Transcript Allele - CARNES AP BIO

Chapter 14
 Genetics:
The scientific study of heredity
 Heredity:
the passing of traits from parents
to offspring
 Inheritance:
You get your genes from your
parents - in meiosis, half of the chromosomes
in a pair come from the Dad, half come from
the Mom
Allele – each form of a gene for a certain trait
(R or r)
 Gene – sequence of DNA that codes for a protein
a thus determines a trait
 Genotype – combination of alleles for a given
trait (RR or Rr or rr)
 Phenotype – Appearance of trait ( round seeds or
wrinkled seeds
 Homozygous - when you have 2 or the same
alleles for a given trait (RR or rr)
 Heterozygous – when you have 2 different
alleles for a trait (Rr)

 Character
– heritable feature that varies among
individuals

ex. Flower color
 Trait

– each variant for a character
ex. Purple vs. white flowers
 Originally
believed that traits of parents
blended together to give offspring results!!!
 Gregor
Mendel – studied pea plants in
monastery garden – COUNTED the plants and
compiled data (QUANTITATIVE APPROACH to
science).
 Mendel
discovered the basic principles of
heredity by breeding garden peas in carefully
planned experiments.
For his experiments, Mendel
chose to CROSS POLLINATE
(mate different plants to each
other) plants that were TRUE
BREEDING (meaning if the
plants were allowed to selfpollinate, all their offspring would
be of the same variety).
P generation – parentals; truebreeding parents that were
cross-pollinated
F1 generation – (first filial) hybrid offspring of parentals that
were allowed to self-pollinate
F2 generation – (second filial) offspring of F1’s
If the blending model of
inheritance were correct, the
F1 hybrids from a cross
between a purple-flowered
and white-flowered pea plants
would have pale purple
flowers (an intermediate
between the two traits of the
parents…BUT:
When F1 hybrids were
allowed to self-pollinate, or
when they were crosspollinated with other F1
hybrids, a 3:1 ratio of the two
varieties occurred in the F2
generation.
So what happened to the
white flowers in the F1
generation?
1.
2.
3.
Alternative versions (different alleles) of
genes account for variations in inherited
characters.
For each character, an organism inherits
two alleles, one from each parent.
If the two alleles differ, the dominant
allele is expressed in the organism’s
appearance, and the other, a recessive
allele is masked.
(Law of Dominance)
4.
Allele pairs separate during gamete
formation. This separation correspondes
to the distribution of homologous
chromosomes to different games in
meiosis.
The gene for a particular inherited character, such as color, resides at a
specific locus (position) on a certain chromosome. Alleles are variants of that
gene. In the case of peas, the flower-color gene exists in two versions: the
allele for purple flowers and the allele for white flowers. This homologous pair
of chromosomes represents an F1 hybrid, which inherited the allele for purple
color from one parent and the allele for white flowers from the other parent.
Figure 11-3 Mendel’s Seven F1 Crosses on
Pea Plants
MENDEL’S TEST
CROSSES ON PEA PLANTS
Seed Coat
Color
Pod
Shape
Pod
Color
Smooth
Green
Seed
Shape
Seed
Color
Round
Yellow
Gray
Wrinkled
Green
White
Constricted
Round
Yellow
Gray
Smooth
Flower
Position
Plant
Height
Axial
Tall
Yellow
Terminal
Short
Green
Axial
Tall
*Flower color – purple (P) vs. white (p)
Seed coat color and flower color are
often put in for one another – thus, the
EIGHT traits!!!
Each true-breeding plant of the parental generation
has matching alleles, PP or pp.
Gametes (circles) each contain only on allele for the
flower-color gene. In this case, every gamete
produced by one parent has the same allele.
Union of the parental gametes produces F1 hybrids
having a Pp combination (because the purple allele is
dominant, all these hybrids have purple flowers.)
When the hybrid plants produce gametes, the two
alleles segregate (separate), half the gametes
receiving the P allele and the other half the p allele.
This Punnett square shows all possible combinations
of alleles in offspring. Each square represents an
equally probable product of fertilization. Random
combination of the gametes results in the 3:1 ratio
that Mendel observed in the F2 generation.
The LAW OF SEGREGATION states that allele pairs
separate during gamete formation, and then
randomly re-form as pairs during the fusion of
gametes at fertilization.
The LAW OF SEGREGATION states that during the formation of
gametes, the two traits carried by each parent separate.
Parent cell with full
gene and Tt alleles.
Traits have separated
during gamete formation
from meiosis.
Grouping F2 offspring from a cross for flower color according to phenotype
results in the typical 3:1 ratio. In terms of genotype, there are actually two
categories of purple-flowered plants (PP and Pp).
 States
that each allele pairs of different genes
segregates independently during gamete
formation;

