Question

In a human genetic study, a family with five phenotypically normal children was investigated. Two children were "homozygous" 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?

Short answer

In summary, the parents of the phenotypically normal children have 45 chromosomes each, being heterozygous for a Robertsonian translocation between chromosomes 19 and 20. The children's chromosome compositions are a result of their parents' genetic makeup, and the stillbirths could be due to monosomy or trisomy of chromosomes 19 or 20. The fact that the parents are first cousins is significant because it increases the likelihood of both parents sharing the same genetic abnormality, resulting in the observed chromosome compositions in their children.

Step-by-step solution

Analyze the information about the children's chromosome compositions

We have the following information about the children's chromosome compositions: - 2 children are homozygous for a Robertsonian translocation between chromosomes 19 and 20 and have 44 chromosomes. - 3 children are heterozygous for the translocation and have 45 chromosomes.

Understand the concept of Robertsonian translocation

Robertsonian translocation is a type of chromosomal rearrangement that occurs when the long arms of two acrocentric chromosomes (chromosomes with the centromere near one end) fuse to form a single chromosome. In this case, the Robertsonian translocation is found between chromosomes 19 and 20.

Determine the parents' chromosome compositions

As we know, humans usually have 46 chromosomes. In this case, we have: - Parents are first cousins. - At least one parent must have the translocated chromosome (19;20) to pass it on to their children. Considering these facts and the information about the children's chromosome compositions, we can determine that: Parent 1 has one normal copy of chromosome 19, one normal copy of chromosome 20, and one translocated chromosome (19;20) - a total of 45 chromosomes. Parent 2 has the same chromosome composition as Parent 1 - one normal copy of chromosome 19, one normal copy of chromosome 20, and one translocated chromosome (19;20) - a total of 45 chromosomes. This means that both parents are heterozygous for the Robertsonian translocation.

Identify the probable reasons for the stillbirths

A possible reason for the stillbirths could be the formation of a non-viable zygote with either: - Monosomy (missing one copy) of chromosomes 19 or 20 - Trisomy (having an extra copy) of chromosomes 19 or 20 These chromosomal abnormalities could result from unbalanced gametes (containing abnormal combinations of chromosomes) produced by the parents during meiosis due to the presence of the Robertsonian translocation.

Explain the significance of the parents being first cousins

The fact that the parents are first cousins is a key piece of information because it increases the likelihood of both parents sharing the same genetic mutation or abnormality. In this case, it increases the chance that both parents have the Robertsonian translocation and thus pass it on to their offspring, resulting in the observed chromosomal compositions of the children.

Key concepts

Chromosome Compositions

Understanding chromosome compositions is crucial when delving into genetic studies, particularly when it comes to identifying genetic anomalies. In humans, the standard chromosome composition consists of 23 pairs, totaling 46 chromosomes. These include 22 pairs of autosomes and one pair of sex chromosomes (XX or XY). Chromosomes are made up of DNA and are found in the nucleus of every cell. They are critical carriers of genetic information.

In the context of the exercise, we come across an atypical chromosome composition resulting from a Robertsonian translocation. This rare form of chromosomal rearrangement involves the fusion of two acrocentric chromosomes — in this case, chromosomes 19 and 20. This leads to a reduced chromosome count because the fusion forms one composite chromosome instead of two separate ones.

When dealing with Robertsonian translocations, it's important to analyze the genetic implications thoroughly. Carriers of a Robertsonian translocation may be phenotypically normal, as seen in two homozygous children from the exercise who have only 44 chromosomes but maintain a complete genetic complement. This translocation can complicate the process of meiosis, potentially leading to offspring with unbalanced chromosome sets, which may explain the stillbirths in the family studied.

Human Genetic Study

Human genetic study involves analyzing the hereditary patterns and genetic variations among humans. Genetics help to understand the complex nature of inherited traits and the occurrence of genetic disorders. In the exercise, the study of a family with a translocation sheds light on the inheritance patterns and possible complications that may arise from such genetic rearrangements.

Genetic studies often involve tracking and interpreting how chromosomes are passed down through generations. Since genetic information is inherited from both parents, it's essential to identify their chromosomal compositions to understand the genetic outlook for their children. The revelation that the parents are first cousins in the exercise intensifies this study as it implies a higher probability of shared genetic heritage and, therefore, shared genetic anomalies.

The significance of the parents' relationship as first cousins cannot be understated in this context; it suggests a higher likelihood of the children inheriting the same chromosomal abnormality, as was the case with the Robertsonian translocation.

Chromosomal Abnormalities

Chromosomal abnormalities are variations in the number or structure of chromosomes and can lead to developmental issues or genetic disorders. These abnormalities include aneuploidies (an abnormal number of chromosomes) and structural changes, such as translocations, deletions, or duplications.

In the given exercise, the stillbirths were likely caused by chromosomal abnormalities resulting from unbalanced gametes produced by the parents. The presence of the Robertsonian translocation can lead to gametes with improper chromosomal sets, either missing or gaining a critical chromosome. For example, monosomy (the absence of one chromosome) or trisomy (one extra chromosome) can be fatal, as the delicate balance of genetic material is disturbed.

Risks associated with such abnormalities are elevated in cases of consanguinity, like the one highlighted where the parents are first cousins. Genetic counseling is often recommended to such families to better understand these risks and aid in family planning. Knowledge of these chromosomal abnormalities also contributes to advancements in medical interventions and prenatal diagnosis.