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Welcome to genetics!
What is genetics?
The scientific study of heredity - the genetic
properties or inherited features of an
organism
• Molecular genetics: study of structure and
function of DNA and RNA
• Population genetics: study of genes within
populations, including gene frequency, the
gene pool and evolution
Like Begets Like
List differences
in physical
characteristics
of you and
your mother
Pre-Mendel Belief of inheritance:
This theory stated that offspring would have traits
intermediate between those of the parents (Blending
Concept)
Mendel’s Law of Segregation:
Each organism contains two factors for each trait,
and the factors segregate during the formation of
gametes so that each gamete contains only one
factor for each trait
Mendel
in his
garden
Mendel observed
over 7,324 peas
for one test cross
The history of genetics…
• Meet Gregor Mendel...
• Timeline of genetics I
Garden Pea
Anatomy and
Traits
Easy to grow, short
generation time, could
control pollination, wide
variety of traits
Mendel looked at 7 major
traits
Plant Height (tall, short)
Pod color (green, yellow)
Pod Shape (Inflated,
constricted)
Seed Shape (round, wrinkled)
Seed Color (yellow, green)
Flower color (white, purple)
What Mendel didn’t know,
was that genetic information
is found on our
chromosomes
© 2007 Paul Billiet ODWS
Let’s learn about the relationship
between chromosomes and genetics
• http://www.youtube.com/watch?v=eOvMN
OMRRm8
CHROMOSOMES
© The Human Genome Project: Biocomputing Admin Ed Yung
Eukaryotic Chromosomes
• Found in the nucleus
• Condensed and visible during cell division
• At the beginning of mitosis they can be seen to
consist of two threads (sister chromatids)
joined by a centromere
• The sister chromatids are identical copies
• During mitosis the sister chromatids separate
and are placed into two nuclei
© 2007 Paul Billiet ODWS
Image believed to be in the Public Domain
Numbers of chromosomes
• Constant for each cell in the body
(except sex cells which only have
half sets).
• Constant throughout the life of an
individual (you don’t lose or gain
chromosomes)
• Constant for all members of a
species
© 2007 Paul Billiet ODWS
Human chromosomes
Image believed to be in the Public Domain
The chromosomes of a human female
© 2007 Paul Billiet ODWS
Image believed to be in the Public Domain
Down’s syndrome
Image believed to be in the Public Domain
Mouse
Maize
Image believed to be in the Public Domain
© A. Lane Rayburn
Organism
Human
Chromosome
numbers
46
Chimpanzee
48
House Mouse
40
Maize
20
Identifying chromosomes
Chromosomes can be identified by:
• Their size
• Their shape (the position of the
centromere)
NB Chromosomes are flexible
• Banding patterns produced by
specific stains (Giemsa)
Chromosomes are analysed by
organising them into a
KARYOTYPE
© 2007 Paul Billiet ODWS
© Biologyreference.com
Essential vocabulary of
genetics:
• Gene: A length of DNA that is the unit of
heredity
• Genotype: The genetic make-up of an
organism, in terms of the allels present
• Allele: Any of two or more alternative forms
of a gene
• Phenotype (trait): Physical or other features
of an organism. Caused by combination of
genotype AND environment
Homologous chromosomes:
Alleles
Each cell has (at least) 2 genes
for any characteristic
Each cell has (at least) 2 genes
for any characteristic
Homozygous
Homologous chromosomes
have identical alleles of a
particular gene.
Two
identical
homozygous
individuals
that
breed
together will always have
the same characteristics
‘Pure breeding’
Heterozygous:
Chromosomes have two
different alleles of a
particular gene (Ff).
•‘not ‘’pure breeding’…
Dominant and recessive alleles
• A dominant allele (e.g. F) will be
expressed if it is present
• A recessive allele (e.g. f) will only be
expressed when there is no dominant
allele of the gene present
Dominant and recessive genes
A dominant gene
(allele) is one that will
be expressed physically
if it is dominant
A recessive allele
(gene) is one that will
only be expressed when
there is no dominant
allele of the gene
present (e.g. t or g)
Cystic fibrosis is disease associated
with expression of a recessive gene
Rules of ‘Mendelian’ Inheritance
1. Inherited traits are determined by genes that are
passed from parents to children (Mendel’s Theory of particulate
inheritance).