applies when genes for two characteristics are
located on different pairs of homologous
chromosomes.
 See
figure 14.7 (page 253)
 http://www.sumanasinc.com/webcontent/anim
ations/content/independentassortment.html
 Device
for predicting the results of a genetic
cross between individuals of a known
phenotype.
 Developed by R.C. Punnett
 Rules:
1.
2.
3.
must predict possible gametes first
male gametes are written across top, female
gametes on left side
when reading a Punnett, start in upper left corner
and read as if a book – WRITE OUT GENOTYPES IN
ORDER!
 Character
– flower color
 Alleles – Purple (P) and white (p)
Genotypic Combos possible –
two dominants: PP (homozygous dominant)
two recessives: pp (homozygous recessive)
One of each:
Pp (heterozygous)
Phenotypes possible –

PP – looks purple, so phenotype is purple

pp – looks white

Pp – looks purple (white is masked, but still part of genotype)
 Designed
to reveal the genotype of an
organism that exhibits a dominant trait

it is homozygous dominant or heterozygous?
 Involves
the breeding of a recessive
homozygote with an organism of dominant
phenotype by unknown genotype
Is the dominant phenotype homozygous or heterozygous? A
testcross will tell us!
Steps to do:

Write out genotypes of parents

Write out possible gametes produced

Draw 4 box Punnett square

Put male gametes on top, female on left side

Fill in boxes

Determine genotypes by reading Punnett starting from
top left

Determine phenotypes by reading from genotype list
Ex.
1.
2.
3.
White flowered plant X Purple flowered plant
Yellow peas X Green peas
Tall plant X short plant
 Developed
following TWO characters at
the same time… Dihybrid cross
Ex.
Homozygous dominant for seed color,
homozygous dominant for seed shape
X
homozygous recessive for seed color,
homozygous recessive for seed shape
 Write
out genotypes of parents
 Write out possible gametes produced –
“hopscotch method”
 Draw 16 box Punnett square
 Put male gametes on top, female on left
side
 Fill in boxes
 Determine genotypes by reading Punnett
starting from top left
 Determine phenotypes by reading from
genotype list
1. heterozygous for shape, heterozygous for
color
X
heterozygous for shape, heterozygous for
color
2. heterozygous for shape, homozygous
recessive for color
X
homozygous dominant for shape, homozygous
recessive for color
Beyond Mendel
 Mendel’s
two laws, segregation and
independent assortment, explain heritable
variations in terms of alternative forms of genes
(hereditary “particles”) that are passed along,
generation after generation, according to simple
rules of probability.


Figure 14.4 in text (be able to explain)
Figure 14.7 B in text (be able to explain)
 Now
let’s go beyond basic Mendelian genetics….
Other Genetic Landmarks



1879 Walther Flemming – German biologist who stained
cells with dye and saw tiny, threadlike structures in the
nucleus  CHROMOSOMES!
 also observed and described MITOSIS and noted that a
full set of chromosomes was being passed on to each
daughter cell.
Sixteen years after Mendel’s death, his paper is
rediscovered and scientists realize that the
chromosomes are the carriers of heredity – Mendel’s
FACTORS are ensuring the passing of traits from parents
to offspring.
1902 Walter Sutton – American biologist who supports
idea that “factors” are located on chromosomes
Other Genetic Landmarks

