Mendelian Genetics
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Transcript of Mendelian Genetics
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Mendelian Genetics
Read through core knowledge.What vocab do you need to learn?
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Terms
Gene Allele Trait Dominant Recessive Homozygous Heterozygous Genotype Phenotype P and F1 and F2
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Definitions
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
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Mendel – why so famous?
Worked with peas Used pure-breeding varieties Came up with idea of ‘gene’ 20 years before
chromosomes were discovered
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Law of segregation
Of the two genes controlling each characteristic, only one is present in each gamete.
During meiosis the two genes are separated.
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Independent assortment
The segregation of one pair of alleles does not affect the segregation of another pair.
There is a random arrangement of parental chromosomes at metaphase of meiosis.
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Monohybrid cross
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
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Test Cross
A cross carried out to determine an organism’s genotype, by mating it with a homozygous recessive organism.
Show how a test cross works
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Dihybrid Cross
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.
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Dominance
Incomplete dominance
Co-dominance
Lethal Alleles
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Incomplete dominance
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
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Eg snapdragons
P1: RR (Red) x rr (white) F1: Rr (pink) F2: ? (You determine the ratios)
1red:2pink:1white R r
R
r
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Co-dominance
Both alleles in heterozygous organism contribute to the phenotype.
Both alleles are independently and equally expressed.
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Eg Human Blood Group AB
P1: AA (type A) x BB (type B) F1: AB (type AB)
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Eg Coat colour in horses and cattle
P1: CRCR(red) x CrCr(white) F1: CRCr(roan) F2: ? You determine the ratios
1 Red: 2 Roan: 1 white
Roan is a blend of both white hairs and red hairs
CR Cr
CR
Cr
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Lethal Alleles
Mutations of a gene that produce a non-functional gene product and affect the organisms survival.
If dominant, may kill in single dose If recessive, kills when homozygote
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Eg Manx cat
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?
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Eg Yellow mice
YY (lethal – yellow – terminates at blastocyst stage) Yy (yellow) yy (not yellow)
Again, what is the phenotypic ratio for a cross of heterozygotes?
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Eg Huntington’s disease
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
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Multiple alleles
More than one allele possible at a gene locus
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Blood groups
There are three different alleles:
– 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.
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Frequency in NZ
Phenotype Genotype Frequency in NZ %
O OO 49
A AA, AO 40
B BB, BO 9
AB AB 2
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Determine the blood typesCross
Parental 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
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Dilemma
If a mother is type A and has a baby type B, can the father be type O?– Explain your answer.
You can now do the
self check for this section.
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Gene-gene interactions
When a characteristic is influenced by more than one gene at two different loci or even on different chromosomes altogether.
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Epistasis (standing upon)
Involves two non-allelic genes (different loci) Action of one gene masks or alters
expression of other genes Three forms – collaboration, complementary,
supplementary
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Eg Albinism
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 gene 2
A B Ccoat colour show one colour/another colour
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Collaboration
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
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Eg comb shape in chickens
P_R_ walnut
P_rr pea
ppR_ rose
pprr single
Carry out a cross for two heterozygotes – PpRr x PpRr
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Supplementary Genes (Epistasis)
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
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Eg Coat colour in mice
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.
gene C gene B
enzyme 1 enzyme 2
no pigment melanin produced Black Brown
C_B_ BlackC_bb Browncc__ No colour
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Complementary Genes
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
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Eg Purple pigment in sweet pea flowers
Gene P makes white intermediate Gene Q converts white to purple
gene P gene Q
enzyme 1 enzyme 2
Colourless Colourless Coloured precursor intermediate
product
(white pp_ _) (white P_ _ _) (purple P_Q_)
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Duplicate genes
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
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Eg Fruit width in Shepherd’s Purse
Gene A and B code for two different enzymes which can form wide fruit.
gene A gene B
enzyme A enzyme B
Substance Active Substance
X Product Y
wide (A_B_,A_bb, aaB_)narrow (aabb)
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Practice
Self check page 111/112 Create a table to compare
Exam questions Study book pg 31 Qb
Type of interaction
F2 phenotype ratio
Number of phenotypes
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 pure-breeding recessive albino parents.
(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
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– 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
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Pleiotropy
A single gene may produce a product that can influence a number of traits in the phenotype.