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MENDEL AND THE
GENE IDEA
CHAPTER 14
GREGOR MENDEL
Austrian monk
Brought experimental and
quantitative approach to
genetics
Bred pea plants to study
inhertance
Why peas?
Control mating (self- vs. cross-
pollination)
Many varieties available
Short generation time
P (parental) generation = true breeding plants
F1 (first filial) generation = offspring
F2 (second filial) generation = F1 offspring
7 CHARACTERS IN
PEA PLANTS
(OBSERVED 3:1
RATIOS)
Dominant vs. Recessive
(expressed) or (hidden)
FIRST- ALTERNATIVE VERSIONS OF GENES ACCOUNTS
FOR VARIATIONS IN INHERITED TRAITS
ALLELES: ALTERNATE VERSIONS OF A GENE
SECOND- AN ORGANISM INHERITS TWO
ALLELES, ONE FROM EACH PARENT. A
GENETIC LOCUS IS REPRESENTED TWICE
THIRD-IF TWO ALLELES AT A LOCUS DIFFER
THEN THE DOMINANT ALLELE DETERMINES THE
ORGANISMS APPEARANCE
dominant (P), recessive (p)
homozygous = 2 same alleles (PP or pp)l; fully
epxressed
heterozygous = 2 different alleles (Pp); no
noticeable effect; gene make a non-functional
protein
FOURTH-LAW
OF
SEGREGATION
The two alleles for a
heritable character
separate (segregate)
during gamete formation
and end up in different
games
MENDEL’S PRINCIPLES
1. Alternate version of genes (alleles) cause variations in inherited characteristics among offspring.
2. For each character, every organism inherits one allele from each parent.
3. If 2 alleles are different, the dominant allele will be fully expressed; the recessive allele will have no noticeable effect on offspring’s appearance.
4. Law of Segregation: the 2 alleles for each character separate during gamete formation.
Law of Independent Assortment:
Each pair of alleles segregates (separates)
independently during gamete formation
Eg. color is separate from shape
Testcross: determine if dominant trait is
homozygous or heterozygous by crossing with
recessive (pp)
Phenotype: expressed physical traits
Genotype: genetic make-up
PUNNETT SQUARE Device for predicting offspring from a cross
Example: Bb x Bb (B=purple, b=white)
Genotypic Ratio: 1 BB, 2 Bb, 1 bb
Phenotypic Ratio: 75% or 3/4 purple and 25 % or ¼ white
Monohybrid cross: study 1 character
eg. flower color
Dihybrid cross: study 2 characters
eg. flower color & seed shape
DIHYBRID CROSS (VIDEO)
Example: AaBb x AaBb
F: Take the FIRST allele of each gene (left eyebrow) to make
one possible gamete.
O: Take the OUTER alleles (big smile) to make one possible
gamete.
I: Take the INNER alleles (nose curve) to make one possible
gamete.
L: Take the LAST allele of each gene (right eyebrow) to make
one possible gamete.
SEGREGATION OF ALLELES AND
FERTILIZATION AS CHANCE EVENTS
Probability Rules:
a) AND= multiply
½ x ½ = ¼
a) OR=add
¼ + ¼= 2/4 =1/2
THE LAWS OF PROBABILITY GOVERN MENDELIAN
INHERITANCE
Rule of Multiplication:
Probability that 2+ independent events will occur together in a
specific combination multiply probabilities of each event
Ex. 1: probability of throwing 2 sixes
1/6 x 1/6 = 1/36
Ex. 2: probability of having 5 boys in a row
½ x ½ x ½ x ½ x ½ = 1/32
Rule of Addition:
Probability that 2+ mutually exclusive events will occur add
together individual probabilities
Ex. 1: chances of throwing a die that will land on 4 or 5?
1/6 + 1/6 = 1/3
EXTENDING MENDELIAN GENETICS The relationship between genotype and phenotype
is rarely simple
Complete Dominance: heterozygote and
homozygote for dominant allele are
indistinguishable
• Eg. YY or Yy = yellow seed
Incomplete Dominance: F1 hybrids have
appearance that is between that of 2 parents
• Eg. red x white = pink flowers
Codominance: phenotype of both alleles is
expressed
• Eg. red hair x white hairs = roan horses
• A dominant allele does not subdue a recessive allele; alleles
don’t interact
• Alleles are simply variations in a gene’s nucleotide
sequence
• For any character, dominance/recessiveness relationships of
alleles depend on the level at which we examine the
phenotype
Tay-Sachs disease is fatal; a dysfunctional enzyme causes an
accumulation of lipids in the brain
At the organismal level, the allele is recessive
At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant
At the molecular level, the alleles are codominant
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
The Relation Between Dominance and Phenotype
BLOOD TYPING
Multiple Alleles: gene has 2+ alleles
• Eg. human ABO blood groups
• Alleles = IA, IB, i
• IA,IB = Codominant
http://www.youtube
.com/watch?v=KX
TF7WehgM8
11 min
BLOOD TYPING
Phenotype
(Blood Group) Genotype(s)
Type A IAIA or IAi
Type B IBIB or IBi
Type AB IAIB
Type O ii
BLOOD TRANSFUSIONS
Blood transfusions must match blood type
Mixing of foreign blood clumping death
Rh factor: protein found on RBC’s (Rh+ = has
protein, Rh- = no protein)
Pleiotropy: single gene has multiple phenotypic
effects (Eg. sickle cell anemia, cystic fibrosis,
dwarfism)
Epistasis: one gene alters the phenotypic
expression of another gene (eg. albinism - white
fur color in mammals)
B- black B- brown E- dep. of pigment E- no dep. of pigment ee overrides the expression of all the B,b genotypes 9:3:4
Polygenic Inheritance: the effect of 2 or more
genes acting upon a single phenotypic character
(eg. skin color, height, weight, eye color)
Nature and Nurture: both genetic and
environmental factors influence phenotype
Hydrangea flowers vary in shade and intensity of color
depending on acidity and aluminum content of the soil.
GENETIC SCREENING AND COUNSELING
Based on Mendelian genetics and probability
rules
Tests for identifying carriers
Fetal testing
Newborn Screening
GENETIC TESTING
Amniocentesis
- Cant be done until 14 to 16
weeks
- Needle inserted through
abdomen
- Sample of amniotic fluid
removed
- Biochemical tests done
immediately or cultured
- Takes weeks, risk of
miscarriage
Chorionic Villus
Sampling (CVS)
- Can be done 8 to 10 weeks
- Suction tube inserted through
cervix
- Biochemical tests/karyotyping
can be done immediately
- Faster results
- Higher chance of miscarriage
PEDIGREE ANALYSIS
Pedigree: diagram that shows the relationship between parents/offspring
across 2+ generations; follows transmission of traits over generations