Living Environment Regents Review Practice Heredity and Genetics Question 21
Saturday, 27 June 2009 10:20
A child is born with an extra chromosome in each of its cells. This condition is usually the result of
Correct Answer: Option A – Nondisjunction
Nondisjunction is the result of the paired homologous chromosomes separating unequally during meiosis. So instead of one of a particular chromosome going to each of the D resulting sex cells, one cell gets two chromosomes of that type. Then during fertilization that zygote gets three of that particular chromosome instead of the normal two, resulting in one extra chromosome. That zygote then undergoes mitosis, resulting in all the cells of that organism containing one extra chromosome.Hence, option A is the correct answer.
Crossing over of chromosomes occurs during the formation of gametes, but this iss not responsible for an additional chromosome in a cell.Hence, option B is incorrect.
Segeregation is separation of the replicated chromosomes to form B new cells – this occurs in asexual reproduction.Hence, option C is incorrect.
Hybridization is the process of combining B different varieties of organisms to produce a hybrid organism.
Living Environment Regents Review Practice Heredity and Genetics Question 23
Saturday, 27 June 2009 10:28
Which combination represents the combination of a homozygous condition?
Correct Answer: Option B – bb
Humans possess 2 genes for every trait that are located on homologous chromosomes. 1 homozygous genotype means that an individual has the same genes for a particular trait on the homologous chromosomes. We represent genes as letters and for any one trait we must use the same letter, a capital letter meaning the dominant gene while a lower case letter meaning a recessive gene.
Hence, option B with the combination bb is the correct answer.
Answers A, C and D are incorrect since they have a combination of a dominant gene denoted by a capital letter and a recessive gene denoted by small letter.
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