1905 E.B. Wilson and Nettie Stevens – Americans
studying insect chromosomes
Saw that male insects always showed a chromosome that
did not seem to have a match (females always had a perfect
matching set of chromosomes.) Thus, they referred to the
non-matching chromosomes as Sex Chromosomes.
In females the sex chromosomes do match
XX
In males, one of the chromosomes looked as if it were missing
a part, so called it a Y
XY



Other Genetic Landmarks

1909 Wilhelm Johannsen – Danish biologist who coined the term
“gene” to define the physical units of heredity.

GENE: segment of DNA molecules that carries the instructions for
producing a specific trait.
Other Genetic Landmarks
 1912
Thomas Hunt Morgan – Showed evidence
that the presence of white eye color in fruit
flies was associated with a particular gene on a
particular chromosome.
 Drosophila melanogaster -- scientific name
for fruit fly .
Why Study Fruit Flies?
Produces about 100 offspring per egg lay – good
statistics!
 Matures in only 15-20 days!
 Only have 8 chromosomes (4 pair) so less to look
at!
 Easy/inexpensive to raise!
 Chromosomes are VERY large and easy to see and
locate!
 Sexes are easily distinguished

female is larger
 shapes of abdomen identify sexes at a glance

Drosophila Crosses
Normally, fruit flies always have RED eyes, but Morgan saw a
white eyed one show up, and it was MALE!! Thought that
this was strange, so he conducted an experiment:
P
white eyed X red eyed
F1
all red eyed offspring
(thus concluded that red is dominant over white for color)
F1
red eyed
X red eyed
F2
¾ red eyed & ¼ white eyed
(AND ALL OF THE WHITE EYED ONES WERE MALE!!!)
 Determined that this was a sex-linked trait – the trait for
eye color in fruit flies is carried on the sex chromosome.
 Examples of other sex-linked traits:
hemophilia & color
blindness
 C = normal vision, c = colorblindness
 Xc Y crossed with XCXc….work this problem out!

Dominance, Multiple Alleles, and
Pleiotrophy
 Involve
effects of alleles for SINGLE GENES
DOMINANT Alleles


See pages 256 and 257
 Definition is NOT clear cut…
Three points:
 They range from complete dominance, through
various degrees of incomplete dominance, to
codominance.
 They reflect the mechanisms by which specific alleles
are expressed in phenotype and do not involve the
ability of one allele to subdue another at the level of
the DNA.
 Dominant alleles are not necessarily more common.
Incomplete Dominance
 Incomplete
Dominance: when BOTH alleles
in an individual affect the appearance of a
trait and you get a brand new color that was
not found in the original parents. Both traits
are written in capitals and have different
letters because BOTH control the
appearance.
 Example: flower color in snapdragons
 Pure red (RR) X Pure white (WW)

Offspring will be pink (RW)
Incomplete Dominance
Codominance

Codominance: when 2 alleles work together and BOTH
are expressed without one masking the other (NO
intermediate phenotype)

TWO ALLELES AFFECT THE PHENOTYPE IN SEPARATE,
DISTINGUISHABLE WAYS!
Multiple Alleles

Multiple Alleles: when more than two possibilities
for a trait are present.
Example: Blood type – see pages 257 and 258
 There are 3 alleles for blood type -- A, B, O
 Here, A and B are dominant over O, but if A and B
are present together, neither dominates!!! This is
codominance – they share the power of expression.

More on Blood Types
 The
letters A, B, and O refer to 2
carbohydrates found on the surfaced of RED
BLOOD CELLS.
Will often see the A,B designation as
superscripts with a base of I;
 O (since is recessive to A and B) is shown as i.

 Matching
compatible blood groups is critical –
proteins called antibodies are produced
against foreign blood factors.

