You run an assisted reproduction clinic. Pam and Martin have a child with cystic

Question

You run an assisted reproduction clinic. Pam and Martin have a child with cystic fibrosis and only want to have a second child if they can be assured that the child will not have cystic fibrosis. You genotype Pam and discover that she is a heterozygote for the most common mutation causing cystic fibrosis. (Pam’s genotype is Ff, where F=normal allele of the CFTR gene and f=mutant allele of the CFTR gene. Cystic fibrosis is inherited in an autosomal recessive manner.)

You suggest that the couple consider first polar body testing, in which several unfertilized oocytes (each with its first polar body) are retrieved from Pam.
Asymmetrical cell division leads to the production of polar bodies during egg formation. To conserve nutrients, the majority of cytoplasm is segregated into the secondary oocyte during meiosis I, then into the egg during meiosis II. The remaining daughter cells generated from the meiotic events are small and contain relatively little cytoplasm; they are referred to as polar bodies. Eventually, the polar bodies degenerate.

Note the position of the first polar body in the diagram above. You will be discussing tests on the first polar body in quiz section.

To see how first polar body testing can help to determine which eggs should be used for in vitro  fertilization, let’s start by modeling meiosis in Pam. Pam is heterozygous for mutations in the CFTR gene and two others that are important for human health:

Part A

Pam is a carrier for hemochromatosis, deafness, and cystic fibrosis. What is her genotype (using the allele notation from the table) and phenotype?

Part B

Look at each replicated chromosome in your drawing on the hard copy. Each replicated chromosome comprises a pair of sister chromatids. The following questions refer to any single pair of sister chromatids.

Are the sister chromatids different from each other? Do the sister chromatids have the same set of genes? Do they have the same alleles?

Part C

The term “homologous chromosome” describes the relationship between Pam’s maternally and paternally derived chromosomes. The following questions refer to any pair of homologous chromosomes in your drawing.

Are the homologous chromosomes different from each other? Do the homologous chromosomes have the same set of genes? Do they have the same alleles?

Primary oocyte Meiosis I division Secondary oocyte O First polar body Meiosis II division Meiosis II division Egg Polar bodies

Explanation / Answer

Part A

For a autosomal recessive disease both the alleles must be recessive alleles, only then the idividual will be diseased otherwise it would be carrier only. Since Pam is carrier for all three diseases which are autosomal recessive diseases hence her genome carries one recessive allele and one dominant allele for all three diseases. Hence her genotype would be HhEeFf and she is a carrier for disease (phenotype) and hence healty individual.

Part B

Sister chromatids are made from copying the same chromosome hence they are same only in context of set of genes in them and alleles as well. This is the case with mitosis. If meiosis is occuring then there occurs homologous recombinationr non homologous recombination which can change the genes and alleles.

Part C

Yes homologous chromosomes are different they are two copies which differ slightly in terms of alleles present in them. One pair of chromosomes is received from father and another one from mother, these two are later called homologous chromosomes and hence they can be different because of their origin from two different individuals but the genes are same on both the copies because unique portion of DNA or chromosome carries same gene.

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