Download Mendelelian Genetics - Kaikoura High School

Mendelelian
Genetics
1
•
Generation “Gap”
P1, patrial generation = the parents or first two
organisims crossed.
F1, first filial generation – the first set of offspring
F2, second filial generation = the result of two of
the F1 generation being crossed.
Developed the terms – gene,allele,
homozygous,heterozygous, dominent, recessive,
genotype and phenotype.
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Mendel’s Laws
1. Law of Dominance
2. Law of Segregation
3. Law of Independent assortment
3
.
•
Law of Segregation
Each paired gene must seperate during gamete formation so alleles
cqn recombine in new pairs.
Law of Independent
Assortment
•
Traits are inherited independantly and are not changed by other
alleles for other traits.
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Results of Monohybrid Crosses
Inheritable factors or genes are
responsible for all heritable
characteristics
Phenotype is based on Genotype
Each trait is based on two genes,
one from the mother and the
other from the father
True-breeding individuals are
homozygous ( both alleles) are the
same
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•
Back or test cross to test for homo or
heterozygous.
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Incomplete Dominance
and
Codominance
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Incomplete Dominance
F1 hybrids have an appearance somewhat
in between the phenotypes of the two parental
varieties. Neither gene is dominent – both are
expressed equally.
RR = red flower
rr = white flower
r
But Rr makes pink flowers!
R
R
r
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Incomplete dominance problems
In Andalusian chickens, the black
Andalusian character is incompletely
dominant to the white-splashed
Andalusian character. The heterozygous
condition produces blue Andalusian
chickens. Determine the genotypes and
phenotypes of the F1 and F2 generations
if a pure breeding, black Andalusian is
crossed with a pure breeding, whitesplashed Andalusian.
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Codominance
Both alleles are equally
expressed. The offspring
have a mix of alleles which
are equally expresses.
Eg. Red cows crossed with
white will generate Roan
cows.
But F2 generation
demonstrate Mendalian
genetics (1:2:1)
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Codominance vs Incomplete
In incomplete dominance only one allele is
active but is reduced in effect.
In codominence both alleles are active. In
human blood this produces a whole new
blood group - AB.
1. type A= IAIA or IAi
2. type B = IBIB or IBi
3. type AB = IAIB
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How we write it - CrCw
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•
OVERDOMINANCE
The phenotype falls outside the range of the
parents e.g if one homozygote is tall (TT) and
the other is short (tt) then the heterozygote
may be extra tall (Tt). If this is a good
adpation then it can become common in the
population.
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LETHAL GENE
A mutation of a gene that produces a
product that is nonfunctional. In some
the homozygous dominant is lethal – it
dies as an embryo so get a ratio of 2:1
instead of 1:2:1. In some it just
affects expression of genes (Manx cats)
and it can also be expressed at
different stages of development e.g.
Huntingtons.
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Manx Cat
Manx Cat – Tailless cat is
another trait caused by
an allele that has
dominant effect in
heterozygous and is
lethal in homozygotes.
The Manx and normal
alleles are denoted by L
and l respectively
MLMl or MLML
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Other
example
is
achondroplasia, the most
common form of dwarfism,
with a normal length body
trunk but shortened limbs.
AA – die, aa – normal, Aa
- dwarf.
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Multiple Alleles
•
•
More than one gene at a locus coding for a trait.
Human blood type is a great example – there are
three alleles for blood type, A,B and O. Allele A
and B are co-dominant and both are dominant
over O.
Reminder – the truth about eye colour.
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Try These
1. If a male has blood type B and a
female has blood type A, what are the
possible blood types in the offspring?
2. Is it possible for a child with Type O
blood to be born to a mother who is
type AB? Why or why not?
3. A child is type AB. His biological
mother is also type AB. What are the
possible phenotypes of his biological
father?
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Gene Interaction and Epistasis
The phenomenon of two or more genes
governing the development of single
character is known as Gene interaction.
The genes can be
on
different
chromosomes.
When one gene masks or alters the
expression
of
another
gene,
the
phenomenon is called Epistasis. (eg and
meaning)
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3.Supplementary Gene Action
This is the masking of a characteristic
(which is determined by one pair of
alleles) by the action of another pair of
alleles.
