Download Chapter 8 Mendel and Heredity

12.4 Bellringer
 Study the animal shown in figure 11 (page 284)
 List possible mechanisms that allow the arctic fox to
change its fur color with changing seasons
12.4 – Beyond Mendelian Heredity
 **Complex control of characters
 A horse with red hair mates with a horse with white hair, and
their offspring has both red and white hair.


If characters are controlled by single genes with simple
dominant and recessive alleles, the colt’s hair should be on
color or the other


How can this be?
NOT ALWAYS!
Most of the time, characters, such as hair color in horses,
display more complex patterns of heredity than the simple
dominant-recessive patterns discussed so far….
Characters influenced by several genes
 Polygenic inheritance – when several genes influence
a character


Can be scattered along the same chromosome or located on
different chromosomes entirely
Determining the effect of these genes is difficult because of
random fertilization and crossing-over


Many different combinations appear
Examples

Eye color, height, weight, and hair and skin color
 All have degrees of intermediate between one extreme and the
other
Intermediate characters
 Some organisms display a
trait that is intermediate
between the two parents

Known as incomplete
dominance

SNAPDRAGONS
 When a red snapdragon is
crossed with a white
snapdragon, a pink
snapdragon is produced
 **What if a plant breeder
crossed a pink snapdragon
with a white snapdragon?
Characters controlled by genes with three or
more alleles
 Multiple alleles – genes controlled by three or more
alleles

BLOOD TYPE

Even for traits controlled by three or more genes, an individual
can still only have two of the possible alleles for that gene
Characters with two forms displayed at the same
time
 For some traits, two dominant alleles are expressed a
the same time

CODOMINANCE

Different from incomplete dominance because both traits are
displayed
Practice
Characters Influenced by the Environment
 Phenotype often depends on conditions in the
environment

HYDRANGEA


Range in color from blue to pink
 Plants in acidic soil bloom blue, while those in neutral to basic
soil will bloom pink flowers
ARCTIC FOX

Color is affected by temperature
 During summer, the fox produces enzymes that darken its coat
to a reddish brown, enabling it to blend in
 During winter, the pigment-producing genes do not function
because of the cold temps resulting in a white coat, enabling it
to blend
**Environment, cont.
 More examples
 FUR COLOR IN SIAMESE CATS

Influenced by temperature
 Fur on ears, nose, paws, and tail is darker than the rest of its body
• Has a genotype that results in dark fur at locations on its body that are cooler than
the normal body temperature
 Darkened parts have a lower body temp than lighter parts

HUMANS



Height – influenced by nutrition
Color of skin – exposure to sun
Human personality – aggression – strongly influenced by environment
 Identical twins are often used to study environmental
differences because their genes are identical to each other

Downfall…. Twins must be separated.
**Genetic Disorders
 In order to be “normal” a persons genes must encode
and function precisely

Sometimes they are damaged or copied incorrectly resulting in
fault proteins
 Changes in genetic material are called mutations
 Rare because cells have efficient systems for correcting errors
 Many mutations are carried by recessive alleles in
heterozygous individuals

Two phenotypic ally normal people who are heterozygous
could produce children who are homozygous recessive
Some Human Genetic Disorders
Genetic Disorders explained…
 Sickle Cell Anemia
 The recessive allele that causes sickle-shaped blood cells, helps
protect heterozygous individuals from malaria

Malaria – disease caused by protozoan in dirty water
 Cystic Fibrosis (CF)
 Most common fatal, hereditary, recessive disorder among
Caucasians

One in 25 Caucasians carry at least one copy of defective gene
Disorders, cont.
 Hemophilia
 More than a dozen genes code for genes involved in blood
clotting

Receive the defective gene from your mother, because it is located
on the X chromosome
 Huntington’s Disease (HD)
 First appears between ages 30-40.
 Loss of muscle control, uncontrollable physical spasms, severe
mental illness, and eventual death
 Most people don’t know they have it until they have had
children

Unknowingly passed from generation to generation
Treating Genetic Disorders
 Most cannot be cured
 In some cases, can be treated if diagnosed early enough
 People with a family history of severe genetic
disorders will often undergo genetic counseling to
understand risks and probabilities
Gene Therapy
 Could soon allow scientists to correct recessive
genetic disorders by replacing defective genes with
copies of healthy ones

Gene therapy

The first and essential step to gene therapy is to isolate the gene
Mendel’s Hypotheses
 The four hypotheses Mendel developed as a result of
his experiments now make up the Mendelian theory
of heredity—the foundation of genetics.




For each inherited trait, an individual has two copies of the
gene—one from each parent.
There are alternative versions of genes. Today the different
versions of a gene are called its alleles.
When two different alleles occur together, one of them may be
completely expressed, while the other may have no observable
effect on the organism’s appearance.
When gametes are formed, the alleles for each gene in an
individual separate independently of one another. Thus,
gametes carry only one allele for each inherited trait. When
gametes unite during fertilization, each gamete contributes
one allele.