Chapter 19 & 20
Biology 25: Human Biology
Prof. Gonsalves
Los Angeles City College
Based on Mader’s Human Biology,7th
edition and Fox’s 8th ed
Powerpoints
Heredity: The transmission of traits from
one generation to another.
Variation: Offspring are different from their
parents and siblings.
Genetics: The scientific study of heredity
and hereditary variation.
Involves study of cells, individuals, their
offspring, and populations.
I. History of Genetics

Blending Hypothesis: In 1800s biologists and plant breeders
suggested that traits of parents mix to form intermediate traits in
offspring.
Parents
Offspring
Red flower x White flower
Pink flower
Tall height x Short height
Medium height
Blue bird x Yellow bird
Fair skin x dark skin
Green birds
Medium skin color
If blending always occurred, eventually all extreme characteristics
would disappear from the population.

Gregor Mendel: Established genetics as a science in 1860s.
Considered the founder of modern genetics.
II. Modern Genetics
Began as a science in 1860s.

Gregor Mendel: An Austrian monk, who was a farmer’s son.
He was trained in mathematics, chemistry, and physics.

Studied the breeding patterns of plants for over 10 years.

Artificially crossed peas, watermelons, and other plants.

Kept meticulous records of thousands of breedings and
resulting offspring.

Rejected blending hypothesis, and stressed that heritable
factors (genes) retain their individuality generation after
generation.
II. Modern Genetics
Gregor Mendel:

Calculated the mathematical probabilities of
inheriting many genetic traits.

Published results in 1866. They were largely ignored
due to fervor surrounding Darwin’s publications on
evolution.

Discouraged by the lack of attention from the
scientific community, he quit his work and died a few
years later.

Importance of Mendel’s work was not appreciated
until early 1900s when his paper was rediscovered.
III. Mendel’s Experiments


Used “true-breeding” or purebred plant varieties for seven
pea characteristics. Self-pollination produces all identical
offspring.
Using artificial pollination, he crossed true-bred varieties.
Trait
Varieties
Flower color
Purple or white
Seed color
Yellow or green
Seed shape
Round or wrinkled
Pod color
Green or Yellow
Pod shape
Smooth or constricted
Flower position
Axial or terminal
Plant height
Tall or short
III. Summary of Mendel’s Results
All plants displayed one trait only.
Trait
Varieties
Offspring
Flower color
Purple or white
100% Purple
Seed color
Yellow or green
100% Yellow
Seed shape
Round or wrinkled
100% Round
Pod color
Green or Yellow
100% Green
Pod shape
Smooth or constricted
100% Smooth
Flower position
Axial or terminal
100% Axial
Plant height
Tall or short
100% Tall
The trait that prevailed was dominant, the other recessive.
IV. Mendel’s Conclusions
1. Results indicate that blending
hypothesis is not true.
2. Only one of the two traits appeared
in the first generation. He called this
the dominant trait. He called the
trait that disappeared the recessive
trait.
IV. Mendel’s Conclusions
1. Results indicate that the recessive trait is
intact.
2. The crossbred plants with purple flowers
must be carrying the genetic information to
produce white flowers.
3. The crossbred plants with purple flowers
are genetically different from the purebred
plants, even though they look the same.
IV. Mendel’s Conclusions
4. Must distinguish between:
Phenotype: Physical appearance of individual.
Example: Two phenotypes for flower color.