2. A child inherits two
parent.
3.
sets of genes—one from each
A trait may not be observable, but its gene can
be passed to the next generation
Mendelian Inheritance
Each person has 2 copies of every
gene—one copy from mom and a
second copy from dad. These
copies may come in different
variations, known as alleles, that
express different traits.
For example, 2 alleles in the gene for freckles
are inherited from mum and dad:
– allele from mum = has freckles (F)
– allele from dad = no freckles (f)
– child has the inherited gene pair of alleles, Ff
(F allele from mom and f allele from dad).
Genes, dominance and hybrids
Mendel bred peas with contrasting physical
characteristics and compared their offspring.
•Each original pair of chromosomes are
termed the P(parental) generation.
•The offspring are called the F1 (filial)
generation (filius/ filia – son/daughter)
•Offspring of parents with different traits are
called hybrids (F1 hybrids)
Testing inheritance: The F1
cross…
• To test for recessive genes, Mendel
allowed all 7 kinds of F1 hybrid plants to
produce a second (F2, second filial)
generation by self-pollination
• He crossed the F1 generation with itself to
produce the F2 offspring
• Roughly 25% of the F2 plants
demonstrated the recessive allele!
What type of cross did Mendel perform
when he needed to determine the
geneotype of an organism?
Definitions
Autosome: any chromosome
that is NOT a sex
chromosome
Homework for Tuesday…
Play the Dragon Game...
Research about a type of genetically
inherited disease:
Find out about a type of inherited disease
Genetic Diseases
Autosomal Dominant
Disorders
Neurofibromatosis
Huntington’s Disease
Autosomal Recessive
Disorders
Tay Sachs
Cystic
Fibrosis
Inherited diseases
•
•
•
•
Phenylketonuria
Sickle Cell disease
Huntingdon’s disease
Cystic fibrosis
Inherited diseases
Simple inheritance
• ‘Single gene
inheritance’
• ‘mono-hybrid cross’
• How Mendel worked
(1)
• How Mendel worked
(2)
Law of
Probability
What are the chances of
tossing two heads in a
row?
Five heads in a row?
How about having two
red heads in a row?
Mendel’s Work
All of these produce
approximately a 3:1
ratio.
Testing inheritance: The F1
cross…
• To test for recessive genes, Mendel
allowed all 7 kinds of F1 hybrid plants to
produce a second (F2, second filial)
generation by self-pollination
• He crossed the F1 generation with itself to
produce the F2 offspring
• Roughly 25% of the F2 plants
demonstrated the recessive allele!
One-Trait
Inheritance
Monohybrid
cross
Working out inheritance: The
Punnett Square…
Let’s meet Reginald Punnett…
The rules of genetic inheritance...
Probability - likeliness that something
will or will not happen
Hey, Keith, how's the
weather?
Weather forecast
(90% chance of rain)
Winning lottery
(1 in 2 million chance
)
Well, there is
a 50% chance
of a snow.
Probability is used in genetics to predict what
offspring will look like when 2 parents are
crossed
.
X
You can figure
out the probability
of getting a liger or
a tigon.
A tool that can help you understand the
laws of probability and
how it is related to genetics is called a
Punnett square, which
is a chart that shows all the possible
combinations of alleles that
can result from a genetic cross.
Steps in Making a Punnett Square:
1. Draw a square and divide it into four.
2. The alleles for one parent go on the top. The
alleles for the other parent go down the side.
3. The boxes are like a multiplication problem, with
one allele contributed by each parent.
I can only give 1/2 of my
DNA to my offspring, so I
will either give my baby plant
a T OR a t.
Tt
One Trait Inheritance
• T = tall, t = short
• Complete the punnet
square in your notes.
