Transcript Genetics

Genetics
Mendel’s Laws
Mendel
•
•
•
•
Gregor Mendel was a monk.
He grew peas in the monastery garden.
He studied how they grew and reproduced
He counted and calculated the outcome of
many generations of pea plants
• He published his conclusions about
genetics, but they were not read by other
scientists for many years
Mendel’s Laws and Principles
• Genetic factors (genes) occur in pairs. One
factor of each pair comes from the male parent
and one from the female parent.
• Principle of Dominance and Recessiveness:
The genetic factor (gene) from one parent may
mask (or dominate) the genetic factor from the
other parent.
• Law of Segregation: pairs of genetic factors
(genes) are separated during the formation of
gametes (egg or sperm)
• Law of Independent Assortment: genetic
factors (genes) are distributed to gametes
independently.*
* We now know that this is only strictly true when the factors are located
on different chromosomes. Genes that share a chromosome may “travel
together” to some extent. The “crossing-over” that occurs during the first
division of meiosis does give some validity to the law of independent
assortment.
What is so amazing
about Mendel is that he figured
all this out without ever seeing
a chromosome!
Quick Review of Mendel
• Your traits are controlled by genetic factors
(genes for short)
• You have two of each gene (you got one of
them from each parent).
• One gene sometimes overpowers your
other one (dominance vs. recessiveness)
• Your genes separate (segregate) when
you produce gametes (eggs or sperm)
• Your children only get half of your genes.
• Its pretty much random which of your
genes they get (independent assortment)
Terminology: Allele
• An allele is one of several forms of a gene.
• Explanation: A gene can have several forms.
– One form of a hair-colour gene might cause the
organism to produce a darker pigment than the other.
– One form of another gene might produce more growth
hormone than its alternate.
The different types of allele
can be represented by
letters (uppercase, lowercase
or superscripted)
There can be
several
types
Everybody
has two alleles
Of each gene.
They can be the
same or different
An Allele
is a Gene
Examples of alleles
• examples:
– Pea plants have two alleles for the height
gene:
• Tall height
• Dwarf height
T
t
(the dominant allele)
(the recessive allele)
– Peas can get any combination of the 2 alleles
– (TT)=tall plant, (Tt)=tall plant, (tt)=short plant
– Humans have 3 alleles for a blood type gene:
• A type blood
• B type blood
• O type blood
IA
IB
i
(dominant to O type)
(dominant to O type)
(recessive to A and B)
– You can get any combination of 2 of these 3 alleles
– (IA IA)or(IA i) = type A;
(IBIB)or(IBi)=type B
– (IA IB) = type AB;
(i i) = type O
Genotype & Phenotype
• Phenotype: The traits that an organism actually
displays (eg dark hair)
• Genotype: the underlying genes that control the
trait, (usually represented by UPPER or lower case letters.)
– Eg:
DD- homozygous dark
hair
Same Phenotype
Dd- heterozygous dark
hair
Different Genotype
dd- homozygous blond
hair
– Homozygous: having two identical alleles of a gene.
– Heterozygous: having two different alleles of a gene
(alleles are alternate forms of the gene)
Generation Names
• The Parental Generation (P)
– This is the two original individuals that were
crossed
• The First Filial Generation (F1)
– These are the offspring (the children) of the
parental generation
• The Second Filial Generation (F2)
– These are the offspring of two individuals from
the F1 generation. Essentially, they are the
grandchildren of the parental generation
Punnett Squares
• Developed by Reginald Punnett to help
illustrate Mendel’s laws.
• A Punnett square shows the various
possible outcomes of a genetic cross
Writing Genotype
• The genotype of an organism is its
arrangement of its two alleles of a gene.
• If it has two alleles for tallness, its
genotype will be TT, If it has one allele for
tallness, and one for shortness, it will be
Tt, if it has both alleles for shortness, it will
be tt.
To make a Punnett Square
• Determine the genotype of the parental
generation, eg Rr x Rr, or BB x bb, or YY x Yy
• Draw a square, divide it into four smaller R r
squares.
R
r
• Above the square write the two letters
corresponding the paternal genotype
• To the left, write the two letters corresponding to
the maternal genotype
• Go through the four boxes, picking the
genotype letter from directly above, and
the one from directly left, and copying
them in each box
R r
R
r
RR
Rr
Rr
rr
• If a genotype has an upper and lowercase
of the same letter, always write the
uppercase one first
rR  is equivalent to  Rr
Preferred way of
writing it!
