Human Genetics and Karyotyping: Did you know about the ACHOO syndrome, also known as the photic sneeze reflex? People with this syndrome start to sneeze when they are suddenly exposed to light after spending some time in darkness. You can observe this outside the theatre. As soon as people come out of the theatre after the movie, some people start to sneeze, and this is because they have genes responsible for the trait which they have inherited from either of the parents.

Male pattern baldness is hereditary that causes men to lose hair in a well-recognized pattern. If both the parents have O positive blood group, the child will certainly be O-positive. It’s fascinating how we can predict these traits simply by noticing a few characteristics of an individual. Also, it makes you wonder if we can diagnose a genetic condition? Well, the key to all this information is hidden in our genes and chromosomes, and we can access the information by a rather complex process called karyotyping.  How did karyotyping and human genetics bring a revolution in biological science?  Read along to learn.

Study Deviations From Mendelian Genetics

Human Genetics Definition

Human genetics can be defined as a tool to understand the theoretical framework for understanding the pattern of inheritance of the human species. It is a rapidly growing branch of science. New insights into the biochemical basis of heredity and the development of human cytogenetics have led to Improvement in our knowledge and awareness of the genetic cause of human diseases.  This knowledge could be used to refine diagnostics and help to find new therapeutic approaches, even prevent genetic diseases.

Genetic Variation and Patterns of Inheritance

Everyone is different, unique, with different faces, heights, hair, and skin colour. This difference is caused due to differences in genetic makeup, also called genetic variation. Despite having so many genetic variations, people look similar to their parents and other family members in some ways. Characters can be inherited from generation to generation. But the transfer of characters is not random, but there are few patterns of inheritance.

1. Autosomal Dominant Inheritance

Autosomal dominant traits are those where a single copy of a mutated gene located in autosomes is required to affect an individual. If a trait is autosomal dominant, it will occur in every generation. If one of the parents is affected with the autosomal dominant trait, there is a 50% chance that the child will be affected too. Example: Huntington’s disease, Myotonic dystrophy.

Fig: Autosomal Dominant Inheritance

2. Autosomal Recessive Inheritance

The autosomal recessive disease is one where both copies of a mutated gene located in the autosome are required to affect an individual. If only one copy of a gene is present, the person becomes a carrier but does not affect the individual. Since the trait is recessive, it does not appear in every generation typically. If one of the parents is affected by this, there is a 25% chance that the child will be affected, 50% will be a carrier, and 25% will be unaffected. Example: Sickle-cell anaemia, Phenylketonuria.

Fig: Autosomal Recessive Inheritance

3. X-linked Dominant Inheritance

Sex-linked dominant traits are those present on the sex chromosomes (X or Y). Since it is sex-linked, it has a different impact on different gender. X – linked dominant traits can be seen in both males and females of the same generation. For example, if the father carries the abnormal X gene, there is a 100% chance that the daughter will be affected, but none of his sons will be affected. And, if the mother carries the abnormal X gene, there is a 50% chance that the child, male or female, will be affected. Example: Rett syndrome.

Fig: X- Linked Dominant Inheritance

4. X-linked Recessive Inheritance

Males are more frequently affected. Sex-linked recessive diseases affect mostly males. A male with a mutation in a gene on the X chromosome is typically affected by the condition as they possess only one X chromosome. If the mother is a carrier of the affected gene, chances are 50% of the male children will be affected and 50% normal. 50% female children will be the carrier and 50% normal. It can be seen in males of every generation. Example: Haemophilia, Colour Blindness.

Fig: X- Linked Recessive Inheritance.

5. Mitochondrial/ Cytoplasmic Inheritance

Mitochondria has its own genome, and it affects the phenotype of an individual. These genes can only be passed by the mother to the next generation as these are present in the cytoplasm of the oocyte. Mitochondrial genes can affect both males and females of every generation. For example, if the mother has a faulty gene but the father has normal genes, it will affect all of her progeny. While if the father has the faulty gene and the mother is normal, it will not affect any of the progeny.

Fig: Cytoplasmic Inheritance

Application of  Human Genetics

Knowledge of human genetics can be applied in two main ways: Medical genetics and genetic counselling.

