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BTEC APPLIED SCIENCE UNIT 11 - Learning aim C
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C: Explore the principles of inheritance and their application in predicting genetic traits. Grade Distinction.
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Emilia HawkinsUnit 11: Genetics and genetic engineering
C: Explore the principles of inheritance and their application in predicting genetic traits.
DROSOPHILA FLY LAB COLLATED RESULTS
Monohybrid and dihybrid inheritance
Monohybrid inheritance refers to the inheritance of one characteristic which is controlled by a single
gene. It involved crossing two individuals with either a heterozygous or homozygous gene locus and
can be demonstrated using a punnet square. There are three possible genotypes which can be
present; two dominant alleles or homozygous dominant, two recessive alleles or homozygous
recessive and both a dominant and recessive allele or heterozygous. Crossing two individuals with a
heterozygous gene locus will result in the offspring having a 75% chance of presenting the dominant
trait and a 25% chance of presenting the recessive trait. When homozygous individuals are crossed,
where one parent has two dominant alleles and another has two recessive alleles, the offspring will
always carry one dominant and one recessive allele.
Dihybrid inheritance refers to the inheritance of two separate genes or two pairs of alleles which is
therefore observed as two separate characteristics. When crossing two individuals where one parent
is homozygous dominant for both pairs of alleles and the other is homozygous recessive for both
pairs of alleles, it will always result in a child being heterozygous. If both the parents are
heterozygous for the two pairs of alleles, there are nine different possible combinations of pairs of
alleles. But many of these result in the same presented characteristics as this characteristic is ruled
by the dominant gene.
Monohybrid cross between flies with normal wing and flies with vestigial wing
Predicted outcome
W w
W WW Ww
w Ww ww
The predicted outcome suggests that 3/4 or 75% of the offspring will have a dominant allele for the
normal wing characteristic, and 1/4 or 25% of the offspring will have both recessive alleles and
therefore have a vestigial wing. The probability of the flies having normal or vestigial wings is
calculated using a punnet square which acts off Mendel’s law of dominance. Outcomes which have a
dominant allele present as the phenotype of having normal wings whereas recessive alleles
represent vestigial wings. The results of the experiment may be different to the predicted results
because the alleles are randomly paired and therefore will not fully match.
Observed outcome
Fly Phenotype Count
Normal wing 737
Vestigial wing 279
Chi-squared test
, Emilia Hawkins
Observed = O
Expected = E
Degrees of freedom = the number of classes – 1
Total = 1016
Ratio 3:1
Phenotype Observed (O) Expected (E) O-E (O-E)2 (O-E)2 / E
Normal wing 737 762 -25 625 0.82
Vestigial wing 279 254 25 625 2.24
Total = 3.06
With 1 degree of freedom, and where the probability of exceeding the critical value of 0.05 or 95%
the chi-squared test gives the value 3.84.
Dihybrid Cross between wild type flies with normal wing.
Predicted outcome
BW Bw bW Bw
BW BBWW BBWw BbWW BbWw
Bw BBWw BBww BbWw Bbww
bW BbWW BbWw bbWW bbWw
bw BbWw Bbww bbWw Bbww
The predicted outcome shows that 9/16 or 56.25% of the offspring will have both a wild body and
normal wing characteristics. It also shows that 3/16 or 18.75% of the offspring will have a wild body
and vestigial wing, along with a further 3/16 or 18.75 of the offspring which will have an ebony body
and normal wing characteristics. The prediction also shows that 1/16 or 6.25% of the offspring will
have an ebony body and vestigial wing. The probability of the flies representing any of the four
combinations of characteristics depends on how the alleles pair up as the genes are formed.
Mendel’s law of independent assortment says that in meiosis each pair of alleles segregates
independently of other pairs. This means that when calculating predictions for dihybrid crosses, the
alleles will have no effect on each other and will pair up independently. Also, because of Mendel’s
law of dominance, some flies may have the alleles for certain characteristics but, due to the
dominant alleles controlling the phenotype, they don’t present physically. For example, a fly with the
genes BbWw has alleles for all four characteristics but would look like a fly with a wild body and
normal wings.
Observed outcome
Fly Phenotype Count
Ebony body, vestigial wing 166
Ebony body, normal wing 480
Wild body, vestigial wing 490
Wild body, normal wing 1467
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