Question

genetics question 2

QUESTION 2 Which of the following statement(s) is/are true? A test cross is a cross between two F1s arising from pure-breeding parents. Barbara McClintock and Harriet Creighton established the physical reality of crossing over by observing the distribution of cytological markers and phenotypes in controlled crosses of corn (Zea mays) The first person to demonstrate crossing-over experimentally was Curt Stern. The closer two genes are on a chromosome, the higher the probability of recombination between them. The expected phenotypic ratio in a testcross with a an individual heterozygous for two loci (assuming complete dominance) is 1:1:1:1

Answer 1

The following statements are true:

• Barbara McClintock and Harriet Creighton established the physical reality of crossing over by observing the distribution of cytological markers and phenotypes in controlled crosses of corn (Zea mays)
• The expected phenotypic ratio in a testcross with an individual heterozygous for two loci (assuming complete dominance) is 1:1:1:1

Here's why the others are incorrect:

• A test cross is a cross between an individual possibly Heterozygous for the genes in question with another individual that is recessive at all the loci being tested. For example (AaBbCc x aabbcc)
• Curt Stern first demonstrated that crossing over takes place is Mitotic divisions as well. It was previously thought that crossing over only occurs during Meiosis.
• The closer two genes are on a chromosome, lesser is the probability of recombination between them. This is because closely linked genes are unlikely to have a crossing over event between them.