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LIFE SCIENCES: CONSOLIDATION TASK 8
TOPIC: GENETIC CROSSES
1. COMPLETE DOMINANCE (Mendel’s experiments/principles)
1.1 Learners want to investigate eye colour in fruitflies (Drosophila melanogaster). Fruit flies can have red (R) eyes or white (r) eyes. Red
eye colour is dominant and white eye colour is recessive.
Male fruit flies, homozygous for red eye colour, were bred with female fruit flies with white eye colour.
Represent a genetic cross to determine the possible phenotypes and the genotypes of the F1 generation for eye colour. (6)
1.2 In humans the ability to roll the tongue is due to a dominant allele. A father heterozygous for tongue rolling and a mother who cannot roll her tongue have children. Use the symbols T and t for the alleles of the tongue rolling characteristic and represent a genetic cross to determine the possible genotypes and phenotypes of the children. (6)
1.3 Fur colour in mice is controlled by a gene with two alleles. A mouse with black fur was crossed with a mouse with brown fur. All offspring had black fur.
Using the symbols B and b to represent the two alleles for fur colour, show diagrammatically a genetic cross between the two mice described above. Show the possible genotypes and phenotypes of the offspring. (6)
1.4 When flies with grey bodies were crossed with flies with black bodies, all offspring in the F1 had grey bodies.
Use the letters G and g to represent a genetic cross to show the F2 genotypes and phenotypes if the F1 were interbred. (7)
1.5 A geneticist wanted to find which corn colour is dominant in a species of maize. The species has two phenotypes for colour, yellow and white. She performed four genetic crosses and record the colour of the offspring as shown in the table below.
Genetic crosses / Parent phenotypes / Offspring phenotypes1 / yellow x yellow / all yellow
2 / white x white / 51 white and 17 yellow
3 / white x yellow / 32 white and 34 yellow
4 / white x white / all white
1.5.1 According to the results, which colour is dominant? (1)
1.5.2 Which ONE of the genetic crosses (1, 2, 3 or 4) from the table allows the
conclusion suggested in QUESTION 5.1? (1)
1.5.3 Give a reason for your answer to QUESTION 5.2. (2)
1.5.4 Use the symbols G and g to represent genetic cross 2. Also indicate the
proportions of the F1 phenotypes. (6)
DIHYBRID CROSSES
1.6 In pea plants the allele for round seeds (R) is dominant over the allele for
wrinkled seeds (r). The allele for yellow seeds (Y) is dominant over the allele for green seeds (y).
Plant A, heterozygous for both seed shape and seed colour, was crossed with plant B, which had wrinkled, green seeds.
1.6.1 Write down the genotype of:
(a) Plant A (1)
(b) Plant B (1)
1.6.2 Write down all the possible genotypes of the gametes of plant A. (2)
1.6.3 State the phenotype of an offspring having the genotype:
(a) rrYy (2)
(b) RrYy (2)
1.6.4 When plant B was crossed with plant C, all the offspring had round, yellow seeds. Use this information and write down the genotype of plant C. (2)
1.7 About 70% of people get a bitter taste when a substance called PTC is placed
on their tongue. They are referred to as ‘tasters’. All other people are unable to taste PTC and are referred to as ‘taste-blind’. The ‘taster’ allele is dominant and the ‘taste-blind’ allele is recessive.
Also in humans, normal skin pigmentation is dominant to the albino condition
(no pigmentation).
The letters in the key below must be used to represent the alleles for the
different characteristics above.
Key :T – taster
t – taste-blind
N – normal skin pigmentation
n – no skin pigmentation (albino)
A man who is heterozygous for both tasting PTC and skin pigmentation
marries a woman who is taste-blind for PTC and is an albino.
1.7.1 State why the example above represents a dihybrid cross. (1)
1.7.2 Write down
(a) The genotype of the woman (1)
(b) All the possible gametes of the man (2)
1.7.3 The man and woman have a child whose genotype is ttNn. What is the
child’s phenotype? (2)
1.7.4 A man and a woman are only able to produce children with the genotype TtNn. State the possible genotypes of the man and the woman. (2)
2. INCOMPLETE DOMINANCE (Variations from Mendel’s principles)
2.1. A homozygous snapdragon plant with red flowers (R) was cross-pollinated with a homozygous snapdragon plant with white (W) flowers. All the plants that grew from the cross had pink flowers. Represent a genetic cross to determine the genotype of the F1 generation of plants. (6)
3 CO-DOMINANCE (Variations from Mendel’s principles)
3.1. A bull homozygous for white fur (W) and a cow that is homozygous for reddish- brown fur (R) have had three offspring in their lifetime and all of
them were roan.
Represent a genetic cross to determine the probability of their next
offspring being white, reddish-brown or roan. (7)
3.2. A mother with blood group A and a father with blood group B are both heterozygous in terms of this characteristic. Represent a genetic cross to determine the different possible blood group genotypes and phenotypes of their children. (6)
4 INHERITANCE OF SEX (Variations from Mendel’s principles)
4.1. A couple has three sons and the woman is pregnant again. Show diagrammatically by means of a genetic cross the percentage chance
for them to have a baby girl. (7)
5 INHERITANCE OF SEX-LINKED CHARACTERISTICS (Variations from Mendel’s principles)
5.1 Haemophilia is a sex-linked hereditary disease that occurs as a result of a recessive allele on the X –chromosome (Xh). A normal father and heterozygous mother have children. Represent a genetic cross to determine the possible genotypes and phenotypes of their children. (6)
5.2 Colour-blindness is the inability to distinguish between certain colours. The allele for colour-blindness is recessive and it is carried on the X-chromosome (Xc). A colour-blind father and a homozygous normal mother have children. Represent a genetic cross to determine the possible genotypes and phenotypes of their children. (6)
6 PEDIGREE DIAGRAMS
6.1 The pedigree diagram below shows the inheritance of Daltonism in a family.
Daltonism (red-green colour blindness) is sex-linked.
The allele for Daltonism is recessive to the allele for normal colour vision.
6.1.1 Use the symbols XD, Xd and Y to state the genotype of the following:
(a) Individual 2 (1)
(b) Individual 3 (1)
6.2.1 How many family members not affected by Daltonism are
definitely carriers? (2)
6.3.1 Use a genetic cross to determine the possible genotypes and
phenotypes of the offspring that may be formed by individuals 5 and 6. (6)
7 DIHYBRID CROSSING (Combination of Mendel’s principles/experiments and variation from Mendel’s principles/experiments)
7.1 In some flowers, a true-breeding, red flower (RR) gives all pink flowers when
crossed with a white-flower (WW): If in this case flower position (axial or terminal) is inherited as it is in Mendel’s peas, what will the genotypes and phenotypes of the generation resulting from the following cross:
axial-red (RRAA) x terminal-white (WWaa) be?
A central-pink: RWAa
B axial-red: RRAa
C axial-pink: RWAa
D axial-white: WWaa (2)
Consolidation task 8