Transcript Mendelian Genetics - Mill Creek High School

Extending Mendelian Genetics,
moving into Human Genetics
Chapter 7
• 1879
Walther Flemming –
*German biologist
*stained cells with dye and saw tiny, threadlike
structures in the nucleus  CHROMOSOMES!!!
*observed and described MITOSIS -- noted that a full
set of chromosomes was being passed on to each
daughter cell.
• 1900 -- Sixteen years after Mendel’s death in 1884, his paper is
rediscovered -- scientists realize that the chromosomes are the
carriers of heredity – Mendel’s FACTORS are ensuring the
passing of traits from parents to offspring
(remember, Mendel’s pea plant research spanned
1856-1863)
• 1902
Walter Sutton –
*American biologist; supports idea that “factors” are located on
chromosomes
*thought that Mendel’s concepts could be applied to all
chromosomes at a cellular level
• 1905
E.B. Wilson and Nettie Stevens
-- Americans studying insect chromosomes
-- they worked independently
-- both 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 developed the
chromosomal theory of sex
*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
See board for Punnett square for sex determination:
Autosomes vs. sex chromosomes
The first 22 pairs of human chromosomes are autosomes
ONLY the last pair are sex chromosomes.
• 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
• 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
Chose to study fruit flies for
following reasons:
• 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
-- color of abdomens identify
sexes at a glance
• 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
• XcY
crossed with
XC Xc
When are showing sex linked
genes, MUST use the X and Y
as base letters…
Sex-linked genes in female
mammals…
In each cell of female mammals, one of the
two X chromosomes is randomly “turned off”
by the process of
X chromosome inactivation
Because of this, females are a patchwork of
two types of cells:
*one type with an active X chromo that
came from the mother, and a second type
with an active X chromo that came from
the father
Ex. Tortoiseshell and calico cat
fur colors
• X inactivation
Epistasis
• In epistasis, a gene at one locus alters the phenotypic
expression of a gene at a second locus
• For example, in mice and many other mammals, coat color
depends on two genes
• One gene determines the pigment color (with alleles B for
black and b for brown)
• The other gene (with alleles C for color and c for no color)
determines whether the pigment will be deposited in the
hair
Ex. double heterozygous black mouse (BbCc) X double heterozygous black mouse (BbCc)
gametes: BC, Bc, bC, bc for each
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-12
BbCc

BbCc
Sperm
1/
4 BC
1/
4 bC
1/
4 Bc
1/
4 bc
Eggs
1/
1/
1/
1/
4 BC
BBCC
BbCC
BBCc
BbCc
BbCC
bbCC
BbCc
bbCc
BBCc
BbCc
BBcc
Bbcc
BbCc
bbCc
Bbcc
bbcc
4 bC
4 Bc
4 bc
9
: 3
: 4
• 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 possibilities are written in capitals and
have different letters because BOTH control the
appearance. The trait being studied is shown as a base
letter
• Example: flower color in snapdragons (C)
Red = R, so CR is used
White = W so CW is used
• Cross a homozygous white snapdragon
with a homozygous red snapdragon.
Incomplete dominance is in effect.
C WCW
x
C R CR
Offspring are all CRCW, and are pink!
Codominance
• situation in which both alleles are equally
strong and both alleles are visible in the
hybrid genotype.
• An example of codominance is found in
chickens.
• When white chickens are crossed with
black chickens, the result is not a gray
chicken, but a chicken with both black and
white feathers.
Codominance: when 2 alleles work together and BOTH
are expressed without one masking the other (NO
intermediate phenotype)
Multiple Alleles: when more than two possibilities for a
trait are present.
