Question

Answer the following questions:

(a)

1. What is a test cross? How can we find out the zygosity of a plant by the use of test cross? 
2. 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? 

OR 
(b) Name the two events ‘a’ and ‘c’ shown in the diagram below. 

 

1. Mention where in the cell these events take place. 
2. Explain the changes taking place during event ‘b’ and mention why these are needed.

Hint:

Test cross helps identify zygosity. Haemophilia is X-linked recessive. Splicing removes introns; translation builds proteins.

Answer 1

(a)

• [(i)] A test cross is a cross between an individual of unknown genotype with a homozygous recessive individual to determine the genotype (zygosity). If the progeny shows all dominant traits, the test plant is homozygous dominant; if a 1:1 ratio of dominant and recessive traits appears, it is heterozygous.
• [(ii)] Human females are rarely haemophilic because the disorder is X-linked recessive. For a female to be haemophilic, she must inherit the defective gene from both parents (i.e., mother must be carrier and father must be haemophilic). Haemophilic individuals suffer from defective blood clotting due to absence of clotting factors, resulting in prolonged bleeding.

OR
(b)

• Event a: Transcription 
  Event c: Translation
• [(i)] These events take place in the nucleus (transcription and splicing) and cytoplasm (translation).
• [(ii)] Event b is RNA splicing, where introns (non-coding sequences) are removed and exons (coding sequences) are joined. This process is necessary to produce a functional mRNA transcript that can be translated into a proper protein.