7.1 genetics, pass paper question revision Flashcards
The effects of the fur colour alleles can be further modified by
a second gene which is not sex-linked. The allele r changes
the brown colour to tan, and white colour to grey. Allele R has
no effect on fur colour in rabbits.
A grey coloured male rabbit was crossed with a roan female
who is heterozygous for the second colour modifying gene
State the genotypes of both parents.
Male: XWy-rr; [1 mark]
• Female: XBXWRr: [1 mark]
This is an example of epistasis. The male phenotype has been modified and since the
modifying allele is recessive, he must inherit 2 r alleles for the modified phenotype to
be expressed. He must have inherited X” since the second, modifying gene, will
change the white to grey.
A roan female rabbit is crossed with a white male rabbit.
*Use a genetic diagram to state the ratio of phenotypes
expected in the offspring of this cross.
Parental genotypes; male: XWY-; female: XBX (allow 0 or blank for Y=); Il mark]
• Parental gametes; male: XW, Y-; female: XB, XW ; [1 mark]
• Offspring phenotypes and ratios; female roan, female white, male brown, male
white; 1:1:1:1; [1 mark]
Fur colour in rabbits is controlled by a gene inherited on the X
chromosome. Some rabbits can be either brown or white.
Female rabbits can be described as roan’ if they have
patches of brown and white fur.
Explain why male rabbits cannot be roan.
• They only inherit 1X chromosome and both (brown and white) alleles must be
inherited; [l mark]
• (Brown and white) the alleles are codominant; [l mark]
• Only female rabbits inherit 2 alleles (for fur colour); Il mark]
The clue is in the final sentence of the introductory paragraph; females can be both
brown AND white. This is an indirect statement suggesting that there is a third,
heterozygous, phenotype. Since females carry XX and males carry XY, males canno
be heterozygous for this particular trait.
An investigation on fruit flies, Drosophila melanogaster, was
carried out to determine the relationship between body
colour and wing length. Scientists found the genes controlling
these characteristics are inherited on different autosomal
chromosomes. Fruit flies are either black or grey and have
-either long or short wings.
Two homozygous fruit flies were crossed: a fruit fly with a
black body and long wings was crossed with a grey fruit fly
with short wings. All of the offspring produced had black
bodies with long wings
Using this information and a genetic diagram, show how these
offspring were produced. Use B/b and L/I to represent the
alleles.
Black body and long wings are dominant; [l mark]
• Parent 1 genotype: BBLL (black body and long wings); [l mark]
• Parent 2 genotype: bbll (grey body and short wings); [l mark]
• All offspring BbLI; [1 mark]
The offspring from part a) were crossed with grey bodied fruit
flies with short wings. Use a genetic diagram to show the
expected ratios of the phenotypes expected from this cross.
Genotypes of offspring: BbLI, Bbll, bbLI, bbll; [l mark]
• Phenotypes of offspring: black and long, black and short, grey and long, grey and
short; [l mark]
• 1:1:1:1; [1 mark]
The genotype of the offspring from part a) was BblI and the second parent from this
question has to be homozygous recessive in both traits, bbll
The scientists claimed that independent segregation had
taken place. Chi? can be used to determine the significance
of the data. The critical value for this data is 7.82. Use the
following formula to calculate the value of Chi squared (y2)
and comment on the scientists claim
X2= Sum of (observed -expected)2 /expected
Any five of the following:
• (O-E)2/E = 0.1125, 0.3125, 0.05, 0.45 [2 marks for all 4, 1 mark for 3]
• Value of Chi2: 0.925/0.93; [1 mark]
• (The scientists claim is correct because) there is no significant difference between
the observed values and expected values; [l mark]
• (Because) the Chi? value is smaller than the critical value; [l mark]
• There is a greater than 5% probability that the differences in the data are due to
chance [1 mark]
A scientist crossed a grey male cat with a grey female cat
multiple times. There were 12 kittens born of which 8 were grey
and 4 were white. The trait is not sex-linked.
-Explain how the information provides evidence that white fur
is recessive.
Both parents are heterozygous; [l mark]
• Kittens must receive a white fur allele from both parents; [l mark]
Both parents each passed on an allele to the white kitten, which means they must have
passed on the white fur allele. This was not expressed in their phenotype showing us it
is recessive. In order for two grey cats to pass on the allele coding for white fur they
must have been heterozygous
