Transcript Dominant & Recessive Traits

Dominant & Recessive Traits
Chapter 12
DOMINANT TRAITS
eye coloring brown eyes
RECESSIVE TRAITS
grey, green, hazel, blue eyes
vision
farsightedness
normal vision
normal vision
normal vision
normal vision
nearsightedness, night blindness,
color blindness*
hair
dark hair
non-red hair
curly hair
full head of hair
widow's peak
blonde, light, red hair
red hair
straight hair
baldness*
normal hairline
facial
features
dimples
unattached earlobes
freckles
broad lips
no dimples
attached earlobes
no freckles
thin lips
Dominant Trait
Recessive Trait
Appendages
extra digits
fused digits
short digits
fingers lack 1 joint
limb dwarfing
clubbed thumb
double-jointedness
bent pinky
normal number
normal digits
normal digits
normal joints
normal proportion
normal thumb
normal joints
straight pinky
other
immunity to poison ivy
normal pigmented skin
normal blood clotting
normal hearing
normal hearing & speaking
normal- no PKU
*sex-linked traits
susceptibility to poison ivy
albinism
hemophilia*
congenital deafness
deaf mutism
phenylketonuria (PKU)
Longer 2nd toe is dominant over
2nd toe shorter than big toe.
Mendel’s experiment
Gregor Mendel conducted experiments that
used monohybrid crosses; carried out in 3
steps
Monohybrid cross- studies 1 trait
Step 1: Mendel allowed pea plants to selfpollinate for several generations to get
offspring that are true-breeding (purebred).
He used true-breeding plants as the first
generation in his experiment. These were
called the parental (P) generation
Mendel’s experiment
Step 2: Mendel crossed 2 P generation plants
with contrasting traits. This produced offspring
called the first filial generation (F1 generation).
Mendel recorded the # of F1 generation plants
with each trait.
Step 3: Mendel allowed the F1 generation to
self-pollinate, producing a second filial (F2)
generation. He then recorded the # of F2
generation plants with each trait.
Mendel’s Results
F1 generation- 100% displayed the same trait
for a given character (the dominant trait)
F2 generation- the trait that disappeared in
the F1 generation reappeared. The ratio of
the characters in this generation were
approximately 3:1 (dominant:recessive).
Notice this # is approximate. Why?
Law of Segregation
When an organism
produces gametes,
each pair of alleles
is separated and
each gamete has an
equal chance of
receiving either one
of the alleles.
Patterns in Inheritance?
Mendel conducted another experiment to
see if traits were passed down in a pattern.
He did this by conducting a dihybrid cross.
dihybrid cross- involves 2 characters
He found that the inheritance of one trait did
not affect the inheritance of any other trait.
This is because of the Law of Independent
Assortment.
Law of Independent Assortment
During gamete
formation (meiosis)
the alleles of each
gene segregate
independently.
Alleles can “mix
and match”
i.e. Round seeds
can be green or
yellow (as can
wrinkled seeds)
Exceptions:
Genes that are located close together
on the same chromosome will rarely
separate independently. These genes
are linked genes.
Sex Linked Traits
•Genes located on one of the sex chromosomes (X or Y)
•Most sex linked traits are on the X chromosome
because it is much longer than the Y chromosome
•More common in males since they only have one X
chromosome
Examples: colorblindness, hemophelia
More complicated traits
Most traits do not follow Mendel’s pattern of
inheritance because most genes either have
more than 2 possible alleles OR are controlled by
more than 1 gene.
Examples:
Polygenic inheritance
Incomplete dominance
Multiple alleles
Codominance
Polygenic Inheritance
Characters that are controlled by more
than one gene are polygenic traits.
Examples: eye color (amount of greenness
or browness of the eye), height, skin color
Incomplete dominance
2 dominant alleles occur. Neither allele is
completely dominant over the other.
Example: snapdragons
Multiple Alleles
Genes that have 3+ possible
alleles have multiple alleles.
Only 2 alleles for a gene can be
present in one individual. The
determination of dominance in
these cases can be very complex.
Example: blood type (alleles= IA,
IB, i)
Codominance
2 traits can appear at the same time for some
characters, leading to codominance. In these
cases, both alleles for the same gene are fully
expressed.
Examples: applies to blood type AB, roan fur color
in cattle
Pedigree
A family history that shows how a trait is inherited over
several generations.
A pedigree chart is a diagram that shows the occurrence
and appearance or phenotypes of a particular gene or
organism and its ancestors from one generation to the
next.
A pedigree can
help us learn
about sexlinked traits,
dominance, &
heterozygosity.
Affects of the environment
Some phenotypes can be affected by conditions
in the environment (such as nutrients &
temperature).
Examples: color of the arctic fox is affected by
temperature, height in humans can be affected
by nutrition, exposure to too much oxygen in
premature babies leads to blindness while too
little oxygen leads to brain damage, drugs that
cause birth defects (thalidomide prescribed to
treat morning sickness in pregnant women),
sunlight & butterfly wing color