Question

(a) (i) What is a test cross? How can we find out the zygosity of a plant by the use of test cross? (ii) Why are human females rarely haemophilic? Explain with the help of a cross. Under what conditions can a haemophilic female be born? How do haemophilic patients suffer?

Accepted Answer

(a) (i) • It is a cross to determine unknown genotype of an organism showing dominant phenotype by crossing with a recessive parent. • A cross between plant with dominant phenotype (example violet colour flower) and plant with recessive phenotype( white colour flower )(monohybrid cross) is done - If the F1 individuals show 1: 1 ratio between dominant and recessive phenotype(violet flower: white flower),then the given plant is heterozygous (Vv) If the F1 individual show all dominant phenotype (all violet flower),then plant is homozygous (VV) / Cross can be explained by taking any other dominant phenotype (ii) • Haemohilia is a sex linked recessive trait and human females are rarely haemophilic because mother of such a female has to be at least carrier and father should be haemophilic. • Mother Father P1 generation - XXh x XhY F1 generation - XXh, XY, XhXh, XhY Carrier female, normal male, haemophilic female, haemophilic male / Mother Father P1 generation - XXh x XY F1 generation - XX, XY, XhX, XhY Normal female, normal male, carrier female,haemophilic male • A haemophilic female can be born if both the X chromosomes are having haemophilic gene / XhXh • clotting of blood is affected /In an affected individual a simple cut results in nonstop bleeding.

Marking-scheme solution

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(a) (i) What is a test cross? How can we find out the zygosity of a plant by the use of test cross? (ii) Why are human females rarely haemophilic?…

Marking-scheme solution

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