Parental P1
The Punnett Square
Monohybrid cross
• BB x bb gives the
gametes:
– B-B and b-b
– All offspring are Bb
First Filial F1
• Bb X Bb gives the
following gametes
– B-b and B-b
• The next generation:
2nd Filial F2
– Bb, BB, and bb
Corrections for Practice Sheet
Parental Generation: YY x yy
1st filial
YY x yy
Y
2nd filial
Yy x Yy
Y
Y
y
y
Yy
Yy
Y
YY
Yy
y
Yy
Yy
y
Yy
yy
Predictions:
Phenotype Yellow:
Phenotype green:
Genotype Yy
Genotype YY
Genotype yy
100%
0%
Predictions:
Phenotype Yellow:
Phenotype green:
75%
25%
100%
0%
0%
Genotype Yy
Genotype YY
Genotype yy
50%
25%
25%
Practice sheet, Question 2
1st filial
tt x Tt
t
2nd filial
Tt xTt
t
T
t
T
Tt
Tt
T
TT
Tt
t
tt
tt
t
Tt
tt
Predictions:
Phenotype Tall:
Phenotype short:
Genotype Tt
Genotype tt
Genotype TT
50%
50%
Predictions:
Phenotype Tall:
Phenotype short:
75%
25%
50%
50%
0%
Genotype Tt
Genotype tt
Genotype TT
50%
25%
25%
Punnett Square
Dihybrid Cross
Twice as wide by twice as high
• A dihybrid cross involves
That means for each new trait the
two traits at the same time
Square is four times as complicated
• The Punnett Square gets a
bit more complicated
the parental generation
• RrYy X RrYy
– Gives the following gametes:
• RY-Ry-rY-ry (paternal)
• RY-Ry-rY-ry (maternal)
• How? It’s a bit like FOIL in
math. (shown for paternal gametes)
First
Last
RrYy
Inner
outer
Since the
RY parent
Maternal
Ry
Has the
same
rY
Genotype, we’ll
Show it quicklyry
Ry
RY
rY
ry
Now just take
an R
Genotype
Ratios
grom=the
top 6.25%
and
RRYY
1/16=
an R =2/16
from the
side
RRYy
= 12.5%
and a=2/16=
Y from the
top
RrYY
12.5%
and a Y from the
side
RRyy=1/16=
6.25%
to fill= up
each
little
RrYy
4/16
= 25%
box.= 12.5%
Rryy = 2/16
rrYY = 1/16 = 6.25%
The =
results
show
rrYy
2/12 =will12.5%
rryyall
= the
1/12possible
= 6.25%
genotypes, and their
probabilities
Phenotype Ratios
56.25%
18.75%
18.75%
6.25%
Incomplete Dominance & Co-dominance
(2 types of trait blending)
• Sometimes an allele does not completely
overpower the other allele.
– If the traits blend, to produce an “in-between
trait” we call it incomplete dominance:
• Eg. White flowers x red flowers  pink flowers
– If both traits show up, we call it codominance
• Eg. White hair horse x red hair horse  roan horse
black dog x white dog spotted dog
A type blood x B type blood  AB type blood
Sex Determination in Mammals
I’m
different!
(except for the platypus)
• In all mammals* and many vertebrates,
the sex of an organism is determined by
the sex chromosomes.
• Identical sex chromosomes = (XX) female
• Different sex chromosomes = (XY) male
The y chromosome is “stunted” or
smaller than the corresponding X
chromosome, so there are a few
traits carried on the X chromosome
that males have only one allele for.
*the platypus has a slightly different chromosome system
Sex Determination in other
organisms
• Many other organisms use X and Y chromosomes
to determine sex, but not all do.
– Some amphibians (frogs, salamanders) can change sex
based on environmental condition, regardless of their
chromosomes.
– Birds have Z and W chromosomes instead of X and Y
• Females have ZW, males have ZZ, a sort of reversal of the
mammal system
– Some insects have a haploid male, diploid female sex
determination, which has many strange outcomes:
• Males have no fathers, and can have no sons, but they can have
grandfathers and can become grandfathers.
• There are more than two sex possibilities (drone, queen, worker)
– For some species (like the zebrafish) we still don’t know
how sex is determined.
Human Genetic Abnormalities
(diseases and conditions due to genes)
• Diseases caused by a single gene
– Huntington’s Disease is a neurological condition
caused by a rare dominant gene with 96% chance of
expression. Since symptoms only develop in adults,
it is easily passed on, even though it is frequently
fatal.
– Cystic Fibrosis is a lung disease caused by a
recessive gene
– Sickle cell anaemia is a blood condition caused by a
recessive gene. It is more frequent in people whose
ancestors came from tropical regions. This may be
because carriers (people with one normal and one
sickle cell allele) are more resistant to malaria.
• More diseases caused by a single gene
– Tay-Sach’s Disease is a degeneration of the nervous
system that usually causes death in early childhood
(c. Age 4). Tay-Sach’s disease is most common in
people whose ancestors came from Eastern Europe,
Eastern Quebec, and Louisiana.
– Phenylketonuria is the inability of the body to
process the amino acid phenylalanine. It causes
gradual brain damage, and if untreated, can lead to
seizures and death by the end of childhood.
