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Chapter 10
Mendel & Meiosis
10.1: Mendel’s Laws of Heredity
Heredity- passing on of characteristics
from parent to offspring
Traits- characteristics that are inherited
Genetics- branch of biology that studies
heredity
Gregor Mendel
Austrian monk, 1860’s
Carried out thousands of genetic
experiments with garden peas
Why peas?
1. Peas reproduce sexually
 Form gametes- sperm & eggs
 Fertilization unites these gametes to form
a zygote which develops into a seed
2. Peas normally self-pollinate or fertilize
Mendel could cross-pollinate them
manually & be sure of the parents in a
given cross
3. Peas are easy to grow & reproduce
quickly
Pea plant traits
Pea plants have many distinct traits that are
either/or characteristics
Seed shape- round or wrinkled
Seed color- yellow or green
Flower color- purple or white
Plant height- tall or short
Etc.
Hybrids
Hybrid- offpsring of parents that have
different forms of a trait
Mendel crossed parents with different
traits and recorded data on what the
resulting hybrid offspring were like
What Mendel discovered:
1. The rule of unit factors- each organism
has 2 factors that control each of its traits
 Alleles- alternate forms of a gene
 Dominant allele- represented by capital
letter (A, B, C…)
 Recessive allele- represented by
lowercase letter (a, b, c…)
2. The rule of dominance- between alternate
forms of a gene, one will be dominant over
the other- the dominant form will show up
when paired with the recessive form
3. The law of segregation
Every individual has two alleles for each
gene
When gametes are produced, each
gamete receives only one of these alleles
More vocabulary:
Phenotype- the way an organism looks
Genotype- the alleles an organism has
(the letters)
Homozygote (homozygous)- an organism
with two alleles that are identical (AA, aa)
Heterozygote (heterozygote)- an organism
with two alleles that are different from one
another (Aa, Bb, Dd)
Let’s imagine….
We have a pea plant with purple flowers.
Purple (F) is dominant over white (f)
The plant’s genotype could be homozyote,
______, or heterozygote, ______.
The plant’s phenotype is ________.
If we have a pea plant with white flowers,
the plant’s genotype must be ________.
It is a _____________.
The plant’s phenotype is _________.
Gametes
Diploid cells- cells whose nuclei contain
two sets of homologous chromosomes
Diploid number- total number of
chromosomes in a diploid cell
Abbreviated 2n
46 in humans, 44 autosomes & 2 sex
chromosomes
Haploid cells- cells with a single set of
chromosomes (1 of each homologous
pair)
Gametes- sperm or eggs
Haploid number- total number of
chromosomes in a haploid cell
Abbreviated n
23 in humans, 22 autosomes & 1 sex
chromosome
Fertilization of egg (n) by sperm (n) results
in a fertilized egg called a zygote (2n)
Gametes are the result of a division
process called meiosis
Meiosis is the process that separates the
letters in a genotype into separate
genotypes
Meiosis
Meiosis is a reduction division- the number
of chromosomes is reduced in the division
Occurs in reproductive organs- ovaries &
testes
Involves two divisions
Four daughter cells result
Each daughter cell has only ½ as many
chromosomes as the starting cell
Punnett Squares
Tool used by geneticists to predict
expected results of crosses between
parents of known genotypes.
Monohybrid cross- cross between parents
that differ in one trait
Punnett squares determine the probability
that a certain genotype will occur in the
offspring of a certain cross
10.2 Meiosis
In mitosis, each new cell produced has the
exact number of chromosomes as the
original cell
Meiosis- the making of sperm or eggs
Homologous Chromosomes
Somatic cell- typical body cell
46 chromosomes that come in 23 matched
pairs
Homologous chromosomeschromosomes in a matched pair that have
genes controlling the same inherited
characteristics
Locus- location of a particular gene on a
chromosome
Homologous chromosomes have genes
for the same trait at the same locus, but
they may have different versions of that
gene
Types of Chromosomes
1. Autosomes
 Found in both males and females
 22 pairs
2. Sex chromosomes
 Determine gender
 1 pair- XX or XY
 Only small parts are homologous, but they
behave like homologous pairs during meiosis
We inherit one chromosome from each
pair from our mother and one from our
father.
Meiosis I: Homologous Chromosomes
Separate
Interphase
 Chromosomes duplicate
Prophase I
 Synapsis- homologous chromosomes (each
composed of 2 sister chromatids) come together
as pairs to forma tetrad
 Crossing over- legs of homologous
chromosomes flop over each other and may
exchange pieces- source of genetic variation
 Spindle forms
Metaphase I
Tetrads line up on metaphase plate
Anaphase I
One homologous chromosome from each
tetrad moves to opposite pole
Only tetrads split up, not sister chromatids
Telophase I and cytokinesis
Each pole has haploid set of
chromosomes, one from each homologous
pair
Cytokinesis occurs
Interphase again?
In some species, interphase occurs between
Meiosis I and II
Chromosomes uncoil
Nuclear membranes reform
No chromosomal duplication
Meiosis II: Sister chromatids separate
Prophase II
 Spindle forms
Metaphase II
 Chromosomes line up on metaphase plate
Anaphase II
 Sister chromatids are pulled apart
Telophase II
 New nuclear membranes form
 Cytokinesis occurs
Results of Meiosis
Four new haploid daughter cells
In males- four viable sperm
In females- one viable egg, three polar
bodies
Oogenesis