Transcript Mendelian Genetics

Mendelian Genetics
Read through core
knowledge.
What vocab do you
need to learn?
Terms
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Gene
Allele
Trait
Dominant
Recessive
Homozygous
Heterozygous
Genotype
Phenotype
P and F1 and F2
Definitions
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Unit of hereditary
One of 2 or more forms of a gene at a given locus
Genetically inherited characteristic of organism, varies amongst
individuals
Allele that is expressed in heterozygotes
Allele that is only expressed in homozygotes
Carries two copies of the allele
Carries different allelic forms of a given gene
Organism’s hereditary make-up
Physical characteristics of an organism
Patrial generation, first and second filial generation
Mendel – why so famous?
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Worked with peas
Used pure-breeding varieties
Came up with idea of ‘gene’ 20 years before
chromosomes were discovered
Law of segregation
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Of the two genes controlling each
characteristic, only one is present in each
gamete.
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During meiosis the two genes are separated.
Independent assortment
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The segregation of one pair of alleles does
not affect the segregation of another pair.
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There is a random arrangement of parental
chromosomes at metaphase of meiosis.
Monohybrid cross
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Carry out a cross for a dominant and a recessive
homozygote where P is for pink flower and p is
for white
Cross the F1 and give the ratios of the F2.
P
p
p
P
Test Cross
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A cross carried out to determine an
organism’s genotype, by mating it with a
homozygous recessive organism.
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Show how a test cross works
Dihybrid Cross
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Carry out a cross for a dominant and a
recessive homozygote where Y is for yellow
and y for green, and R for round and r for
wrinkled.
Cross the F1 and give the ratios of the F2.
Dominance
Incomplete dominance
Co-dominance
Lethal Alleles
Incomplete dominance
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Action of one allele does not completely
mask the action of the other.
Neither allele has dominant control over the
trait.
Heterozygous offspring is intermediate in
phenotype
Eg snapdragons
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P1:
F1:
F2:
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1red:2pink:1white
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RR (Red) x rr (white)
Rr (pink)
? (You determine the ratios)
R
R
r
r
Co-dominance
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Both alleles in heterozygous organism
contribute to the phenotype.
Both alleles are independently and equally
expressed.
Eg Human Blood Group AB
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P1:
F1:
AA (type A) x BB (type B)
AB (type AB)
Eg Coat colour in horses
and cattle
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P1:
F1:
F2:
CRCR(red) x CrCr(white)
CRCr(roan)
? You determine the ratios
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1 Red: 2 Roan: 1 white
CR
Cr
CR
Cr
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Roan is a blend of both white hairs and red
hairs
Lethal Alleles
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Mutations of a gene that produce a nonfunctional gene product and affect the
organisms survival.
If dominant, may kill in single dose
If recessive, kills when homozygote
Eg Manx cat
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MM (normal tail)
MML (manx - no tail)
MLML (lethal – deformity of spine in embryo)
Carry out a cross for two heterozygotes.
What is the phenotypic ratio?
Eg Yellow mice
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YY (lethal – yellow – terminates at blastocyst stage)
Yy (yellow)
yy (not yellow)
Again, what is the phenotypic ratio for a
cross of heterozygotes?
Eg Huntington’s disease
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A dominant allele is lethal.
Nerve cell death in brain causing jerky
involuntary movements and dementia.
Why does it persist in the human population?
Shows in adults 30-40 years
Multiple alleles
More than one allele
possible at a gene locus
Blood groups
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There are three
different alleles:
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A, B and O
The alleles code for
making the enzyme that
hold the sugars
together that make the
different antigens on
the RBC.
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O is nonfunctioning
(recessive)
A is A antigen
(dominant)
B is B antigen
(dominant)
A and B antigens can act
with other antibodies so
must be matched for
transfusion.
Frequency in NZ
Phenotype
Genotype
Frequency in NZ
%
O
OO
49
A
AA, AO
40
B
BB, BO
9
AB
AB
2
Determine the blood types
Cros Parental
s
genotype
Ratio F1
genotype
Ratio F1
Phenotype
1
ABxAB
1AA:2AB:1BB
1A:2AB:1B
2
OOxOO
3
ABxAO
4
AAxBO
5
AOxOO
6
BOxOO
Dilemma
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If a mother is type A and has a baby type B,
can the father be type O?
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Explain your answer.
You can now do the
self check for this section.