Phenylketonuretics who receive treatment to survive
childhood, must restrict their intake of proteins
containing phenylalanine, and avoid the sweetener
aspartame (AKA. NutraSweet) which also contains
phenylalanine.
• X-linked (AKA. Sex-linked) Conditions.
• These are usually prevented by genes
carried on the X chromosome, so they are
more frequent in males than females.
– Haemophilia is a condition that prevents the
blood from clotting normally. A victim can die
of minor cuts unless treated quickly.
– Red/Green Colourblindness is the most
common form of colour deficiency in human
males.
– Muscular Dystrophy is a disease of the
muscles fibres that causes weakness and
reduced life expectancy
Colourblindness test
25
29
(everyone)
45
56
6
8
How X-Linked Traits Work
• Most X-linked traits are caused by a recessive allele located
on the X-Chromosome. Because Y chromosomes do not
have all the genes found on an X chromosome, men are
less likely to have the dominant normal gene that would
prevent the condition. Therefore, X-linked traits are several
times more likely to be expressed in a male than in a female
• Two well known sex-linked traits are R/G colourblindness
and hemophilia
– XC = gene for normal vision 81% XCXC= normal female
– Xc = gene for colourblindness 18% XCXc= female carrier (normal)
cXc= colourblind female
– Y = no gene
X
1%
CY= normal male
X
90%
Let’s say 90% of all genes are normal and
c
10% X Y =colourblind male
10% are not. That means a female has
a 99% chance of normal vision, but a male
has only a 90% chance of normal vision, so
the male is about 10 times more likely to be
Colourblind.
Missing Chromosome (monosomy) and
Extra Chromosome (trisomy) Diseases
• Down Syndrome (trisomy-21) results from
having an extra copy of chromosome 21.
• Symptoms include impaired mental development,
characteristic facial features, and occasionally
muscle weakness.
• Klinefelter’s Syndrome (trisomy-XXY)
• Affects a small percentage of men. Symptoms
include small testicles, reduced male hormone
production and higher incidence of male cancers
and some other diseases, such as diabetes and
rheumatoid arthritis.
• Turner’s Syndrome (monosomy-XO)
• About 1 in 5000 females has a missing sex
chromosome. This can result in several
characteristics, including:
• Short stature, low-set ears, low hairline, webbed neck,
amenorrhea, and sterility.
• Trisomy-X (trisomy-XXX)
• An extra X chromosome produces a female that is
normal to most external appearances, apart from a
tendency to be taller than average. There is some
risk of increased minor disorders, including:
– behavioural issues, clumsiness, poor coordination,
underdeveloped facial muscles, wide-set eyes, etc.
Other chromosome related conditions
• Cri-du-chat syndrome
– Characterized by delayed development, small head size, and
characteristic facial features. Infants often have high pitched cry.
– Caused by a loss of part or all of chromosome #5
• Angelman Syndrome
– Characterized by intellectual difficulties, seizures, and small
head size.
– Caused by malfunction of a gene on chromosome #15 or by a
missing maternal chromosme#15
– This is one of the few cases where it seems to matter who you
inherited the chromosome from. The maternal chromosome
always seems to dominate.
– Children with Angelman syndrome typically have a happy, excitable demeanor
with frequent smiling, laughter, and hand-flapping movements. Hyperactivity and
a short attention span are common. Most affected children also have difficulty
sleeping and need less sleep than usual. Some affected individuals have
unusually fair skin and light-colored hair
• XYY Syndrome (trisomy-XYY, Jacob’s Syndrome)
• An extra Y chromosome produces a male that is in
most respects normal. He may have slight
symptoms of excess male hormones, such as
being slightly taller than average, and having a
tendency towards acne.
• At one time it was believed that men with this condition had a
higher tendency towards violent crime, but this hypothesis
has not been born out by further studies. It may affect as
many as 0.1% of the male population.
In Summary
• Phenotype is the actual appearance of a certain
trait (eg: dark hair, blond hair)
• Genotype is the underlying pair of genes
controlling the trait (eg: BB, Bb or bb)
• Alleles are alternate forms of a gene, that occur
in “pairs of genes”
A pair of genes
• Homozygous for a trait means having the same
alleles (eg: BB or bb) of the gene of the trait.
• Heterozygous means having different alleles
(eg: Bb)
• All genes (except a few on the Y chromosome)
occur in pairs - two alleles for each trait.
• One allele may overpower another (ie. It is
dominant)
• If you are heterozygous for a trait, the
dominant allele wins!
Assignments
• Read p. 165-178
• Read the vocabulary lists on page 179.
look up any terms that you don’t
recognize.
• Do the exercises on page 180 to 181
questions # 6-25 (except #18 in the green
book). Put the answers on paper, to be
handed in next class. Write the question
as well as the answer.