1a) Consider the topic of familial Down syndrome. A person with a normal phenoty

Question

1a) Consider the topic of familial Down syndrome. A person with a normal phenotype who carries a translocated chromosome can produce _____ gamete types, with respect to chromosome content of the involved chromosomes, and only ____ of these types will produce phenotypically normal offspring with fertilized with a normal gamete.

1b) Allelic variation focuses primarily on differences in specific ______.

1c) What can result in the position effect?

2) An individual that is heterozygous for an inversion has the following chromosomes (• is the centromere):

M N O P Q •R S T U
m n o t   s r • q p u  

Assume that a crossover occurred between R and S. Starting with “upq...” region list the remaining genes in order (NO spaces between the letters) of the chromosome resulting from crossing over. You must use upper and lower-case letters correctly. Don’t include any notation relating to the centromere.

What type(s) of gametes is/are produced if nondisjunction occurs in meiosis II?

Explanation / Answer

1)

Familial Downsyndrome occurs due to Robertsonian translocation in heterozygous carriers of the chromosome. The individuals with Down syndrome can produce offspring with other carrier individual of translocation. Heterozygous individuals who arre carriers of Robertsonian translocation can produce phenotypically normal offspring. A person with normal phenotype and is carrier of translocation can produce viable gametes and only the gametes with heterozygous genotype can produce phenotypically normal individuals.

b)

The alleles are variant forms of a gene. The allelic variation focuses on differences in specific gene. This may result in heterozygous genotype and phenotypic effects may or may not be exhibited distinctly with heterozygous genotype.

c)

Position effect results in enhances or reduced expression of gene after translocation from one position to another. The difference in gene expression is due to enhancers within DNA sequence that regulate expression of adjacent genes after translocation. Therefore, position effect results in change in gene expression.

2)

In heterozygous gene inversion of chromosomes takes place and crossover occurs between R. S

M N O P Q •R S T U
m n o t   s r • q p u

The resultant gene order is upq rst M N O P Q

The nondisjuction during meiosis II may result in both viable and nonviable gametes. This is because the gene is heterozygous for inversion.

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