• Label the P gametes
• Label the F1
generation
• What is the
phenotypic ratio?
• What is the
geneotypic ratio?
One Trait Inheritance
P gametes
F1 generation
Phenotypic Ratio = 3:1
Genotypic Ratio = 1:2:1
TT
Tt
Tt
tt
Punnett squares are written like multiplication tables.
X
1
2
3
4
1
2
3
4
Cystic fibrosis Punnet Squares
(1)
Cystic fibrosis Punnet Squares
(2)
Practice Questions on simple
inheritance
1.
2.
Jack and Jill are having a baby. What is the probability that their
baby will be a BOY (XY)? What is the probability that their baby will be a
GIRL (XX)?
Jill - XX
Jack - XY
probability of having
a baby girl (XX):
__________________
probability of having
a baby boy (XY):
___________________
3. Use the Cats below to create a Punnett Square.
Black = Dominant
Blue/Grey = Recessive
d
D
Dd
D
Dd
d
Dd
Dd
Parent One Genotype ________________
Parent Two Genotype ________________
What % is Hybrid? __________________
What % Homozygous? _______________
What % are Black? ___________________
What % are Blue/Grey? _______________
4.
5.
Going beyond Mendel
1. Test crosses
2. Multiple alleles – dihybrid and
trihybrid crosses
3. Codominance
4. Incomplete dominance
5. Sex linkage
Beyond Mendel
• Incomplete dominance
• Codominance
• Polygenic traits
• Multiple allelic traits
• Genetic disorders
• Testing for genetic disorders
Working out inheritance
What type of cross did Mendel perform when he
needed to determine the genotype of an
organism?
Further genetics
What happens if a number (say 2 or 3) traits
are mixed?
Di-hybrid and tri-hybrid crosses
Mendel’s Law of independent assortment
Multiplication rule for inheritance
Inherited traits
• This trait is due to a
single gene.
• Unattached earlobes
are dominant
• Utah inherited traits
Inherited traits: Dimples
• Single gene
• Dimples dominant
• No dimples recessive
Inherited traits: Freckles
• Freckles dominant
• No freckles recessive
What is the pattern of
inheritance of multiple traits?
Dihybrid Cross (Two-factor
cross)
• These involve TWO genes which control
two different charaxteristics
• Unlinked genes are found on different
chromosomes
• (autosomal) are those chromosomes
which are not gender chromosomes
• Dihybrind cross
Mendel’s Law of Assortment
Each pair of
alleles
segregates
independently.
2. A Dihybrid Cross
Mendel’s Law of independent
assortment
• The principal of independent assortment
states that genes for different traits can
segregate independently during the
formation of gametes (sperm and eggs).
• Independent assortment helps to account
for the many genetic variations observed in
plants, animals and other organisms
What about testing multiple
genotypes?
• Multiplication Rule: multiply the probability
of one event with the probability of another
event.
Ex: coin toss heads = ½
coin toss tails = ½
½x½=¼
Multiplication Rule
• What is the probability of a cross between a YyRr
and a YyRr producing a YYRR?
Y
y
• The probability of YY = ¼
Y
YY Yy
y Yy yy
• The probability of RR = ¼
Therefore ¼ x ¼ =
1/16
R
R
r
RR Rr
r Rr rr
Using the multiplication rule for
a trihybrid cross
You have freckles, dimples, and a
unattached earlobes. Your S.O. has freckles
and dimples, and attached earlobes. In other
words,
FfDdUu x FfDduu
Question: What is the chance your darling
child would have all three recessive
phenotypes: no freckles (ff), no dimples (dd)
or attached earlobes?
Beyond Mendel
• Incomplete dominance
• Codominance
• Multiple alleles
Beyond Mendel: Incomplete
Dominance
• Sometimes, 3
different phenotypes
can exist!
• In incomplete
dominance, neither of
2 alleles is dominant
(or recessive)
• They DO ‘blend’ to
produce a new
phenotype
Example: Incomplete dominance
Genotype Ratio:
• 25% R1R1
• 25% R2R2
• 50% R1R2
Phenotype Ratio
• 25% Red
• 25% White
• 50% Pink
Codominance
• Both alleles are
equally dominant!
