UNIT 3
GENETICS
Chapter 6
Meiosis and Mendel
6.1 - Chromosomes
and Meiosis
6.1 - Chromosomes and Meiosis
A. You have body cells and gametes.
1. Cells can be divided into two major groups:
a. Somatic Cells - body cells.
1) Make up most of your body tissues
and organs.
2) DNA is not inherited.
b. Gametes - sex cells
1) Derived from germ cells in reproductive
organs, i.e., ovaries or testes
2) Form egg and sperm cells
3) DNA can be inherited
2. Each species has a characteristic number of
chromosomes per cell.
a. Number not linked to complexity of the
organism.
b. Cells differ due to way genes are
expressed, not because they contain
different genes.
B. Your cells contain autosomes and sex
chromosomes.
1. Your body cells have 23 pairs of chromosomes.
a. Each pair referred to as a homologous pair.
b. Homologous
chromosomes are
two chromosomes one from mother
and one from father.
c. Each pair contains copies of same
genes, but the copies may differ.
2. Autosomes - chromosome pairs
numbered 1-22.
(they are homologous).
3. Sex chromosomes - a pair of chromosomes that
directly control development of sexual
characteristics.
a. Very different in humans
b. XX chromosomes female
c. XY chromosomes male
d. Considered 23rd pair,
but X and Y not
homologous
Sex chromosomes
C. Body cells are diploid (2n); gametes are haploid (n).
1. Sexual reproduction involves fusion of two
gametes.
a. Results in genetic mixture of both parents.
b. Fertilization - Fusion of egg and sperm.
c. Each egg and
sperm cell has
half the usual
number of
chromosomes.
2. Diploid and Haploid cells
a. Body cells are diploid (2n) - two copies of
each chromosome.
b. Gametes are haploid (n) - have one copy of
each chromosome.
3. Maintaining the correct number of chromosomes is
important to the survival of all organisms.
4. Germ cells (sex cells) undergo the process
of meiosis to form gametes (egg / sperm).
a. Diploid nucleus divides
into haploid nucleus.
b. Sometimes called
reduction division.
Haploid cells
Chapter 6
Meiosis and Mendel
6.2 - Process of
Meiosis
6.2 - Process of Meiosis
A. Meiosis begins with a diploid germ cell
containing chromosomes replicated during
S-phase of Interphase.
B. Cells go through two rounds of
division in meiosis.
1. Nuclear division produces four
haploid cells from one diploid cell.
2. Two rounds of cell division Meiosis I and Meiosis II, each
with four phases.
C. Homologous Chromosomes and Sister Chromatids
1. Need to distinguish between the two to
understand meiosis.
2. Homologous chromosomes - two separate
chromosomes: one from mother, one from
father.
a. Very similar to each other, e.g., same length and
carry same genes (but are not copies of each
other).
b. Each half of duplicated chromosome is called a
chromatid. (together called sister chromatids).
1). Homologous
chromosomes
divided in meiosis I.
2). Sister chromatids
not divided until
meiosis II.
Sister chromatids
3. Results in two haploid (n) cells each containing
replicated chromosomes, i.e., sister chromatids
still held together at centromere.
D. Meiosis I (first of two rounds of cell division):
1. Occurs after DNA was duplicated during
interphase (during Synthesis phase).
2. Separates homologous pairs of
chromosomes in four phases. (PMAT)
E. Meiosis II (second of two rounds of cell division):
1. Sister chromatids divide in four phases. (PMAT)
2. DNA is not replicated between meiosis I and
meiosis II.
F. Meiosis differs from mitosis in significant ways.
1. Meiosis has two cell divisions while mitosis
has one.
2. In mitosis, homologous chromosomes never
pair up.
3. Meiosis results in haploid cells; mitosis
results in diploid cells.
G. Haploid cells develop into mature gametes.
1. Gametogenesis - production of mature
gametes.
2. Differs between the sexes
a. Sperm - male gamete, smaller than
female egg.
1) Develops flagellum
2) Contains no organelles
b. Egg - female gamete
1) Uneven cell divisions result in one large
egg cell and three polar bodies.
2) Egg provides organelles to embryo.
Chapter 6
Meiosis and Mendel
6.3 - Mendel &
Heredity
6.3 - Mendel and Heredity
A. Mendel laid the groundwork for genetics.
1. Traits - distinguishing
characteristics that are inherited,
e.g., eye color.
2. Genetics - study of biological
inheritance patterns and
variation.
3. In 1800s scientists thought traits
were blended in offspring.
4. Gregor Mendel showed traits are
inherited as discrete units from
parental generation.
B. Mendel’s data revealed patterns of inheritance.
1. Mendel studied plant variation in a
monastery garden.
2. Mendel made three key
decisions in his experiments:
a. Control over breeding
b. Use of purebred plants
c. Observation of “eitheror” traits (only appear in
two alternate forms).
3. Experimental design
a. Mendel chose pea plants - they reproduce
quickly; he could control how they mate.
b. Pea plant flowers contain both male and
female reproductive organs (called a
complete flower).
c. Used cross pollination to cross different pea
plants.
d. Studied seven traits, each with either-or
characteristics.
4. Results:
a. Crossed (mated) two purebred (homozygous)
pea plants, e.g., crossed white-flowered with
purebred purple-flowered plants .
1). Called parental, or P generation.
2). Resulting plants (first filial or F1 generation)
all had purple flowers (heterozygous).
b. Allowed F1 generation (e.g., all purple) to selfpollinate/fertilize.
1). Produced F2 generation - some had all
purple flowers, others had all white flowers.