Gene-gene interactions
When a characteristic is
influenced by more than
one gene at two different
loci or even on different
chromosomes altogether.
Epistasis (standing upon)
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Involves two non-allelic genes (different loci)
Action of one gene masks or alters
expression of other genes
Three forms – collaboration, complementary,
supplementary
Eg Albinism
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Occurs in rodent that are homozygous
recessive for colour even if they have alleles for
agouti or black fur.
The gene for colour is epistatic
gene 1
A
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coat colour show
gene 2
B
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C
one colour/another colour
Collaboration
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Ratio 9:3:3:1
(Although the ratio is typical, it is unusual that
some of the phenotypes may not have been
shown in the parents)
Four different phenotypes depending on the
presence or absence of certain genes
Eg comb shape in chickens
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P_R_
P_rr
ppR_
pprr
walnut
pea
rose
single
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Carry out a cross for
two heterozygotes –
PpRr x PpRr
Supplementary Genes (Epistasis)
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Ratio 9:3:4
A dominant allele at one locus is necessary
for the expression of alleles at another
Typically three phenotypes
Carry out a cross for two heterozygotes –
CcBb x CcBb
Eg Coat colour in mice
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Gene C controls the production of melanin
Gene B indicates whether the colour is black or brown
Without the production of melanin, there will be no colour.
no pigment
gene C
gene B
enzyme 1
enzyme 2
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C_B_ Black
C_bb Brown
cc__ No colour
melanin produced
Black
Brown
Complementary Genes
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Ratio 9:7
Development of a characteristic requires the
presence of at least one dominant allele at
both of 2 loci
Typically there are two phenotypes
Carry out a cross for two heterozygotes –
PpQq x PpQq
Eg Purple pigment in sweet pea
flowers
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Gene P makes white intermediate
Gene Q converts white to purple
Colourless
precursor
(white pp_ _)
gene P
gene Q
enzyme 1
enzyme 2
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Colourless
intermediate
(white P_ _ _)
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Coloured
product
(purple P_Q_)
Duplicate genes
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Ratio 15:1
A characteristic is developed if EITHER or
BOTH of the dominant alleles at two loci
is/are present.
Carry out a cross for two heterozygotes –
AaBb x AaBb
Eg Fruit width in Shepherd’s Purse
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Gene A and B code for two different enzymes which
can form wide fruit.
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Substance
X
gene A
gene B
enzyme A
enzyme B
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Active
Product
wide (A_B_,A_bb, aaB_)
narrow (aabb)
Substance
Y
Practice
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Self check page 111/112
Create a table to compare
Type of
interaction
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F2
phenotype
ratio
Number of
phenotypes
Exam questions
Study book pg 31 Qb
Example
organism
Example
trait
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A women who owned a purebred female albino (lacking pigments) poodle (an
autosomal recessive phenotype) wanted white puppies, so she took the dog to
a breeder, who said he would mate her female with an albino stud male, also
from a pure stock. When six puppies were born they were all black, so the
women sued the breeder, claiming that he replaced the stud with a black dog,
giving her six unwanted puppies. You are called in as an expert witness, and
the defence asks you if it is possible to produce black offspring from two purebreeding recessive albino parents.
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(a)
Discuss what evidence you would give by explaining what gene-gene
interrelationship is involved in each of the parents and using appropriate allele
symbols, draw biochemical pathways to obtain an albino phenotype and a
black phenotype.
Clear well-labelled diagrams may be used to help you answer this question.
(b)
Explain the expected possible F2 phenotypes ratios if two of the black
puppies were allowed to interbreed.
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Exams 4U 2007 Q5
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It is definitely a form of epistasis (that is – there is
obviously more than one gene involved.
We know of 3 types of epistasis; collaboration,
supplementary, complementary
Use the process of elimination
There are only 2 phenotypes, therefore it is not
collaboration
There is no intermediary product, therefore it is not
supplementary
Both genes are required to create colour, therefore it
is complementary
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So, both parent dogs are claimed to be pure
breeding – therefore homozygous
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What are our options for this with 2 genes? –
AABB, aabb, but also AAbb and aaBB
Note a P1 cross of AABB x aabb and a P1
cross of AAbb x aaBB both make the F1
generation AaBb, which produces the
complementary ratio of 9:7 in the F2
Pleiotropy
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A single gene may produce a product that
can influence a number of traits in the
phenotype.