Antibodies bind to foreign molecules and
cause donated blood cells to clump together
(agglutination).
Figure 14.10 Multiple alleles for the ABO blood groups
Pleiotropy
Most
genes have MULTIPLE phenotypic
effects

Ability of a gene to affect an organism
in many ways is called PLEIOTROPHY
This
is due to molecular and cellular
interactions that are responsible for
an organism’s development

Ex. Sickle-cell disease (page 262)
Figure 14.15 Pleiotropic effects of the sickle-cell allele in a homozygote
Sickle cell is a disease
caused by the
substitution of a single
amino acid in the
hemoglobin protein of red
blood cells. When
oxygen concentration of
affected individual is low,
the hemoglobin
crystallizes into long rods.
Heterozygotes for sickle
cell have increased
resistance to malaria
because the rod shape of
blood interrupts the
parasites life cycle. So,
sickle cell is prevalent
among African
Americans.
Epistasis
Involves
MORE THAN ONE GENE
Defined as when a gene at one locus
alters the phenotypic expression of a
gene at a second locus
Mouse coat color – page 258
 coat color – B = black, b = brown
 second gene determines whether
pigment will be deposited in the
hair: C = color, c = albino
Figure 14.11 An example of Epistasis
One gene determines whether the
coat will be black (B) or brown (b).
The second gene controls
whether or not pigment of any
color will be deposited in the hair,
with the allele for the presence of
color (C) dominant to the allele for
the absence of color (c).
Polygenic
Inheritance
• Additive effect of
two or more
genes on a single
phenotypic
character
• Ex. Skin color in
humans – page
259
Nature vs. Nurture


Phenotype depends on nature AND genes…
See NORM OF REACTION: phenotypic range of
possibilities due to environmental influences on
genotype…READ TEXT PAGE 259!
 Ex. Blood count of RBC’s and WBC’s depends on
altitude, physical activity, presence of infection
 Ex. Color of hydrangea blooms depends on soil acidity
Figure 14.13 The effect of environment of phenotype
Human Genetics
 Humans
are difficult to study…but we have
developed ways to approach these
difficulties.





Pedigree analysis – family history for a particular
trait
Study of Genetic diseases
Twin studies – Nature vs. nurture
Population Sampling
Genetic Technology
Figure 14.14 Pedigree analysis
•Males are shown as squares, Females are shown as circles
•Horizontal lines – “marriage” or mating lines
•Vertical lines – offspring lines
•Shaded symbols represent individuals with the trait being studied
•CARRIERS of the trait are those individuals that are heterozygous (Ww OR Ff)
because they may transmit the recessive allele to their offspring even though they do not
express the trait.
•See text page 261 – PEDIGREE ANALYSIS
Errors in Chromosomes
1.
Mistakes in numbers of chromosomes:
nondisjunction -- members of a pair of
homologous chromosomes do not move apart
properly…result in offspring that have:


Aneuploidy – abnormal chromosome number:
 Can be…Trisomy or Monosomy or Polyploidy
Chromosomal Mistakes
2. Mistakes in shape of chromosomes:
deletion – part of chromosome is broken off and lost
completely
 duplication – broken fragment of chromosome attaches to
sister chromatid so section is repeated on that chromatid
 inversion – when fragment reattaches to original chromosome
but in reverse order
 translocation – broken fragment attaches to a nonhomologous
chromosome


(can exist as reciprocal or nonreciprocal)
Figure 15.13 Alterations of chromosome structure
Technology is Providing New Tools
for Genetic Testing and Counseling
 Carrier
recognition with genetic screening
and Fetal testing:
-ultrasound and sonograms
-amniocentesis
-chorionic villi sampling
-fetoscopy
-blood/urine tests of newborns
Figure 14.17 Testing a fetus for genetic disorders
Probabilities Practice
• What is the probability that the genotype Aa will
be produced by the parents Aa x Aa?
–½
• What is the probability that the genotype ccdd
will be produced by the parents CcDd x CcDd?
– 1/16
• What is the probability that the genotype Rr will
be produced by the parents Rr x rr?
–½
• What is the probability that the genotypes TTSs
will be produced by the parents TTSs x TtSS?
– 1/4
Genetics Practice Problems
• How many unique gametes could be
produced through independent assortment
by an individual with the genotype
AaBbCCDdEE?
–8
• What is the expected genotype ratio for a
dihybrid heterozygous cross?
– 9:3:3:1