Purple flowers

White flowers.
Genotype: Genetic makeup of an individual.
Not all purple flowers are genetically identical.
IV. Mendel’s Conclusions
5. Each individual carries two genes for a given
genetic trait. One gene comes from the
individual’s mother, the other from the father.
There are two alternative forms of genes or
hereditary units.
The alternative forms of these hereditary
units are called alleles.
P: Allele for purple flowers
p: Allele for white flowers
IV. Mendel’s Conclusions
6. In a given individual, the two genes for a
given trait may be the same allele (form of a
gene) or different.
Phenotype
Genotype:
Purple
PP (Homozygous dominant)
Purple
Pp (Heterozygous dominant)
White
pp (Homozygous recessive)
Homologous Chromosomes Bear the Two
Alleles for Each Characteristic
Phenotype and Genotype of Mendel’s Pea Plants
Punnet Square:
Used to determine the outcome of a cross between
two individuals.
Heterozygotes make 1/2 P and 1/2 p gametes.
P
p
P
PP
Pp
p
Pp
pp
Offspring:
Genotype: 1/4 PP, 1/2 Pp, and 1/4 pp
Phenotype: 3/4 Purple and 1/4 white
Genotypic and Phenotypic Ratios of F2 Generation
VI. Principles of Mendelian Genetics
1. There are alternative forms of genes, the
units that determine heritable traits.
These alternative forms are called alleles.
Example:
Pea plants have one allele for purple flower
color, and another for white color.
VI. Principles of Mendelian Genetics
2. For each inherited characteristic, an
individual has two genes: one from each
parent.
In a given individual, the genes may be the
same allele (homozygous) or they may be
different alleles (heterozygous).
VI. Principles of Mendelian Genetics
3. When two genes of a pair are different alleles,
only one is fully expressed (dominant allele).
The other allele has no noticeable effect on the
organism’s appearance (recessive allele).
Example:
Purple allele for flower color is dominant
White allele for flower color is recessive
VI. Principles of Mendelian Genetics
4. A sperm or egg cell (gamete) only contains
one allele or gene for each inherited trait.
Principle of Segregation: Alleles segregate
(separate) during gamete formation.
(When? During meiosis I)
During fertilization, sperm and egg each
contribute one allele to the new organism,
restoring the allele pair.
VI. Principles of Mendelian Genetics
5. Principle of Independent Assortment:
Two different genetic characteristics are
inherited independently of each other.*
*As long as they are on different chromosomes.
Mendel did not know about meiosis, but
meiosis explains this observation.
Why?
How are chromosomes shuffled during
meiosis I?
VII. Human Genetics
Inheritance of human traits.
Most genetic diseases are recessive.
Dominant Traits
Recessive Traits
Widow’s peak
Straight hairline
Freckles
No freckles
Free earlobe
Attached earlobe
Normal
Cystic fibrosis
Normal
Phenylketonuria
Normal
Tay-Sachs disease
Normal
Albinism
Normal hearing
Inherited deafness
Huntington’s Disease
Normal
Dwarfism
Normal height
Eucaryotic cell division is a more complex and time
consuming process than binary fission
Features of Eucaryotic DNA
1. DNA is in multiple linear chromosomes.
 Unique number for each species:
• Humans have 46 chromosomes.
• Cabbage has 20, mosquito 6, and fern over 1000.
2. Large Genome: Up to 3 billion base pairs (humans)
 Contains up to 50,000-150,000 genes
 Human genome project is determining the sequence
of entire human DNA.
3. DNA is enclosed by nuclear membrane.
Correct distribution of multiple chromosomes in each
daughter cell requires a much more elaborate process
than binary fission.
DNA: Found as Chromosomes or Chromatin
Chromosomes
Tightly packaged DNA
Found only during cell
division
DNA is not being used
for macromolecule
synthesis.
Chromatin
Unwound DNA
Found throughout cell
cycle
DNA is being used
for macromolecule
synthesis.
Eucaryotic Chromosomes Duplicate Before
Each Cell Division
Cell Cycle of Eucaryotic Cells
Sequence of events from the time a cell is formed, until
the cell divides once again.
 Before cell division, the cell must:
 Precisely copy genetic material (DNA)
 Roughly double its cytoplasm
 Synthesize organelles, membranes, proteins, and
other molecules.
 Cell cycle is divided into two main phases:
 Interphase: Stage between cell divisions
 Mitotic Phase: Stage when cell is dividing

Eucaryotic Cell Cycle:
Interphase + Mitotic Phase
Mitosis: The Stages of Cell Division
1. Prophase

Chromatin condenses into chromosomes, which appear as two
sister chromatids joined by a centromere.

Nucleoli disappear.

Nuclear envelope breaks apart.

In animal cells, mitotic spindle begins to form as mictotubules
grow out of two centrosomes or microtubule organizing centers
(MTOCs).
• Each centrosome is made up of a pair of centrioles.

Microtubules attach to kinetochores on chromatids and begin to
move chromosomes towards center of cell.

Centrosomes begin migrating to opposite poles of cell.
Interphase and Prophase of Mitosis in Animal Cell
Mitosis: The Stages of Cell Division
2. Metaphase

Short period in which chromosomes line up along
equatorial plane of cell (metaphase plate).

Chromosomes are completely condensed and easy to
visualize.

Mitotic spindle is fully formed.

Kinetochores of sister chromatids face opposite sides
and are attached to spindle microtubules at opposite
ends of the cell.
Metaphase, Anaphase, and Telophase of Mitosis
in an Animal Cell
Mitosis: The Stages of Cell Division
3.Anaphase

Centromeres of sister chromatids begin to separate.

Each chromatid is now an independent daughter
chromosome.

The separate chromosomes are pulled toward opposite
ends by spindle microtubules, attached to the
kinetochores.

Cell elongates as poles move farther apart.

