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Sigenio Dinybrid crosses
Digeniocrosses are breedingexperiments involving the inheritance of two pairs of contrasting
characteristics at the same time
Round peas R are dominant to wrinkledpeas r and
yellowpeas
Y are dominant to greenpeas y
A dihybrid cross between a dominant homozygous Rr Y4 and a
recessive one reyy will mean that F offsprings will all be
heterozygotes
and display the dominant phenotype
When oesprings have their parents phenotypes it is called parental
phenotypes when they are different recombinant phenotypes
Dihybrid crosses will show inheritance of two separates genes
independently following mendel s law of independentassortment
n peas genes for colour and shape are on different autosomes As
a result tour potential
gametes are corned Theysegregate
independently oe each other thus can recombine in all possibleways
When ratios are not what youexpect it maybe because a small sample size is being observed
an experimental error has ocurred the process is random so the unexpected
might also occ
genes examined also linked both on the same chromosome
may
Using probability to predict the outcome oe complex crosses
When crosses become complex simplediagrams
may become difficult to analyse
In Elies Greybody is 3 dominant Longwings are also dominant therefore the
oe
probability
any eyinheriting a greybody and longwings is t
Calculations can t be applied to linked genes
s
Chi squared X test
The chi squared test is a simple statistical method used to establish nether differences
between observed and expected results are
significa or simply due to chance
The null hypothesis states there is no significa diferente thus due tochange IE calculated
value a critical valve No significant difference between o and E accept hypothesis
Valve o X is not enough you will need the degrees of freedom This will
always be one less
than the number of categories available In this example there are two categories thus 1
degree of freedom
in most cases a
probability p o o os is used This means that 9si
Of the time you will get the results youexpected
osobserved
X s E OIE y Es Expected
IE X2 was less than o os it would mean there was a statisticallysignifican difference be twee
observed and expected and we reject the null hypothesis
This might mean we have to look for errors in the experimentaltechnique or revise the
entire hypothesis
, Genelinkage
cenelinkage
when F Elies are crossed the normal 9 3 3 I dihybrid ratio of parental and recombinanttypes does
not occur There is a 3 I ratio of dominant recessivephenotypes which is what we would expectir
a monogenic cross
The explanation of this is that genes are linked This means they are sited on the same
chromosome
and inherited as if they were one unit Mendelian ratios for a dihybrid cross only occur
when the genes are on separate chromosomes
dentieying linked genes
IEgenes are found on the same chromosome they are linked and inherited to a greateror lesser
degree as iethey were a singlegene
Whengenes are closely linked close to each other on the chromosomes recombination events on meios
rarely occur in the gametes
IE genes are further apart the number of recombinations will be
higher
Tightness of the linkage relates to how close togetherthe
genesare on the chromosome Tightlinks are lesslikelyto be
splitduringcrossing over Themaybe so tightlylinked that
theymay never besplitduringmeiosis thus gametes
will always have parentalphenotypes formed
Clue that ocurred in a dihybridcross is when the
linkage has
expected 9 3 3 I ratio for a heterozygote cross does not occur
Largerdiscrepancies especially when there is an unexpectedlyhigh number ofparentaltypes
this indicate that linkage between genes is involved
may
Chromosomemapping
Usinggenelinks we can build a chromosome map to do this we will need to work out the crosso
value
number of recombinantoffspring
Crossover values y yo
total number of offspring
Closelylinkedgenes will result in low crossover valves Genes that are further apart produce
more recombinante thus have a higher crossover value
Sex linkage
Chromosomes indiploidorganisms occur in homologou
pairs the last pair are the sex chromosomes F XX M
XY All female eggs contain X chromosomes thus
are referred to as homogametic Halt of the male sperm will contain X chromosome
and the other half will contain Y They are referred to as heterogametic
in females one of the X chromosomes is inactivated at random inevery cell to forma
Barr cell
Genes carried in the X chromosome are said to be sex linked
Any recessive or mutant
alleles on the X chromosome passed from mother to offspringswill be expressed on their
phenotype as there is no corresponding allele on the Y chromosome
In Drosophila eye colour is sex linked It is the result of multiple alleles and the most
common
eye colour being inherited through a dominant allele on the X chromosome
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