1. Medical Genetics

It is a branch of practical genetics that involves applied genetics to prevent and manage the genetic disease. We have not found any cure for genetic diseases, but given the understanding of the pattern of inheritance, it can be prevented or managed. The management can be done before pregnancy, during pregnancy, or after the birth of a baby. During pregnancy, amniocentesis or chorionic villus sampling can be done to know the genetic conditions of a baby. After birth, precautionary measurement and treatments like gene therapy can minimise the symptoms of genetic disease.

2. Genetic Counselling

It is a communicative approach of genetics that considers science and psychology to deal with risk management due to someone’s genetic condition. Genetic counsellors work on the familial history, karyotype, and biostatistics to calculate the chance of occurrence of a disease and even the age of onset of a condition. Couples can opt for genetic counselling and learn the chances of having an affected child.

Karyotyping

Human genome karyotyping is defined as the process of sorting and pairing all the chromosomes in a given cell, and the karyotype is a photograph obtained after the process. Chromosomes are sorted based on type and structure to make a karyotype. Karyotype provides a genome-wide snapshot of an individual’s chromosomes. A Karyotype can tell us about the number, size, position of the centromere, and features of the sex chromosome.

Process of Karyotyping: The purpose of karyotyping is to obtain images of chromosomes, so dividing cells are preferred with stable metaphase; the nucleus is stained and photographed to obtain the images of the chromosome. This image comes with scattered chromosomes, which are later arranged manually or using computer tools. 

The process of karyotype follows a standardised procedure to reveal characteristic structural features for each chromosome. Stains like Giemsa-stain are used to obtain banding patterns on chromosomes. Stained chromosomes present with alternating black and white bands, unique to each chromosome and can give information about genetic abnormality. Chromosomes are classified into seven groups, A to G, by the length and centromere position.

Fig: Karyotype (female)

The karyotype is often confused with Idiogram, which is incorrect. Idiogram is a schematic diagram of a karyotype that illustrates all chromosome maps.

Fig: Ideogram of human chromosomes

Significance of Karyotyping

1. It can be used in clinical diagnosis.
2. The karyotype tells about the structure of each chromosome and determines the genetic changes in an individual. A karyotype can detect both structural and numerical anomalies in an individual.
3. Karyotypic analysis can give information like detection of birth defects, genetic disorders, and even some forms of cancers.

Uses of Karyotyping

1. To detect the chromosomal mutation.
2. To detect aneuploidy condition.
3. Diagnosis of specific congenital disabilities, genetic disorders, and even cancers.
4. Forensic testing.
5. Paternity testing.

Summary

Human genetics is defined as the study of patterns of inheritance and genomes in humans. We got a better understanding of human genetics after the human genome project. Every human share about 99.9% genomic similarity. Human traits and characters are affected by both nuclear as well as mitochondrial genomes. The pattern of inheritance can be autosomal dominant, autosomal recessive, or sex-linked. Different patterns of inheritance can be studied to determine various characters of human genetics like diseases etc. 

Human genome karyotyping is one of the tools to study the chromosomes of an individual. A karyotype can tell about structure, number, and genetic anomalies. Karyotyping is a highly specialized process that involves staining to obtain banding patterns and sorting the chromosomes based on structure and types. The karyotypic study can give us information that helps identify paternity,  forensics, and even genetic anomalies. Genetic diseases can not be treated but can be managed with the help of human genetics and genetic counselling.

Frequently Asked Questions (FAQs) from Human Genetics and Karyotyping

Q.1. What is human genetic variation?
Ans: Human genetic variation is defined as variation present among the human population. This is caused due to polymorphism in genes.

Q.2. Why is human genetics important?
Ans: Human genetics provides us with the resources to understand human disease, diagnosis, and even prevention. It helps in improving community health.

Q.3. What type of cells are used for karyotyping?
Ans: Karyotype can be studied only in a dividing cell; hence, cells that are naturally dividing are used for karyotyping. For example, human chorionic villus samples,  bone marrow, etc. In case these samples are difficult to find, lymphocytes can be taken and induced to divide.

Q.4. What happens if a karyotype test is abnormal?
Ans: Abnormal karyotype results may indicate the presence of a genetic anomaly in an individual.

Q.5. What is the difference between autosomal and sex-linked traits?
Ans: Gene for autosomal traits are those which are present on autosomes. These affect every individual. Gene for sex-linked traits is present on the sex chromosomes, naturally dividing cells, producing gender-specific impact.

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