Anaphase ends when a complete set of chromosomes
reaches each pole.
Mitosis: The Stages of Cell Division
4. Telophase

Cell continues to elongate.

Cell returns to interphase conditions:
• A nuclear envelope forms around each set of
chromosomes.
• Chromosomes uncoil, becoming chromatin
threads.
• Nucleoli reappear.
• Spindle microtubules disappear.

Cytokinesis usually occurs at the end of this stage
Mitotic Phase: Mitosis + Cytokinesis

Cytokinesis

The division of cytoplasm to produce two daughter
cells. Usually begins during telophase.
• In animal cells: Division is accomplished by a
cleavage furrow that encircles the cell like a ring
in the equator region.
• In plant cells: Division is accomplished by the
formation of a cell plate between the daughter
cells. Each cell produces a plasma membrane and
a cell wall on its side of the plate.
Cytokinesis in Animal and Plant Cells
Animal Cell
Plant Cell
External Factors Control Mitosis
1. Anchorage
 Most cells cannot divide unless they are attached to a
solid surface.
 May prevent inappropriate growth of detached cells
2. Nutrients and growth factors
Lack of nutrients can limit mitosis
 Growth factors: Proteins that stimulate cell division.
3. Cell density


Density-dependent inhibition: Cultured cells will stop
dividing after a single layer covers the petri dish.
Mitosis is inhibited by high cell density.
 Cancer cells do not demonstrate density inhibition
Density Dependent Inhibition of Mitosis
Normal Cells Stop Dividing at High Cell Density
Cancer Cells are Not Inhibited by High Cell Density
Cell-Cycle Control System
There are three critical points at which the cell cycle is
controlled*:
1. G1 Checkpoint: Prevents cell from entering S phase and
duplicating DNA.
 Most important checkpoint.
 Amitotic cells (muscle and nerve cells) are frozen
here.
2. G2 Checkpoint: Prevents cell from entering mitosis.
3. M Checkpoint: Prevents cell from entering cytokinesis.
*Cells must have proper growth factors to get through
each checkpoint.
Cell Division is Controlled at Three Key Stages
Growth factors are
required to pass
each checkpoint
Cancer is a Disease of the Cell Cycle



Cancer kills 1 in 5 people in the United States.
Cancer cells divide excessively and invade other body tissues.
Tumor: Abnormal mass of cells that originates from
uncontrolled mitosis of a single cell.
 Benign tumor: Cancer cells remain in original site. Can
easily be removed or treated


Malignant tumor: Cancer cells have ability to “detach”
from tumor and spread to other organs or tissues
Metastasis: Spread of cancer cells form site of origin to
another organ or tissue.
 Tumor cells travel through blood vessels or lymph
nodes.
Functions of Mitosis in Eucaryotes:
1. Growth: All somatic cells that originate after
a new individual is created are made by
mitosis.
2. Cell replacement: Cells that are damaged or
destroyed due to disease or injury are
replaced through mitosis.
3. Asexual Reproduction: Mitosis is used by
organisms that reproduce asexually to make
offspring.
Meiosis: Generates haploid gametes

Reduces the number of chromosomes by half, producing
haploid cells from diploid cells.

Also produces genetic variability, each gamete is
different, ensuring that two offspring from the same
parents are never identical.

Two divisions: Meiosis I and meiosis II. Chromosomes
are duplicated in interphase prior to Meiosis I.

Meiosis I: Separates the members of each
homologous pair of chromosomes. Reductive
division.

Meiosis II: Separates chromatids into individual
chromosomes.
STAGES OF MEIOSIS
Interphase:
Chromosomes
replicate
Meiosis I:
Reductive division.
Homologous
chromosomes separate
Meiosis II:
Sister chromatids
separate
Meiosis I: Separation of Homologous Chromosomes
1. Prophase I:
 Most complex phase of meiosis (90% of time)

Chromatin condenses into chromosomes.

Nuclear membrane and nucleoli disappear.

Centrosomes move to opposite poles of cell and
microtubules attach to chromatids.

Synapsis: Homologous chromosomes pair up and
form a tetrad of 4 sister chromatids.

Crossing over: DNA is exchanged between
homologous chromosomes, resulting in genetic
recombination. Unique to meiosis.
Chiasmata: Sites of DNA exchange.

Prophase I: Crossing Over Between
Homologous Chromosomes
Meiosis I: Separation of Homologous
Chromosomes
2. Metaphase I:

Chromosome tetrads (homologous
chromosomes) line up in the middle of the
cell.

