Exceptions to Mendel`s Laws:

Genetics
S
Examples of Traits
Attached Ear Lobes
Hand clasping
Widows peak
Tongue rolling
Cleft Chin
Gregor Mendel, an Austrian monk,
discovered the basic principles of
heredity through experiments in
his garden. The observations he
made while growing peas in his
monastery's garden became the
foundation of modern genetics
and the study of heredity.
http://www.dnalc.org/view/16182-Animation-4Some-genes-are-dominant-.html
Genetics &
Heredity
Heredity – The
passing of traits
of parents to
offspring.
Genetics - The
study of how
traits are passed
on from parents
to their
descendents.
Started 150
years ago with
Gregor Mendel.
Mendel used true breeding
(self pollinating) pea plants.
Quick reproduction
Easily observed
Controlled reproduction
Cross pollination
seed shape
seed color
flower color
 http://www2.edc.org/we
blabs/mendel/mendelIns
tructions.html
Hybrid
http://www2.edc.org/weblabs/mendel/mendelInstructions.html
Mendel’s Conclusions –
2. The chromosome exists in pairs. 1 factor
(chromosome) from each parent.
 Chromosome section
that has genetic
information for a trait.
 Different forms of a gene.
Genotypes You can’t see
2 alleles that control the
phenotype (looks)
Homozygous – same –
dominant – RR (red)
recessive – rr (white)
Heterozygous –
different – Rr (red)
Phenotype –
how it appears/expressed
or looks (red or white)
Mendelian Genetics
http://www.britannica.com/EBchecked/topic/374739/Gregor-Mendel
Independent Assortment
Dominant & Recessive Traits
Law of Segregation
3 : 1 Ratio
Exceptions to
Mendel’s Laws:
Co-dominance
Incomplete Dominance
Multiple Alleles
Lethal Genes
Test Crosses
Environment
Environment
Environmental
factors such as soil
type, exposure to
drugs, alcohol,
radiation, diet, will
change the
phenotype.
Examples:
Hydrangea, heart
disease, cancer.
Lethal Genes:
 Some genes are lethal when present in the
homozygous condition (dominant or recessive)
 Ex. In chickens, when an embryo contains two
copies of the recessive gene known as creeper,
the embryo dies inside the eggshell.
 Chicks that are heterozygous for the gene
survive
 In mice, yellow coat color is dominant to grey
 Mice that have YY (pure yellow) coat color
do not survive
Multiple Alleles:
Polygenic Inheritance
 Some genes have 3 or more alleles BUT you can
only have two alleles for a trait
(Multiple genes determine the phenotype!)
 Ex. Skin color or height in humans
 Human blood types ABO
 There are 4 phenotypes A, B, AB & O
produced from 3 alleles IA-dominant, IB –
dominant, & i - recessive
Coat Colour in Rabbits
Four phenotypes and four alleles
Allelic series is C > cch > ch > c (which is most dominant)
Coat Color Phenotype
Full Color
Chinchilla
Himalayan
Albino
Genotype
CC, Ccch, Cch, Cc
cchcch, cchch, cchc
chch, chc
cc
Incomplete Dominance:
 Two alleles of a gene appear to be blended into the
phenotype
 Red snapdragons X white snapdragons = pink!
Test Cross:
B=brown
b= blue
 Designed to reveal the genotype of an organism that
exhibits the dominant trait
 Brown eyes might be BB or Bb… how can we tell??
 Cross the organism with an individual expressing the
recessive trait….bb
Co-dominance:
 Two alleles of a gene are clearly expressed in
the phenotype (equally)
 Ex. Roan color in cattle – coat has both red
and white hairs in it
 AB blood type in humans - IA & IB alleles
are present
DNA Discovered 1869
Friedrich Miescher
1905 Some genes are linked and do not
show independent assortment, as seen by
Bateson and Punnett.
1950 Erwin Chargaff -In human
DNA, for example, the four bases are
present in these percentages:
A=30.9% and T=29.4%; G=19.9%
and C=19.8%. The A=T and G=C
equalities, later known as Chargaff's
rules, helped Watson and Crick to
discover the structure of DNA.
In 1953 Rosalind Franklin, James Watson ,
Francis Crick, and Maurice Wilkins
determined the structure of
deoxyribonucleic acid (DNA).The
molecule that is the basis for heredity,
DNA, contains the patterns for
constructing proteins in the body,
1997
2000
Human Genome Project 2003
HGP was a 13-year project
coordinated by the U.S. Department
of Energy and the National Institutes
of Health. During the early years of
the HGP, the Wellcome Trust (U.K.)
became a major partner; additional
contributions came from Japan,
France, Germany, China, and others.
Project goals were to:
•
identify all the approximately
20,000-25,000 genes in human
DNA
•
determine the sequences of the 3
billion chemical base pairs that
make up human DNA
•
store this information in databases
•
improve tools for data analysis
•
transfer related technologies to
the private sector
•
address the ethical, legal, and
social issues that may arise from
the project
http://www.uvm.edu/~cgep/Education/Sequence.html#Mutations
Proving the genotype:
 If you are asked to prove a genotype of an
individual (with a dominant trait), use a testcross
 Cross the unknown genotype with an individual
who is homozygous recessive
The Results:
 If all the offspring of the cross show the dominant
trait (brown eyes), then the parent must be
homozygous for the dominant allele BB
 If both phenotypes appear among the offspring
then the parent must have been heterozygous for
the trait Bb
Punnett Square – Roan Cattle
•In certain cattle, the hair colour can be red- RR, white- R’R’, or
roan- RR’
•Cross a red bull with a white cow
•Cross an offsrping from the F1 generation with a white cow
Punnett Square – Blood Groups:
 Show a cross between a person who has
heterozygous A type blood & a person who has
homozygous B type blood.
IAi x IBIB
Genotypes:
Phenotypes:
Example with Mouse Coat Colour
 Yellow is dominant to grey
 Cross hybrid yellow mice Yy x Yy
Resulting offspring:
2 yellow to 1 grey
One genotype is missing
Somehow the YY genotype is lethal
The 2:1 ratio is the typical ratio for a lethal gene.
Answers to samples:
A •child’s phenotype is O, the genotype must be ii (recessive)
•father (A) and mother (B) must both carry the recessive
allele
•Father’s genotype IAi and mother’s genotype IBi
B
C
Chance of having an
AB type baby is
25%
a) The offspring occur in three types, classified as long, round
and oval (intermediate), suggesting incomplete dominance
• Long (L) Round (L’) Oval (LL’) - heterozygous
Test this idea…. LL’ x LL’ Gives a ratio of 1 long to 2 oval to 1
round radishes.
b) (LL x LL), all the offspring will be
long.
c) (L’L’ x L’L’), all the offspring will be
round.
A)  Since ¼ of the eggs don’t hatch – lethal gene
 Crested (C) and non-crested (c) then both parents would
have to be Cc ( eg. Cc x Cc)
All the eggs with CC genotype would not hatch
Viable offspring would hatch in a ratio of 2
crested to 1 non-crested
B)
Cc x cc
Ratio: 1Cc to 1 cc
1 crested to 1 non
A)
Bull’s genotype RR, Cow’s genotype R’R’
B)RR’ x R’R’
All of the offspring have RR’
genotype and will be roan
Ratio of 2 roan to 2 white calves