An example is mouse fur – one allele gives
black (B) – lots of melanin, when
recessive (b) it gives brown – less
melanin. Another gene allows how much
melanin is deposited so C allows colour to
show but c does not.
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Collaboration
•
••
••
••
This is where one characteristic is controlled by two or more pairs of
alleles. The two genes interact to produce a novel phenotype. One well
known example is comb shape in chickens. The four shapes are
single,pea,walnut,rose.
The ratio is the same as a normal dihybrid cross 9:3:3:1
P_R_ - two dominants gives walnut
P_rr – one dominant + one recessive gives pea
ppR_ - other dominant and other recessive gives rose
pprr – two recessives gives single.
The big difference to a normal dihybrid cross is the fact that the
phenotypes are four differnt, not variations of the same two traits.
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The ratio is 9:4:3.
B_C_ - black
B_cc – white
bbC_ - brown
bbcc - white
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Types of Epistasis
1. Collaboration
2. Complementary
3. Supplementary
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2. Complementary gene action
In this case two dominant allelees from two
different genes are needed to get expression
of the gene. The genes are complementary to
each other. The ratio is 9:7.
As an example: sweet peas the development of
purple flowers requires the presence of 2
dominant genes, C and R, e.g., CCRR.
When either C or R alone present purple
flowers cannot be produced; as a result white
flowers are obtained
e.g., ccRR or CCrr or ccrr
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Rose
Single
Pea
Walnut
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Linkage and sex determination
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What is so different between the X and Y
chromosomes?
X- over 1000 genes identified
Y- 330 genes identified, many are inactive
One gene on the Y is very important: SRY. The SRY
gene is the primary determinant of sex.
If SRY is present, testes develop in the early embryo.
The testes secrete the hormone testosterone, which
causes development as a male.
If SRY is absent (no Y chromosome), ovaries develop
instead of testes, and the embryo develops into a
female.
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The sex chromosomes
•
Many organisims like humans with a
homogamteic sex (female XX) and a
heterogametic sex (males XY).
•
XY- female, XX- Male This system
of sex determination operates in
birds, reptiles, some insectsThe
bird sex chromosomes are called Z
and W.
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In some insects there is no Y chromosome so
you get XO and XX.
Diploid (2n) female, Haploid (n) maleThis
system of sex determination is found
mainly in Hymenoptera – honey bees,
ants, termites, etc.
The unfertilized haploids develop male and
called drones.
These drones carry only half the number
(16) of chromosomes of the female (32)
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In reptiles and some fish sex is
determined by the environment (
temperature, abilty to change sex
etc).
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Family trees
•
3 types – sex-linked
–
autosomal recessive and autosomal dominant
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Sex Linked Inheritance
Because the X chromosome is larger than the Y
there are parts of the X chromosome that have
no matching part on the Y. Any gene carried on
the non-homologous part are called sex linked.
Examples are red-green colour blindendss,
haemophilia, all tortiseshell cats are female.
For males, any faulty gene on the X will show up as
there is no gene on the Y to mask the effect. In
females both parents must have the recessive
trait to pass it on.
We write the alleles above the X and Y symbol.
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Sex-linked notation
XBXB normal female
XBXb carrier female
XbXb affected female XBY normal male
XbY affected male
SEX LINKED DOMINANCE
Dominant gene on X chromosome
Affected males pass to all daughters and
none of their sons
Genotype= XAY
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If the mother has an X- linked dominant
trait and is homozygous (XAXA) all
children will be affected
If Mother heterozygous (XAXa) 50% chance
of each child being affected
E.g. dwarfism, rickets, brown teeth enamel
SEX LINKED RECESSIVE
Gene located on the X chromosome
More males than females affected (males
inherit X from mother)
Females can only inherit if the father is
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Sex linked dominant disorders
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affected and mother is a carrier (hetero)
or affected (homo)
An affected female will pass the trait to
all her sons
Daughters will be carriers if father is not
affected
Males cannot be carriers (only have 1 X so
either affected or not)
E.g. colour blindness, haemophilia, Duchene
muscular dystrophy
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2. Duchene type Muscular Dystrophy:
Is also depends on sex linked recessive gene.
If mother is carrier, about half of the male
children are expected to be affected.