Each homologous chromosome faces opposite
poles of the cell.
Meiosis I: Homologous Chromosomes
Separate
Stages of Meiosis: Meiosis I
3. Anaphase I:
 Chromosome tetrads split up.

Homologous chromosomes of each pair separate,
moving towards opposite poles.

Random assortment: One chromosome from each
homologous pair is shuffled into the two daughter cells,
randomly and independently of the other pairs.

Random assortment increases genetic diversity of
offspring. Possible combinations: 2n.

One human cell can generate 223 or over 8.3 million
different gametes by random assortment alone.
Random Assortment of Homologous Chromosomes
During Meiosis I Generates Many Possible Gametes
Meiosis I: Separation of Homologous
Chromosomes
4. Telophase I and Cytokinesis:

Chromosomes reach opposite poles
of the cell.

Nucleoli reorganize, chromosomes
uncoil, and cytokinesis occurs.

New cells are haploid.
Meiosis II: Separation of Sister Chromatids
During interphase that follows meiosis I, no DNA
replication occurs.
Interphase may be very brief or absent.
Meiosis II is very similar to mitosis.
1. Prophase II:

Very brief, chromosomes reform.

No crossing over or synapsis.

Spindle forms and starts to move
chromosomes towards center of the cell.
Meiosis II: Separation of Sister Chromatids
2. Metaphase II:

Very brief, individual chromosomes line up
in the middle of the cell.

Kinetochores of chromatids face opposite
poles.
3. Anaphase II:

Chromatids separate and move towards
opposite ends of the cell.
Meiosis II: Separation of Sister Chromatids
Meiosis II: Separation of Sister Chromatids
4. Telophase II:

Nuclei form at opposite ends of the cell.

Cytokinesis occurs.
Product of meiosis:
Four (4) haploid gametes, each
genetically different from the other.
Meiosis Produces Four Genetically Different Gametes
Mitosis versus Meiosis (Review)
Mitosis
One cell division
Meiosis
Two successive cell divisions
Produces two (2) cells
Produces four (4) cells
Produces diploid cells
Produces haploid gametes
Daughter cells are genetically
identical to mother cell
Cells are genetically different from
mother cell and each other
No crossing over
Crossing over*
Functions: Growth,
Functions: Sexual reproduction
cell replacement, and
asexual reproduction
*Crossing over: Exchange of DNA between homologous chromosomes.
Meiosis in Males and Females
Spermatogenesis:
 Four sperm cells are made.


Starts in puberty and occurs continuously.
Males produce millions of sperm cells a month.
Oogenesis:




Only one large egg is produced. The other three
cells are small polar bodies.
Oogenesis starts before birth in females, stops at
Prophase I, and resumes during puberty.
Meiosis is completed only after fertilization.
Females make one mature egg/month.
Sources of Genetic Variability in Sexual Reproduction
1. Crossing Over: After crossing over and synapsis, sister
chromatids are no longer identical.
2. Independent Assortment: Each human can produce
over 8.3 million different gametes by random shuffling
of chromosomes in meiosis I.
3. Fertilization: A couple can produce over 64 trillion (8.3
million x 8.3 million) different zygotes during
fertilization. This figure does not take into account
diversity created by crossing over.
Accidents During Meiosis Can Cause Chromosomal
Abnormalities

Nondisjunction: Chromosomes fail to separate.

Members of a pair of homologous chromosomes fail
to separate during meiosis I or:

Sister chromatids fail to separate during meiosis II.

Nondisjunction increases with age.

Gametes (and zygotes) will have an extra chromosome,
others will be missing a chromosome.

Trisomy: Individuals with one extra chromosome,
three instead of pair. Have 47 chromosomes in cells.

Monosomy: Missing a chromosome, one instead of
pair. Have 45 chromosomes in cells.
Nondisjunction of Chromosomes During
Meiosis Produces Abnormal Gametes
Accidents During Meiosis Can Result in a Trisomy or
Monosomy

Most abnormalities in numbers of autosomes are very
serious or fatal.


Down’s syndrome: Caused by a trisomy of
chromosome number 21 (1 in 700 births). Mental
retardation, mongoloid features, and heart defects.
Most abnormalities of sex chromosomes do not affect
survival.

Klinefelter Syndrome: Males with an extra sex
chromosome (XXY) (1 in 1000 male births).

Turner Syndrome: Females missing one sex
chromosome (XO) (1 in 2500 female births).
Down’s Syndrome is More Common in
Children Born to Older Mothers
Abnormal Numbers of Sex Chromosomes Usually
Do Not Affect Survival
Klinefelter Syndrome (XXY)
Incidence: 1:1000 male births
Turner Syndrome (XO)
Incidence: 1 in 2500 female births