Transcript Chapter 23 - Cloudfront.net

Chapter 23
Heredity: The Continuity of Life
Heredity
• God has provided for all things to
reproduce after their kind.
• Characteristics are passed from parent to
offspring.
• This transmission of characteristics takes
place through either sexual or asexual
reproduction.
Asexual Reproduction
• Asexual reproduction is God’s provision
from one generation to the next that
requires on mitotic division.
• Example: A bud of a sponge breaks off
and grows a whole new sponge.
• An amoeba divides into two amoebas.
Differentiation of Cells
• A fertilized egg
undergoes mitosis and
becomes many cells that
are the same.
• The many like cells then
“differentiate” and
become other kinds of
cells…like a nerve cell or
a blood cell.
• The many different cells
work together to be an
organism!
Sexual Reproduction
• In organisms that reproduce sexually, the
heredity of both parents is combined to
provide the heredity of the offspring.
• Male and female contribute equal amounts
of genetic material.
• In Humans each cell has 46
chromosomes…
• However, the gametes (egg and sperm)
contain only 23 chromosomes.
Sexual Reproduction
• This is because the pairs
of chromosomes
separate to make the
gametes.
• Plants also have egg
cells (in ovaries) and
sperm cells in anthers in
the form of pollen.
• When fertilization occurs
(egg and sperm coming
together) the result is a
zygote.
Sexual Reproduction
• The sperm and egg each
having half the
chromosomes are said to
be haploid.
• The zygote having both
sets of chromosomes and
thus having 46 (for
humans) is said to be
diploid.
• The single cell zygote
now divides by mitosis to
grow into the new
organism.
Classical Genetics
• Gregor Mendel
Austrian monk who grew
pea plants…
• He worked in 1853 and
he noticed that traits were
somehow passed on from
one generation to the
next.
• He discovered that his
plants bred true for each
characteristic.
Classical Genetics
• For example: red blooms
always produced red
blooms.
• For example: wrinkled
seeds always produced
plants with wrinkled
seeds.
• Mendel would say these
plants were “true” for
these traits – they were
pure breeding lines.
Classical Genetics
• The reason pea
plants worked so well
for genetic research is
because he could
produce (grow) a lot
of plants in a short
period of time.
Classical Genetics
Classical Genetics
• Mendel would cross
pure bred lines for
wrinkled seeds with
pure bred lines for
round seeds.
• He would call this first
generation of
offspring the first filial
generation (F1).
Classical Genetics
• This F1 generation
would have all round
seeds!
• What happened to the
wrinkled seed
genes?!
The Law of Dominance
• Mendel concluded that traits are paired.
• Each parent contributes one gene for each
trait.
The Law of Dominance
• If both genes for the trait are the same in
the offspring that offspring is homozygous
for that trait.
• If the genes are different the offspring is
heterozygous for that trait.
The Law of Dominance
• Dominant genes for a trait show up even if
the offspring is heterozygous.
• A dominant gene will “mask” or hide the
recessive trait.
• In modern genetics, different forms of a
gene for a particular trait are called alleles.
Hybrids and Hybridization
• Hybrid is used in
genetics to describe
an organism with two
different traits.
• A hybrid is not a pure
strain.
Punnett Squares
• Geneticist use a
Punnett Square to
represent crosses
between traits.
Incomplete Dominance
• When traits blend and neither one is
dominant it is called incomplete
dominance.
• The traits mix for offspring that are
something more of a mix.
Incomplete Dominance
• For example: a red
pea blossom plant
and a white pea
blossom plant will
combine to have
offspring that are
pink.
• Neither trait is hidden.
The Law of Segregation
• Mendel found that if he
took the F1 generation
plants and crossed them
with each other then the
recessive trait would
show up again.
Law of Segregation
• He determined
that the
recessive
traits
segregate
or separate
in some
of the offspring.
The Law of Segregation
• He called this
generation the F2
generation.
