Meiosis produces gametes with half the normal number of chromosomes. It involves two cell divisions which results in four haploid cells from one diploid cell. During meiosis, homologous chromosomes pair up and may exchange genetic material through crossing over. This introduces genetic variation. Mendel's experiments with pea plants established basic principles of heredity including dominant/recessive traits and independent assortment of traits. Genes located on chromosomes code for traits and exist in different alleles that are inherited according to Mendelian genetics.

1. BIOLOGY CHAPTER 6 &
7
Meiosis and Mendelian Genetics

2. 6.1 & 6.2 CHROMOSOMES AND
MEIOSIS
6.6 MEIOSIS AND GENETIC VARIATION
Mitosis
Somatic cells undergo mitosis
One diploid cell produces 2 genetically
identical, diploid cells
There are 4 stages (prophase, metaphase,
anaphase, telophase)
DNA is copied once and divided once
Mitosis is used for development, growth, and
repair

3. Meiosis
Germ cells produce gametes (sex cells) during
meiosis
One diploid cell produces 4 genetically
different haploid cells
There are 8 stages (prophase I, metaphase I,
anaphase I, telophase I, prophase II,
metaphase II, anaphase II, telophase II)
DNA is copied once and divided twice

4. Homologous Chromosomes
Half of an organism’s chromosomes come from the
mother, and half from the father. The chromosomes
pair up to form homologous chromosomes
Homologous chromosomes are two chromosomes
with the same length, general appearance, and the
same type of genes
The pairing of the genes on the homologous
chromosomes are what code for all of an organism’s
traits (see examples in section 6.4)

5. All humans have 23 total pairs of
chromosomes, for a total of 46 chromosomes.
22 of the pairs are autosomes (body
chromosomes)
1 pair is the sex chromosomes (XX= female,
XY=male)

6. Meiosis I
Same steps as mitosis, except during
Prophase I crossing over can occur(see
below),
during Metaphase I homologous chromosomes
pair up in the middle and
during Anaphase I, the sister chromatids DO
NOT separate
By the end of Meiosis I, there are 2 haploid
cells

7. Crossing Over
Homologous chromosomes exchange genes
This results in a new combination of genes
(called recombination)
Genes that are located near each other on a
chromosome are more likely to be moved (or
crossed over) together. This is called genetic
linkage (refer to page 191)
Meiosis II
This is exactly the same as mitosis

8. 6.3 MENDEL AND HEREDITY
Gregor Mendel
An Austrian monk who is known as the “father
of genetics”
He discovered that traits (genes) are inherited
from parents by working with pea plants
He made both purebred plants and crossed
plants to see what the results would be.

9. WHY PEA PLANTS?
They reproduce quickly
They can both self-pollinate(creating
purebreds) and cross-pollinate(creating a mix
of traits)
They have “either-or” traits (the traits do not
mix to make a 3rd trait, ex. The purple flowers
and white flowers don’t mix to make a pink
flower)
(refer to page 179 to see the traits and results
of Mendel’s experiment)

10. MENDEL’S CONCLUSIONS
Mendel’s Law of Segregation:
 Organisms inherit two copies of each gene,
one from each parent
Organisms donate only one copy of each gene
in their gametes. Thus, the two copies of each
gene segregate, or separate, during gamete
formation (Law of Independent Assortment)
In other words, the Law of Ind. Assortment is
saying that traits are inherited separately.

11. 6.4 TRAITS, GENES, AND ALLELES
6.5 TRAITS AND PROBABILITIES
Genes
Each gene has a specific locus, or place on
the chromosome
A gene is a piece of DNA that provides a set of
instructions to make a certain protein. The
proteins then code for traits.
Each trait is represented by alleles.

12. Letters are used to represent different alleles.
There are two alleles per trait (one from
mother, one from father)
Dominant alleles are represented by capital
letters and Recessive alleles are represented
by lowercase letters
Dominant alleles will always overpower the
recessive alleles.

13. EXAMPLE: P= purple flowers
p= white flowers
PP=purple flowers (homozygous)
pp= white flowers (homozygous)
Pp= purple flowers (heterozygous)
Genotype-the alleles/genetic code (PP or Pp)
Phenotype-the physical trait (purple or white)

14. PUNNET SQUARES
Punnet squares are used to predict the outcome of
genotypes when crossing two organisms
Monohybrid cross- crosses one trait (always yields a
3:1 phenotypic ratio)
Dihybrid cross- crosses two traits (always yield
9:3:3:1 phenotypic ratio)

15. CHAPTER 7
EXTENDING MENDELIAN GENETICS
Complex Patterns of Inheritance
Incomplete Dominance—the trait is in between
the dominant and recessive
Ex. RR=red rr=white Rr=pink
Codominance—both traits are expressed at the
same time
Ex. RR=red rr=white Rr= both red and white

16. Polygenic Traits—when two or more
genes influence a trait
Ex. Human eye color(at least 3 different
genes), human skin color (4 different genes)
Environmental factors—when the
environment plays a role in gene
expression
Ex. Sea turtle eggs become female in warmer
temps and male in colder temps

17. SEX-LINKED TRAITS—TRAITS CAUSED
BY GENES LOCATED ON THE SEX
CHROMOSOMES
The X contains many genes
The Y only determines sex (male)
A male is XY, so all of the alleles on the X are
expressed in a male, even the recessive ones
A female is XX, so dominant alleles can mask
the recessive ones
Carriers—those who carry a recessive genetic
disorder, but do not express the phenotype. They
can pass on the disorder to their offspring