Dihybrid Cross in Biology

This document discusses the concept of dihybrid cross in biology, which involves the cross between two genes or lines that exhibit different traits. It explains the dominant and recessive alleles for different traits and provides examples of dihybrid crosses. The document also explores the genotypic and phenotypic ratios in dihybrid crosses.

Running head: BIOLOGY
Topic:
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Author’s Note:

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Answer 1:
Black fur: FF
White fur: ff
F f
F FF
(Black fur)
Ff
(Black fur)
f Ff
(Black fur)
Ff
(White fur)
Thusthe expected genotypes and phenotypes from the cross are :
Expected ratio:
Genotypic ratio: 1:2:1
Phenotypic ratio: 3:1
Answer 2:
Round fruit: RR
Oblong fruit: rr
a) Homozygous round x heterozygous round

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R R
R RR
(Round)
RR
(Round)
r Rr
(Round)
Rr
(Round)
Genotypic ratio: 1:1
There is no phenotypic ratio as all the phenotypes are round.
b) ) Heterozygous round x heterozygous round
R r
R RR
(Round)
Rr
(Round)

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r Rr
(Round)
rr
(Oblong)
The probable genotypic and phenotypic ratios of the cross are
Genotypic ratio: 1:2:1
Phenotypic ratio: 3:1
Answer 3:
Tasting chemical (Dominant): TT
Non tester for the chemical (Recessive): tt
The crosses between a carrier mother and affected father showing the genotypes are:
XTX x XTY
XTXT XTY XXT XY
Tester Tester Non taster Non taster
( Carrier)
The phenotypes would be 50% of children being tasters and 50% being non-tasters.
Answer 4:
Since father has been coulur blind woman being unaffected is a carrier the results between a
carrier mother and affected father is:

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Generation 1: XX x XCY
Generation 2: XXC x XY
XX XY XCXC XCY
Normal Normal Affected Affected
(Colour blind) (Colour blind)
Answer 5 :
In this case R is a sex linked allele which is dominant over r.
a) Generation 1: XXr x XRY
XRX XY XRXr XrY
Carrier Normal Affected Normal
Thus the chance of an affected daughter from an unaffected mother is 25%.
b) Generation 1 : XRXR x XY
Generation 2: XRXR x XY

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Gneration 3: XRX XRY XRX XRY
Carrier Affceted Carrier Affected
Thus the chance of being affected from an affected mother and unaffected father is 50%.
Answer 6:
Cystic fibrosis is one of the autosomal recessive diseases in children and found among
Caucasians. It is often lethal and causes chronic recurrent infections (Sanyal et al.2018).
C c
C CC
(Cystic
fibrosis)
Cc
(Carrier)
c Cc
(Carrier)
cc
(recessive
homozygous)
Possible genotypes for 1 and 2 are Cc and Cc where father and mother both are carriers.

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a) Again for 3 and 4 they are Cc and Cc because the son from unaffected father and
mother can only become affected when both of them are carriers.
The same situation for an affected individual from unaffected parents is shown in B 5 and 6
being Cc and Cc. This is a characteristic of autosomal recessive disorders. 75% of the
diseases being expressed is seen phenotypically and 25 % not expressed.
b) If only one of them is a carrier there are two probabilities
Case 1:
C c
C CC
(Cystic
fibrosis)
Cc
(Carrier)
C CC
(Cystic
fibrosis)
Cc
(Carrier)
There is 50% chance of being affected and 50% chance of being a carrier.
Case 2:
C c

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c Cc
(Carrier)
cc
(recessive)
c Cc
(Carrier)
cc
(recessive)
There is 50% chance of being unaffected and 50% chance of being a carrier.
c) There is 25% chance of the child being affected after the first child as cystic fibrosis is
an autosomal recessive disorder and there is 25% of chance of being affected and
75% chance of not being affected.
Answer 7:
Dihybrid cross involves the cross between two gene or lines which is observed in different
traits (Lewis et al.2016).
Dominant alleles for Brown hair: HH
Recessive alleles for blond hair: hh
Dominant alleles for Brown eyes: BB
Recessive alleles for blue eyes: bb
Thus the progeny of the F1 generation is BH x bh

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BbHh
The resulting dihybrid cross is :
BH Bh bH bh
BH BBHH
(Brown eyes
brown hair)
BBHh
(Brown eyes brown
hair)
BbHH
(Brown eyes brown
hair)
BbHh
(Brown eyes brown
hair)
Bh BBHh
(Brown eyes
brown hair)
BBhh
(Brown eyes blond
hair)
BbHh
(Brown eyes brown
hair)
Bbhh
(Brown eyes blond
hair)
bH BbHH
(Brown eyes
brown hair)
BbHh
(Brown eyes brown
hair)
bbHH
(blue eye brown
hair)
bbHh
(blue eye brown
hair)

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bh BbHh
(Brown eyes
brown hair)
Bbhh
(Brown eyes blond
hair)
bbHh
(blue eye brown
hair)
bbhh
( blue eyes blond
hair)
a) The ratio of this dihybrid cross is 9:3:3:1. Thus the chances that a child will be born with
brown eyes and brown hair is 9/12 or75%. The genotypes have been marked in red in the
cross.
b) The chances that the child will be born with blue eyes and brown hair is 3/12 or 25%. The
genotypes have been marked in blue for the cross.

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References
Lewis, R., 2016. Human genetics: the basics. Garland Science.
Sanyal, A., Burrell, A., Chaney, H., Sami, I., Perez, G.F., Koumbourlis, A.C. and Hahn, A.,
2018. Preliminary Analysis of the Impact of Pharmacokinetics and Pharmacodynamics on
Improvement in Pulmonary Function in Children with Cystic Fibrosis.