The Law of Independent
Assortment
• An organism that is a
hybrid for two traits is
called a dihybrid.
• These two traits will
segregate or separate
independent of each
other.
The Law of Independent
Assortment
• Sometimes traits (genes) are located on
the same chromosome.
• In this case, the traits will not segregate.
• These traits are called linkage groups.
• Mostly these are associated with genes on
the sex chromosomes…
X and Y
Genetic Research in the
20th Century
• Mendel’s work was
neglected until 1900.
• In 1902, Walter
Sutton theorized that
Mendel’s “heredity
factors” were particles
carried on
chromosomes.
Genetic Research in the
20th Century
• Sutton explained that Mendel’s round/wrinkled and
green/yellow seed traits were on separate
chromosomes.
• His theories laid the foundation for all modern genetic
research.
Genetic Research in the
20th Century
• In 1904, Thomas
Morgan began
research that would
earn him the Nobel
prize in 1933.
• Morgan began to
study fruit flies
(Drosophila
melanogaster)…
Genetic Research in the
20th Century
• Morgan found a pair of
non-identical
chromosomes.
• He determined that these
pairs are the sex
chromosomes.
• The female had identical
chromosomes he named
the X chromosomes.
• The male had one
chromosome like the
female (he called it X)
and one different (he
called it Y).
Genetic Research in the
20th Century
• In most organisms it is the sperm that
determines the sex of the offspring.
Sex-Linked Traits
• Morgan discovered in 1909 that some
traits are carried on the sex chromosomes.
• He called these sex-linked – they are only
carried on the X chromosome.
• A male who inherits a gene with a
particular trait that is carried on the X
chromosome will always display that trait
since he only needs one gene to display it.
Human Genetics
• The Human Genome Project
begun in the 1990’s has
completely mapped all the
human genes.
• Dominant and recessive genes
are expressed by lower case
and upper case letters.
• These letters describe the
genotype of the organism.
• The expression of these genes
is the phenotype (what the
organism looks like)
Human Genetics
Dominant Gene Inheritance
• There are many
genes in the human
population that are
dominant.
• The gene for free ear
lobe attachment is
dominant.
Human Genetics
Dominant Gene Inheritance
• Cystic fibrosis is a
recessive trait.
Human Genetics
Incomplete Dominance
• Sickle cell anemia is an incomplete
dominant gene.
Human Genetics
Multiple Gene Inheritance
• Multiple gene inheritance
occurs when there are
than two alleles (forms) of
a gene for a trait.
• One example is blood
type.
• A, B, AB, and O are the
four possible blood types
in the human population.
• A person with IA and IB as
the two genes has blood
type AB.
Human Genetics
Multiple Gene Inheritance
• This means the person has proteins A and
B on their blood cells.
• A person with IoIo has blood type O and
has no protein markers on their blood
cells.
Human Genetics
Pleiotrophy Inheritance
• Pleiotrophy is when a gene affects several
other genes that are seemingly unrelated.
Human Genetics
Polygenic Inheritance
• Polygenic inheritance
is when traits are
controlled by different
pairs of genes.
• Skin color, for
example, is controlled
by as many as 4 pairs
of genes.
• Height is controlled by
as many as 4 pairs of
genes.
Sex-Linked Characteristics
• Some characteristics are carried on the X
chromosome.
• Hemophilia is a disease carried on the X.
• Hemophilia is a blood disorder that causes
the blood not to clot.
• Colorblindness is another X gene disorder.
• A female carrier will have the genes XcX.
• A female with color blindness will have the
genes XcXc.
Genetic Advances
• We now can detect genetic disorders at birth.
• Some genetic advances in early detection of disease
have given parents the “option” of aborting a baby who
will be born with a disorder as opposed to letting them be
born alive.
• In the United States, 9 out of 10 children with down
syndrome are aborted.
• Some people have even advocated the practice of
eugenics – the control of human population through
genetics.
• This would involve “selecting” superior individuals for
mating only or sterilizing those deemed “inferior”.