2). Trait for white had been “masked” in the F1
generation, it did not disappear.
c. Began to observe patterns - i.e., each cross
yielded similar ratios of traits in F2 generation
(e.g., he counted three plants with purple
flowers for every one with white flowers (3:1))
4. Mendel made three important conclusions
a. Traits are inherited as discrete units.
(explained why individual traits persisted
without being blended or diluted over
successive generations)
b. Two other key conclusions are collectively called
the law of segregation.
1). Organisms inherit two copies of each
gene, one from each parent.
2). Organisms donate only
one copy of each gene in
their gametes.
(two copies of each gene
segregate, or separate,
during gamete formation)
Chapter 6
Meiosis and Mendel
6.4 – Traits, Genes &
Alleles
6.4 - Traits, Genes, and Alleles
A. The same gene can have many versions.
1. Gene - a “piece” of
DNA that provides
instructions to a cell to
make a specific protein.
.
a. Locus - location on chromosome where a
gene resides.
b. Most genes exist in many forms.
c. Allele - alternative form of a gene.
d. You have two alleles for each gene, one on
each homologous chromosome.
(e.) Each parent donates one allele for every gene.
2. Homozygous - means two of the same allele at a
locus.
3. Heterozygous - two different alleles at a locus.
B. Genes influence the development of traits
1. Genome - all the organism’s genetic
material.
2. Genotype - genetic
makeup of a specific set
of genes.
3. Phenotype - physical
characteristics of an
organism (e.g., white or
purple flower, tall or short
plant, round or wrinkled
seed).
C. Dominant and Recessive Alleles
1. Dominant alleles - allele that is expressed
when two different alleles or two dominant
alleles are present (represented by Capital
letters).
2. Recessive alleles - expressed when two copies
of recessive allele present (represented by
lower-case letters).
3. Dominant does not mean stronger; it is
expressed (makes protein) when it’s without
recessive allele for same trait.
2. Alleles represented by letters, one letter
per allele; uppercase = dominant,
lowercase = recessive.
a. Homozygous dominant = e.g., TT
b. Heterozygous = e.g., Tt
c. Homozygous recessive = e.g., tt
D. Alleles and Phenotypes
1. Both homozygous dominant and
heterozygous genotypes yield a dominant
phenotype.
2. Most traits occur in a
range and do not follow
simple dominantrecessive patterns.
.
Chapter 6
Meiosis and Mendel
6.5 – Traits &
Probability
6.5 - Traits and Probability
A. Punnett squares illustrate genetic crosses.
1. Used to predict possible genotypes
resulting from a cross.
a. Axes of grid
represent
possible gamete
genotypes of
each parent.
b. Boxes show all possible genotypes of
offspring from those parents.
c. Can determine ratio of genotypes in that
generation. (the ratio has to add up to the
total # of boxes)
d. Can also be used to determine ratio of
phenotypes.
Punnett Square
parents
gametes
Dominant Allele
Possible offspring
Recessive allele
homozygous
heterozygous
B. Monohybrid cross - involves one trait
1. Homozygous dominant X Homozygous
recessive
Genotypic ratio =
4 : 0 (100% Ff)
Phenotypic ratio =
4 purple : 0 white
(100% purple)
2. Heterozygous X Heterozygous
Genotypic ratio =
1FF : 2Ff : 1ff
(25% : 50% : 25%)
Phenotypic ratio =
3 purple : 1 white
3. Heterozygous X Homozygous recessive
Genotypic ratio =
2 Ff : 2 ff
(can
reduce to 1Ff : 1ff)
(50% : 50%)
Phenotypic ratio =
2 purple : 2 white
(1 purple : 1 white)
C. Test Cross - cross between organism with an
unknown genotype and an organism with a
homozygous recessive phenotype. (?? x rr)
D. Dihybrid cross involves two traits
1. Dihybrid cross - predicts inheritance of two
different traits, e.g., seed color and shape.
2. Mendel discovered
phenotypic ratio in F2
generation was
always 9:3:3:1,
regardless of
combination of traits
used.
(hetero x hetero)
A dihybrid cross involves two traits.
3. Presence of one trait does not affect
inheritance of another trait.
4. Led to Mendel’s second law of genetics, the
Law of Independent Assortment.
5. Law states that allele pairs separate
independently of each other during meiosis.
E. Heredity patterns can be calculated using
probability.
1. Probability - likelihood that a particular
event will happen.
Probability =
number of ways a specific event can occur
number of total possible outcomes
a. Predicts average number of occurrences.
b. Can calculate probability of two
independent events happening at same
time by multiplying probability of each
individual event.
2. Probability also applies to meiosis and
fertilization events.
What do you think the probability would be?
Chapter 6
Meiosis and Mendel
6.6 – Meiosis &
Genetic Variation
6.6 - Meiosis and Genetic Variation
A. Sexual reproduction creates unique
combination of genes.
1. Sexual reproduction provides much
genetic variation within species.
2. Variation results from:
a. Independent assortment of
chromosomes in meiosis.
b. Random fertilization of
gametes.
3. Meiosis can produce human gametes (egg or
sperm) containing 223 (about 8 million) different
combinations of chromosomes.
4. Due to random fertilization, a human zygote can
contain 223 X 223 (more than 64 trillion) different
combinations of chromosomes.
B. Crossing over during meiosis increases genetic
diversity.
1. Crossing over - exchange of chromosome
segments between homologous
chromosomes during Prophase I of Meiosis I.
2. Results in new combination of genes.
C. Linked genes - genes located on the same
chromosome that are inherited together.
1. Some genes on same chromosome are close
together while others are far apart.
2. The farther apart two genes are the more
likely they will be separated during crossing
over.
3. The closer together two genes are on the
same chromosome, the more likely they will
be inherited together.
a. Genes inherited together are called linked
genes.
b. Gene linkage used to calculate distance
between genes on a chromosome.