Transcript Mendelian Genetics

Mendelian Genetics
Chapter 11
Genetics
• What is Genetics? The scientific
study of heredity
• What is 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
• Chromosome: Thread-like structure within
the nucleus that contains the genetic
information passed from one generation of
cells to the next
• What we know today is based on the work
of Gregor Mendel – Austrian monk
“The Father of Modern Genetics”
Gregor Mendel 1856-1865
– Austrian monk who laid groundwork for
understanding biological inheritance
– studied 7 inherited traits in pea plants (truebreeding) of his monastery garden
– COUNTED the plants and compiled data
(QUANTITATIVE APPROACH to science)
Paper was published in 1866,
but not enough was understood
to truly value this work.
Key terms to know
• Allele – each form of a gene for a certain trait (R or r)
• Gene – sequence of DNA that codes for a protein and
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 of the same alleles for
a given trait (RR or rr)
• Heterozygous – when you have 2 different alleles for a
trait (Rr)
Key terms to know
• Dominant – Allele that is expressed,
Represented by a Capitol letter.
• Recessive – Allele that is masked or
hidden, Represented by a lowercase
letter.
• F1 generation: Filial or First Generation
• Test cross (Punnett square): determine
the genotype of an individual with a
dominant phenotype
Figure 14.1 A genetic cross
Mendel chose to use plants that
were true-breeding…
• P generation – parentals; true-breeding
(On their own create identical
offsprings) parents that were crosspollinated
• F1 generation – hybrid offspring of
parentals that were allowed to selfpollinate
• F2 generation – offspring of F1’s
Figure 14.2 Mendel tracked heritable characters for three generations
Figure 11-3 Mendel’s Seven F1
Crosses on Pea Plants
Section 11-1
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
Axial
Tall
Yellow
Terminal
Short
Green
Axial
*Flower color – purple (P) vs. white (p)
Seed coat color and flower color are often put
in for one another – thus, the EIGHT traits!!!
Go to
Section:
Plant
Height
Tall
Mendel’s 4 ideas…
1. Alternative versions (alleles) of genes account
for variations in inherited characters.
2. For each character, an organism inherits two
alleles, one from each parent.
3. If the two alleles differ, the dominant allele is
expressed in the organisms appearance, and
the other, a recessive allele is masked.
4. The two alleles for each character segregate
during gamete production.
(Law of Segregation)
Figure 14.3 Alleles, alternative versions of a gene
Figure 14.4 Mendel’s law of segregation (Layer 2)
From the law of segregation……
• Came the Law of Independent Assortment
Genes for different traits can segregate
independently during the formation of
gametes.
In other words…..
Just because a seed is round does not
mean that it has to be green.
Figure 14.5 Genotype versus phenotype
Probability & Genetics
Punnett Square
• Device for predicting the results of a genetic
cross between individuals of a known
phenotype.
• Developed by R.C. Punnett
• Rules:
1. must predict possible gametes first
2. male gametes are written across top, female
gametes on left side
3. when read Punnett, start in upper left corner
and read as if a book
4. WRITE OUT GENOTYPES IN ORDER
Example……
• Character – flower color
• Alleles – Purple (P) and white (p)
Note: Purple is dominant with a capital
letter and white is recessive shown with a
lowercase of dominant trait
Genotypic combos possible –
two dominants: PP (homozygous dominant)
two recessives: pp (homozygous recessive)
One of each:
Pp (heterozygous)
Phenotypic possibilities – physical
appearance
• PP – purple
• pp – white
• Pp – purple (white is masked, but
still part of genotype)
Monohybrid crosses –
only one character considered
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.
White flowered plant X Purple flowered plant
2.
Yellow peas X Green peas
3.
Tall plant X short plant
Dihybrid (Two-Factor)Cross
• Because genes separate independently
we can determine the possible outcomes
of a two-factor cross.