Can be identified by chromosome study.
It affects male before they reach teens, with
muscular deterioration .
Muscles of leg and shoulders become stiff and the
children usually become paralyzed and crippled
during their middle or late teens.
Virtually all die before age of 21.
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Sex-influenced Genes
• Some traits even though not on sex
chromosomesappear differnntly in men
and women e.g. pattern baldness. It is an
inherited trait and controlled by one gene.
In females it acts as a recessive so a
woman needs two recessives to show
baldness. In men only one is needed – BB
– full hair both sexes, Bb – bald in men not
in women, bb – bald in both.
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The Adams family and Baldness
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Colour blindness
We
have
3
color
receptors
in
the
retinas of our eyes.
They respond best to
red, green, and blue
light.
Each
receptor
is
controlled by a gene.
The blue receptor is on
an autosome, while the
red
and
green
receptors are on the X
chromosome
(sexlinked).
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Features of Colour blindness
Colour blindness is a sex linked character discovered by
Wilson
It is a hereditary disease and the affected person cannot
distinguish green and red colour.
The red blindness is called Protonopia, these persons cannot
see red colour
While, green blindness is called deuteronopia, such a persons
cannot see green colour.
Colour blindness is a recessive character, represented by cc
The genes are for colour blindness is located on X
chromosome. It is common in male but rare female.
Colour blindness follows criss- cross inheritance as
transmitted from father to grandson through daughter.
It is never transmitted from father to son
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Linkage
It is universally accepted that genes are
located in chromosomes
Linkage: The tendency of two or more
genes
to
stay
together
during
inheritance because they are located on
same chromosome.
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Recombination During Meiosis
Recombinant gametes
Parental gametes
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Complete and Incomplete linkage
Gene show a linkage because they are located on a
chromosome.
For e.g., C and Sh are present in one chromosome, while
their recessive alleles c and sh are situated in the
homologous chromosome.
C
sh
c
Sh
C
sh
c
Sh
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Each chromosome behaves as a unit during cell
division.
Therefore, C and Sh would move to one pole to
while c and sh move to opposite pool.
If this always happened, F1 generation (CcShsh)
would produce 2 gametes viz., C Sh and c sh
When only parental character combinations are
recovered in test progeny , it is called
complete linkage.
However, sometime, allele recombine to produce
recombinant types like C sh, cSh
Such a type are called Incomplete linkage.
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Linkage is never 100%. No matter how
tightly two genes are linked, if you
observe enough individuals, you will find
some recombinants.
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Linkage Mapping
Each gene is found at a fixed position on a
particular chromosome. Making a map of their
locations allows us to identify and study them
better.
In modern times, we can use the locations to
clone the genes so we can better understand
what they do and why they cause genetic
diseases when mutated.
Thus, the percentage of gametes that had a
crossover between two genes is a measure of
how far apart those two genes are.
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Pleiotrophy – one gene with many
different effects. Often seen in
genetic diseases such as sickle cell
aneamia.
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Sickle Cell Anemia
Under conditions of low oxygen
tension, hemoglobin S will
precipitate, causing cells to
sickle
Mutations in one amino acid
HH – normal and gets maleria
Hh – protected from maleria but
gets sickle cells in low O2
Hh – dies young from anaemia.
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Sickle cell anemia may be the result of a
genetic mutation that happened in malariaprone regions like Africa thousands of
years ago.
People with sickle cell trait may have been
more likely to survive malaria epidemics and because they survived when others did
not, this allowed the trait to be passed
down through generations.
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Polygenes – two or more genes have similar
and additive effects on the same
characteristic. E.g. intelligence, height,
skin colour. The phenotypes for these
genes form a bell shaped curve and show
continuous variation.
e.gs
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Genes and Environment
Determine Characteristics
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Gene/Environment Interactions
Genes determine range of genotypes but
environmental factors fine tune which
phenotype is displayed.
Internal and External Environment affect
Internal:
modifier genes – sometimes the expression
of the gene at one locus is affected by
alleles at another locus.
Sex-limited genes
Sexlinked genes – sex hormaones affect.
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External environment
Drugs
Startvation or malnutrition
Lack of light
Temperature
Ionising radiation
Poluuting chemicals
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