•
Example: Blood type – see pages 205
There are 3 alleles for blood type -- A, B, O
Possible combinations:
AA, AO
-- Type A blood
BB, BO
-- Type B blood
AB
-- Type AB blood
OO
-- Type O blood
•
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 carbs found on
the surface of RBC’s
Link to website
• Will often see the A,B designation as
superscripts with a base of I; O is shown as i
(since it is recessive to A and B)
• 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
Rh factor
• Another red blood cell chemical that can cause MAJOR
trouble during pregnancy – all are called antigens
• Antigens – substances that stimulate an immune system
response in the body
• A, B, and Rh are 3 types of antigens on RBC’s, but are 2
different categories of antigens
If Mom is Rh negative and the baby is Rh positive….
• First baby will be fine, but the mother’s body is capable
of “remembering” the mis-match, and if she gets
pregnant again with ANOTHER baby that is Rh positive,
then her body would see that fetus as a foreign invader
and destroy it….basically destroys the baby’s RBC’s
• Can take RoGam during 2nd and subsequent
pregnancies to prevent problems with baby
Polygenic Inheritance
• Additive effect of two or more genes on a
single phenotypic character
• Ex. Skin color in humans
Figure 14.12 A simplified model for polygenic inheritance of skin color
Humans are difficult to study…
Why?
1. # of human genes is extremely large (each cell
has ~100,000 different genes)
2. Humans cannot be easily controlled by an
investigator
3. Time span between generations is long
4. Only a small # of offspring are produced by
each set of parents
5. Environment has a HUGE effect on a person’s
development…
Have developed ways to approach
the difficulties…
• Pedigree analysis – family history for a
particular trait
• Twin studies – Nature vs. nurture
• Population Sampling
• Study of Genetic diseases
• Genetic Technology – start with this in
spring 2014…
• pedigree overview online
Pedigree Analysis
•
A pedigree is a family tree that describes the interrelationships of
parents and children across generations
•
Inheritance patterns of particular traits can be traced and described
using pedigrees
•
Squares indicate males; circles indicate females
•
Roman numerals indicate generations; each individual in a
generation has a separate number – should refer to individuals by the
generation number and then the number that they occur in the
pedigree line (ex. III-2)
•
Fully shaded individuals have a characteristic being studied (may be
dominant or recessive)
•
May see half shaded individuals that indicate carriers (but won’t always
see these)
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-15b
1st generation
(grandparents)
2nd generation
(parents, aunts,
and uncles)
Ww
ww
ww
Ww ww ww Ww
Ww
Ww
ww
3rd generation
(two sisters)
WW
or
Ww
Widow’s peak
ww
No widow’s peak
(a) Is a widow’s peak a dominant or recessive trait?
Fig. 14-15c
1st generation
(grandparents)
Ff
2nd generation
(parents, aunts,
and uncles)
FF or Ff ff
Ff
ff
ff
Ff
Ff
Ff
ff
ff
FF
or
Ff
3rd generation
(two sisters)
Attached earlobe
Free earlobe
(b) Is an attached earlobe a dominant or recessive trait?
Pedigree drag and drop practice
Please note:
This practice site does not use Roman Numerals
and does not number each individual – you WILL
have to do that for me on the exam and on the
practice worksheet tonight…
more Pedigree practice part 1
EVEN MORE practice -- part 2
Nature vs. Nurture
• Phenotype depends on environment (nurture)
AND genes (nature)…
• See NORM OF REACTION: phenotypic range
of possibilities due to environmental influences
on genotype
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
pH of soil effects color
of hydrangea
blooms….
Twin studies
• Fraternal twins – 2 separate eggs, 2
separate sperm fertilized these (no more
similar than siblings born years apart….)
• Identical twins – HAVE SAME DNA!
One egg, one sperm….during mitosis, the
one ball of cells separates into two distinct
sets and continues to develop as separate
organisms…
Population sampling
• Studies humans as a GROUP
• Compares statistics
• Ex. – Tasters vs. nontasters
Chemical – Phenylthiocarbimide (PTC)
Tasters – TT or Tt – bitter taste when
this chemical is present
Nontasters – tt – no taste
Link on discovery of this
Study of Genetic Diseases
• Will do this overview in computer lab
activity – each of you will go through the
diseases given and be familiar with what
they are in terms of inheritance and
symptoms…