• Example: Mendel’s Peas
F1 Hybrids for Shape and Color: RrYy
Foil – First, Last, Inner, Outer
Possible gametes passed on to offspring:
RY, ry, rY, and Ry – Place in order
(dominant to recessive) RY, Ry, rY, ry
then place on cross
Beyond Dominant and Recessive
• Incomplete Dominance
One allele is not completely dominant over
the other – something in the middle is
expressed
Ex. Red and White Snapdragons
Result can be heterozygous (Rr) or two
separate dominant alleles (RW) each
resulting in a mixture of both alleles
Another way that incomplete
dominance can be expressed
• Red= RR
• White= WW
• RW= pink- each allele
is equally expressed
to result in a blended
product
One way to express incomplete
dominance
• RR (Red) X rr
(White)= (Rr)Pink
• Rr- results in a
blended result of
PINK
Beyond Dominant and Recessive
• Codominance
Both alleles are expressed in the phenotype
Ex. Cow Hair Color
RR – Red
WW – White
RW – Roan (Red & White)
Practice
Codominance/Incomplete Dominance #1-4
Beyond Dominant and Recessive
• Multiple Alleles
Genes have more then two alleles
Ex. Blood Type
Type A blood- AA or AO alleles
A is dominant to O
Type B blood- BB or BO alleles
B is dominant to O
Type AB- codominant- A and B alleles
A nor B is dominant so both are expressed on organisms RBC
Type O- recessive- OO alleles
Both alleles must be recessive in order to have type O.
More on blood types…..
• The blood type determines what antibodies are
located within the blood. Type A blood has type B
antibodies. If type B blood is put into their bodies,
their immune system reacts as if it were a foreign
invader, the antibodies clump the blood - can cause
death.
• Type AB blood has no antibodies, any blood can be
donated to them - they are called the "universal
acceptors"
• Type O blood has no surface markers on it,
antibodies in the blood do not react to type O blood,
they are called the "universal donors"
Polygenic Traits
• Traits that are controlled by the interaction
of several genes.
• Example:
– Reddish brown eyes in varying degrees found
in fruit flies is controlled by 3 genes
– Human skin color is controlled by 4 different
genes which result in a variety of skin color.
Sex-linked Genetics
Ex. Colorblindness
Sex Chromosomes- last pair
(23rd) in a karyotype
MALE KARYOTYPE
FEMALE KARYOTYPE
Sex Chromosomes- last pair
(23rd) in a karyotype
• Male – XY and Females – XX
• The 23rd pair of chromosomes will
determine the gender of an individual
• Very few genes are located on the Y
chromosome……Most are located on the
X
• Sex linked alleles will ALWAYS be
tracked on the X chromosome ONLY
when we conduct practice genetic
problems
Sex-Linked Genes
• Ex. Colorblindness is carried on the sexchromosomes
• It is a recessive trait – Xc
How many genes do females need to
express the trait (colorblindness)?
2 Xc Xc
How many genes do males need to
express the trait (colorblindness)?
1 XcY
Sex-Linked Punnett Square
• Let C = Normal Vision and c = Colorblind
• Cross: Normal Male ( ) x Carrier Female (
)
Sex-Linked Punnett Square
C
• Let C = Normal Vision and c = Colorblind
• X C Y x X C X c = Normal Male x Carrier Female
1st put male genotype
C
X
Y
on the top of the table
& female genotype on
the left side
C
X
X
c
Sex-Linked Punnett Square
C
• C – Normal Vision and c - Colorblind
• X C Y x X C X c - Normal Male x Carrier Female
2nd, cross them
XC
Y
X
C
X
c
XC XC
C
X X
c
XCY
c
X Y
Sex-Linked Punnett Square
C
list the sex
• C – Normal Vision and c - Colorblind 3rd,
and appearance
C
C
c
X Y x X X -Normal Male x Carrier Femaleof each possible
offspring
XC
Y
Offsprings:
X
1 Normal Female
C
XC XC
XCY
1 Normal (Carrier)
Female
1 Normal Male
X
c
C
X X
c
c
X Y
1 Colorblind Male
PRACTICE and HW
• Complete problems 1-3 on the sex linked
genetic practice problems sheet NOW!
• Complete the remaining 3 Co-dominant
and Incomplete dominant practice
problems and Sex Linked practice
problems # 4-8 from today’s class for HW