In a human genetic study, a family with five phenotypically normal children was investigated. Two children were "homozy. gous" for a Robertsonian translocation between chromosomes 19 and 20 (they contained two identical copies of the fused chromosome). They have only 44 chromosomes but a complete genetic complement. Three of the children were "heterozygous" for the translocation and contained 45 chromosomes, with one translocated chromosome plus a normal copy of both chromosomes 19 and \(20 .\) Two other pregnancies resulted in stillbirths. It was later discovered that the parents were first cousins. Based on this information, determine the chromosome compositions of the parents. What led to the stillbirths? Why was the discovery that the parents were first cousins a key piece of information in understanding the genetics of this family?

Step by step solution

01

Determine the possible genetic compositions of the offspring

The children have three possible chromosome compositions: - 44 chromosomes - homozygous for the Robertsonian translocation - 45 chromosomes - heterozygous for the translocation - 46 chromosomes - without the translocation Based on the information provided, we know that there are two children with 44 chromosomes and three children with 45 chromosomes in this family.

02

Recognize that the parents must be carriers of the translocation

Since there are children with the translocation, it means that one or both parents must be carriers of the translocation. The fact that two children are homozygous and three are heterozygous for the translocation further indicates that both parents are likely carriers.

03

Determine the parents' chromosome compositions

Given that both parents are carriers of the Robertsonian translocation, their chromosome compositions must include one translocated chromosome and one normal chromosome of each type (19 and 20). Thus, both parents have 45 chromosomes: one translocated chromosome plus normal copies of chromosomes 19 and 20.

04

Explain the stillbirths

The stillbirths likely occurred due to zygotes having mismatched sets of chromosomes, leading to developmental issues. One possibility is that the zygotes had two normal chromosomes 19 and 20 each, and two translocated chromosomes as well, leading to a total of 47 chromosomes. Such an abnormal complement of chromosomes would result in failed pregnancies or stillbirths.

05

The significance of first cousins

The fact that the parents are first cousins is important because it means they share a significant proportion of their genes, increasing the likelihood of inheriting the same rare genetic variant, such as the Robertsonian translocation. If they were not closely related, the chances of both being carriers of the translocation would be much smaller. This information is key in understanding the family's genetics.

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