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Genetics Cheat Sheet (DRAFT) by

cells to s module ec

This is a draft cheat sheet. It is a work in progress and is not finished yet.

Darwin

natural selection
selection due to enviro­nmental stresses - survival of the fittest
pangenesis
a mixing of charac­ter­istics from parents

Lamarckism

epigen­etics
presence of methyl groups attached to DNA bases controls the expression of genes & changes cellular behaviour
methyl­ation status changes in response to ageing, diseases (cancer), & enviro­nmental factors (diet)

Mendel

laws of inheri­tance
study of alleles (hered­itable factors)
deductions
genes come in pairs & are inherited as distinct units (1 from each parent)
 
the segreg­ation of parental genes & their appearance in the offspring tracked as dominant or recessive traits
 
there are mathem­atical patterns of inheri­tance from one generation to the next
peas?
grown in small area
 
lots of offspring
 
produce pure plants when allowed to self-p­oll­inate over several genera­tions
 
can be artifi­cially cross-­pol­linated
self-p­oll­ination
Mendel produced pure strains by self-p­oll­inating for several genera­tions
 
male anther + female stigma, germin­ation occurs
 
example: if p = tall x short, f1 = all tall, f2 = 3/4 tall & 1/4 short
partic­ulate inheri­tance
physical traits are inherited as 'parti­cles' - now known as chromo­somes & DNA
example crossing pure plants:
p = TT x tt
 
f1 = all hydrids: Tt
 
f2 = hybrid x hydrid: TT, Tt. Tt, tt
1st law: principle of dominance
an organism with alternate forms of an allele will express dominant form
 
alleles can be dominant or recessive - one dominant allele will display its phenotype
 
cross pure parents for contra­sting traits results in only one form of the trait in the next generation
 
all offspring hetero­zygous & express only dominant trait
2nd law: principle of segreg­ation
each inherited trait is defined by a pair of alleles - parental alleles segregate during meiosis.
 
during formation of gametes, the two alleles respon­sible for a trait separate - meiosis
 
alleles for trait are recombined at fertil­isa­tion, producing offspr­ing's genotype
3rd law: principle of indepe­ndent assortment
alleles for different traits are not dependent on one another for their expression
 
allele pairs separate indepe­ndently during formation of gametes (meiosis)

Mendel's crosses

1. p
TT x tt
2. f1
Tt x Tt
3. f2 test cross
Tt x tt
he didn't know at f1 whether tall plants were homozygous or hetero­zygous so you use a test cross & use homozygous recessive from p to unmask other traits & determine genotype of f2 - if they all come out the same then it was homozygous

experiment

200 people given sulpha­dim­idine, urine taken after 6 hours, treated so main metabolite from exrcetion stained blue
samples put in colori­meter - intensity of colour is propor­tional to amount of metabolite
data sorted into ranges based on optical density & plotted on histogram
family included: all fast apart from eldest daughter, therefore parents are Ff Ff & she has ff

other crosses

dihybrid
p = RRYY X rryy
 
gametes = RY & ry
 
F1 = RrYy (all yellow & round)
 
F2 = 9/16Y + R, 3/16 Y + r, 3/16 y + R, 1/16 y + r. (four different pea phenot­ypes)
mono hydrid hetero­zygous cross
Aa x Aa
 
genotype ratio: 1:2:1
 
phenotype ratio: 3:1
dihybrid hetero­zygote cross
AaBb x AaBb
 
genotype ratio: 1:2:2:­1:4­:1:­2:2:1
 
phenotype ratio: 9:3:3:1

other crosses

dihybrid
p = RRYY X rryy
 
gametes = RY & ry
 
F1 = RrYy (all yellow & round)
 
F2 = 9/16Y + R, 3/16 Y + r, 3/16 y + R, 1/16 y + r. (four different pea phenot­ypes)
mono hydrid hetero­zygous cross
Aa x Aa
 
genotype ratio: 1:2:1
 
phenotype ratio: 3:1
dihybrid hetero­zygote cross
AaBb x AaBb
 
genotype ratio: 1:2:2:­1:4­:1:­2:2:1
 
phenotype ratio: 9:3:3:1

Mendel's law exceptions

1. incomplete dominance
sometimes neither allele is fully dominant over the other so the two alleles are both capital letters & one has an apostrophe e.g. R R'
 
when these alleles come together they portray a mixing of the two phenotypes
2. codomi­nance
both alleles of a gene are dominant & the hetero­zygous phenotype has both traits expressed equally so the two alleles are two capital letters e.g. W(hite) B(rown)
3. multiple alleles
human blood type is governed by presence of 3 different alleles: A B O & each person has 2/3 in their DNA
 
A & B are codominant with each other
 
A & B are purely dominant over O
 
O is recessive
 
blood type gene is I, e.g. for A = IA, B = IB, O =i
type A = IAIA or IAi
type B = IBIB or IBi
type AB = IAIB
type O = ii

sex-linked traits

traits located on sex chromo­somes, X & Y. XX = females, XY= males. many sex-linked traits carried on X chromo­some.
Hemophilia is caused by recessive gene on X chromosome - severity is related to amount of clotting factor in blood
colour blindness is a recessive sex-linked condition on the X, caused by lack of colour receptors in the eye, results in inability to see some colours correctly. more common in males, patients unable to distin­guish shades of red-green.