• Both alleles are
expressed fully,
resulting in a new
phenotype!
• e.g. Roan
horses/cattle – red
and white hairs are
intermingled
Multiple
alleles
Multiple alleles
• When Mendel studied pea ‘traits’, there were
only 2 alleles for each of the traits he studied:
(round/wrinkled, tall/short, yellow/green)
• In fact, you CAN have > 2 alleles for any trait
• Human blood types are an example of a trait
with multiple alleles (A, B, O)
Human Blood types
• Type O is recessive
• Types A and B are
codominant
• Both Types A and B
are dominant to Type
O
What does ‘blood type’ mean?
We have both antigens (markers)
and antibodies in our blood
• 45% are Type O
• 42% Type A
• 10% are type B
• 3% are type AB
What happens if you receive the wrong
type of blood in a transfusion?
Blood type inheritance (Practice)
Blood type inheritance (Practice)
• A woman who is Blood type A
(homozygous) marries a man who is
blood type O. What are the potential blod
types of their children?
• Genotype?
• Phenotype?
High blood pressure
is a polygenic trait.
• What genes could
combine to contribute
to high blood
pressure?
High blood pressure
is a polygenic trait.
• The phenotype is an interaction between a
person's weight (one or more obesity
genes), cholesterol level (one or more
genes controlling metabolism), kidney
function (salt transporter genes), smoking
(a tendency to addiction), and probably
lots of others too. Each of the contributing
genes can also have multiple alleles.
Genetic Testing and Counseling
• Diagnosis of carriers
– Biochemical (i.e. Tay-Sachs)
– Genetic (i.e. Huntington’s disease)
– Ethical considerations
• Fetal diagnosis– Amniocentesis
– Chorionic villi sampling
Chorionic villi sampling
Chorionic villus
sampling (CVS) is
the removal of a
small piece of tissue
(chorionic villi)
from the uterus
during early
pregnancy to
screen the baby for
genetic defects.
A son with cystic fibrosis
(autosomal recessive) is born to
a couple who appear to be
normal. What are the chances
that any child born to this
couple will have cystic fibrosis?
And the Answer is…..
25%
In humans the allele for short fingers
is dominant over that for long
fingers. If a person with short fingers
who had one parent with long
fingers reproduces with a person
having long fingers, what are the
chances of each child having short
fingers?
And the Answer is…..
50%
Homework: Complete the remain questions
on the genetics problem set and do your
genetics mini project.
Combining multiplication and addition
rules to solve complex problems
• An organism with the genotype BbDD is
mated with one with the genotype BBDd.
Assuming independent assortment of
these two genes, what is the probability
that you will get a BBDD offspring?
• ¼ BBDD
More
Incomplete
Dominance
Sickle Cell
Pedigree Tables (1)
• A ‘pedigree chart’ is a
diagram of family
relaitonships that can
be used to determine
the mode of
inheritance of a trait
• Introduction to
pedigree tables
Pedigree Tables (2)
•
•
•
•
•
Male = square
Female = circle
Affected = dark
Non-affected = light
Carrier = half dark/
half light
Dd
Dd
D
D
A summary of Mendel’s
Principles (1)
1. The inheritance of biological characteristics
is determined by individual units – genes
(alleles). Genes are passed from parents to
their offspring
2. In cases where there are 2 or more forms of
the gene for a single trait, some forms of the
gene may be dominant and others may be
recessive
A summary of Mendel’s Laws of
inheritance (2)
3. In most sexually reproducing organisms,
each adult has 2 copies of each gene –
one from each parent. These genes are
segregated from each other during
meiosis
4. The alleles for different genes usually
segregate independently
Taking Mendelian Genetics one
stage further
• The two factor cross
• Mendel’s Law of Independent Assortment
• Multiplication rule
• Addition rule
• Pedigree charts