Transcript PHYSMendeliangenetics

Mendelian Genetics
Principles of Heredity
Gregor Mendel: The Father of Genetics
Basics of Heredity: Mendel’s Rules
1. Traits are controlled by particles
2. Two genes per trait
3. Heterozygous vs. Homozygous
4. Law of Dominance
5. Law of Segregation
6. Law of Independent Assortment
1. Traits are controlled by “particles”
a. The “particles” are solids in the cells
b. “Particles” = genes
c. Genes are…
d. Gene means…
e. Alleles are…
2. Two genes per trait
a. Most biologists thought it was a single
gene per trait (in the sperm), but really…
b. One gene from dad and one from mom
c. Trait: general description of what is being
controlled by the genes (e.g. seed color)
d. Phenotype: visible expression of the genes
(e.g. yellow or green)
e. Genotype: Actual genes present
3. Heterozygous vs. Homozygous
a. the 2 genes may be the same
= homozygous e.g. EE or ee
(purebred, true, non-carrier)
b. …or different
= heterozygous e.g. Ee
(hybrid, crossed, mixed breed, carrier)
4. Law of Dominance
a. when it comes to showing up,
some genes are more powerful
than others = Dominant
e.g. E =
or
b. Others only show up if both
genes are recessive
e.g. e =
Antonio
Alfonseca (P)
4. Law of Dominance, cont.
c. Some more traits:
Polydactyly
Syndactyly
Achondroplasia
4. Law of Dominance, cont.
d. Some traits are inherited as Recessives:
Sickle-cell anemia
Hitchhiker’s Thumb
Albinism
Phenylketylnuria
5. Law of Segregation
a. “When gametes are made, the two genes for a
trait separate and each gamete has only one
gene for each trait” This happens in meiosis
b.
Punnett squares:
show the possible
offspring from a
cross
A
a
A
a
6. Law of Independent Assortment
a. “The way one pair of genes for a trait is
inherited has no effect on any other trait”
b. This allows us to examine multiple traits on a single (larger)
Punnett Square. This is called a dihybrid cross and
predicts 2 traits at once.
c. Probability II “The chances of two separate events
happening at the same time equals the product of their
separate probabilities”
Parent 2: AaBb
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Parent 1:
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AaBb
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d. Solving a Dihybrid Cross
1.
2.
3.
4.
Make “Gamete Tree” for both parents
Create Punnett Square
Insert Gametes along top and side
Fill in Punnett Square
Exceptions to Mendel’s Rules
I. Linkage
A. genes for two traits carried on the same chromosome
B. example: Freckles and alleles for red hair
II. Mutations
A. random changes in the genetic code
B. may produce “unexpected” offspring that Mendel
couldn’t account for.
C. Ex: achondroplastic kids (Dd) usually come from
two perfectly normal (dd) parents
Exceptions to Mendel’s Rules
III. Polygenic
A.
Controlled by multiple alleles on different chromosomes.
B.
Best examined at the population level
C.
Shown as continuous (bell curve) distribution
D.
Ex.: human height, skin and hair color.
IV. Environmental Effects
A. some genes are affected by environmental influence
B. example: BRCA1 Breast Cancer Gene
Exceptions to Mendel’s Rules
IV. Sex Linkage
A. traits specifically carried on (usually) the X
chromosome
B. Work like recessive traits (needs 2 alleles to show)
C. Show up most often in males (XY versus XX
genotype)
D. Examples: Hemophilia and Color Blindness
Exceptions to Mendel’s Rules
V. Codominance
A. two alleles that express themselves equally in the
presence of each other
B. Example: ABO blood grouping, Roan cattle
VI. Multiple Alleles
A. some traits have more than two possible phenotypes
because there are more than just two alleles for the trait
B. This creates multiple combinations of possibility
C. Example: ABO blood grouping
Multiple Alleles & Codominance:
ABO Blood Groups
I.
II.
A and B alleles code for glycoproteins
(antigens) on red blood cells which can be
detected immunochemically:
A. mix blood sample with type A or type B
antibodies
B. look for clumping (agglutination) of RBC’s
O allele carries neither antigen
ABO Blood Groups
A - A antigen only
B - B antigen only
AB - Both A and B antigens
O - Neither antigen
III. ABO Genotypes and Phenotypes
Genotype
Antigen
Phenotype
IAIA
A O
I I
B B
I I
B O
I I
IAIB
A
A
B
B
A, B
A
AB
IOIO
Neither
O
B
Some Important ABO Factoids
IV.
V.
VI.
IA and IB are codominant
Both IA and IB are dominant to IO
Applications
A. testing compatibility of blood transfusions
A.
B.
B.
C.
D.
Who can donate to who?
What happens in case of incompatibility?
disproving parentage of a child
forensic science
childbirthing (Rhogam and hemolytic disease)
Pedigree Analysis
I. Introduction to Pedigrees
A. Background: What is a Pedigree?
“A diagram that shows appearance of phenotypes
for a single trait in a group of related individuals
from one generation to the next.”
Pedigree Analysis
B. Reading a Pedigree: Symbols
Males (squares or triangles)
Females (circles)
Marriage/Mating
Offspring and Siblings
Shaded or Unshaded
Crossed out
Generation Labels (Roman)
Individual Labels (Arabic)
Birth Order (left to right)
Father 1.
Mother 2.
I.
Dead
Marriage line
Siblings
Marriage into family
Oldest Child
Youngest Child
II.
Daughter 1.
Son 2.
Daughter 3.
Pedigree Analysis
C. Genotypes
in a Recessive Pedigree
There are some rules to follow:
1. Shaded people are homozygous recessive. Fill
them in as such.
2. Unshaded people are either:
Homozygous Dominant OR Heterozygous
So… Assign one Dominant allele to each person
3. Work one generation at a time to determine the
“unknown” genotypes. NEVER “skip”
generations!!!!!
Please copy this
pedigree
I-1
II-1
II-2
III-1
II-3
I-2
II-4
II-5
II-6
III-2 III-3
Pedigree Analysis, continued
II.
Advanced Pedigrees: Unknown Inheritance
A.
B.
C.
First, “flip a coin” i.e. pick a mode ( Dom or Rec)
Assign known genotypes across pedigree
Begin filling in unknowns

D.
E.
F.
Remember to work 1 generation at a time!!! Don’t skip!!!
Look for anomalies (matings that don’t work)
Try the pedigree again with the other mode of inheritance
Use colored pencils, different ink pens, or different letters
to help you solve