy of each chromosome
2) Have 22 autosomes and 1 sex chromosomes
e. Chromosome number is maintained by
mitosis and meiosis
1) Mitosis – body cell division
2) Meiosis
a) Divides diploid cells into haploid cells
b) Germ cells
c) Produce gametes
d) Reduces chromosome number and increases
diversity
B. Process of Meiosis
1.
Cells divide twice
a. Homologous chromosomes
1) 2 separate chromosomes
2) Same length and carry same genes
3) Separated in meiosis I
b. Sister chromatids
1) Duplicated chromosomes attached at
centromere
2) Divided during meiosis II
2. Meiosis I (DNA already duplicated)
a. Prophase I
1) Nuclear membrane breaks down
2) Centrioles move to opposite sides
3) Spindle fibers form
4) Homologous chromosomes pair up
5) Crossing over
a) Parts of chromatids
break off and exchange
parts w/adjacent
chromatids
b) Gives lots of genetic
variation
b. Metaphase I
1) Chromosomes meet in the middle
2) Line up in PAIRS (½ from mom and ½ from dad)
c. Anaphase I
1) Homologous pairs separate and move to poles
2) Sister chromatids remain together
d. Telophase I
1) Chromosomes reach poles
2) Nucleus reforms
3) Cytokinesis takes place
3. Meiosis II
a. Prophase II – same as prophase I but NO crossing
over
b. Metaphase II – chromosomes meet in the middle
(NO pairs)
c. Anaphase II
1) Chromatids move toward poles
2) Chromatids now called chromosomes
d. Telophase II
1) Nucleus reforms
2) Spindle fibers disappear
3) Cytokinesis form 4 cells
4. Mitosis vs. Meiosis
a. Meiosis = 2 cell divisions; Mitosis = 1
b. Mitosis homologous chromosomes never pair up
c. Meiosis = haploid cells; Mitosis = diploid cells
5. Haploid cells developing into mature
gametes
a. Gametogenesis – production of gametes
b. Males
1) Sperm become motile by flagella
2) Primarily contribute DNA to embryo
3) 4 functional cells
c. Females
1) Eggs contribute DNA, cytoplasm,
and organelles for embryo
2) Meiosis I forms 1 cell and 1 polar
body
3) Meiosis II forms 1 egg and 2 polar
bodies
4) 1 functional egg 3 nonfunctional
polar bodies
5) Polar body
a) Cell with little more than DNA
b) Eventually is broken down
C. Mendel and Heredity
Mendel laid groundwork for genetics
a. Austrian monk
b. Studied pea plants for 9 years
c. AKA – Father of Genetics
2. Traits – distinguishing characteristics that are
inherited (eye color, tail length, leaf shape)
3. Genetics – study of biological inheritance patterns
and variation
1.
4. Mendel revealed patterns of inheritance
a. Key decision of Mendel
1) Used purebred plants – only produce 1 trait
2) Control breeding
3) 7 “either-or” traits
b. P – parental generation
c. F1 – offspring of parents
d. Mendel’s traits
Seed
Shape
Round
Wrinkled
Round
Seed
Color
Yellow
Green
Yellow
Seed Coat
Color
Gray
Pod
Shape
Smooth
White
Constricted
Gray
Inflated
Pod
Color
Green
Yellow
Green
Flower
Position
Axial
Terminal
Axial
Plant Height
Tall
Dwarf
Tall
e. Mendel’s Experiment
1) Parental generation crossed to produce offspring
2) Prevented the self-pollination process by
removing male flower parts
3) Mendel allowed the resulting plants to selfpollinate
a) F1 generation
i. All plants had purple flowers
ii. Heterozygous – both traits
b) F2 – Some purple, some white
Mendel controlled the
fertilization of his pea plants
by removing the male parts,
or stamens.
He then fertilized the female
part, or pistil, with pollen from
a different pea plant.
4) Mendel noticed patterns (3:1)
f. Mendel’s Conclusions
1) Traits are passed down as genes
2) Organisms inherit 2 copies of each gene – one
from each parent
3) 2 copies segregate during gamete formation
D. Traits, Genes, and Alleles
1.
Same gene can have many versions
a. Gene – piece of DNA that directs a cell to make
a certain protein
b. Locus – specific position on a pair of homologous
chromosomes
c. Allele – alternative forms of a gene that may
occur at a specific locus (yellow or green)
d. Homozygous
1) 2 of the same alleles
2) TT or tt
e. Heterozygous
1) 2 different alleles
2) Tt
2. Genes influence the development of traits
a. Genome – all of an organism’s genetic material
b. Genotype – genetic makeup of specific genes
c. Phenotype – physical appearance
d. Dominant
1) Allele that is expressed
2) Not always most common
e. Recessive – allele which is masked unless 2 are
present (t)
Polydactyly
 Having
too many fingers
or toes
 Dominant trait
Assigning genotypes
- Dominant trait gets capital
letter
- Recessive trait get
lowercase of the same
letter
- RR
Genotype
Phenotype
RR
Round seed
Yy
Yellow pod
gg
Green seed
E. Traits and Probability
1.
Punnett Squares
a. Axes represent parent
genotypes
b. Boxes represent
offspring genotypes
c. Show ratio of possible
genotypes
2. Monohybrid cross
a. Only one trait is looked at
b. Cross – mating of two organisms
c. Homozygous/homozygous
d. Heterozygous/heterozygous
e. Heterozygous/homozygous recessive
3. Testcross – cross b/w organism w/unknown
genotype and an organism with the recessive
phenotype
4. Dihybrid cross
a. Involves 2 traits
b. Heterozygous plants
yield 9:3:3:1 phenotype
ratio
c. Law of independent
assortment – allele pairs
separate independently
of each other during
meiosis
5. Patterns can be calculated
a. Probability – likelihood of something happening
1) Predicts an ave. number of occurrences, not
an exact number
2) Applies to random events like meiosis and
fertilization
b. Equation
Probability =
number of ways a specific event can occur
number of total possible outcomes
F. Meiosis and Genetic Variation
1.
Sexual reproduction creates unique combinations
of genes
a. Independent assortment
b. Random fertilization of gametes
c. 1 couple can produce 70 trillion different
combinations of chromosomes
d. Unique phenotypes give a reproductive
advantage to some organisms
2. Crossing over increases diversity
a. Exchange of chromosome segments b/w
homologous chromosomes
b. Results in new combinations of genes
c. Happens in prophase I
d. Farther apart genes = more likely to be
separated
e. Genetic linkage
1) Genes being inherited together b/c they are
located close together on a chromosome
2) Allows calculation of distance b/w 2 genes