Table of Contents
Human Genetics: What is that?
As part of genetics, human genetics deals with the genetic constitution of human beings. In this area, medical diagnostics and molecular biological research work closely together. In pre-clinic semesters, the basic concepts, the different inheritance traits, and pedigree analyses are what is most important for you.
Symbols of Genetics
Autosomal Inheritance: Dominant and Recessive
In the autosomal inheritance traits, the genetic information is located on the chromosomes 1—22. Thus, inheritance is not gender-specific.
Autosomal Dominant Inheritance
If traits are inherited as autosomal dominant, they may be expressed in the heterozygous or in the homozygous state. An organism can be homozygous dominant if it carries two copies of the same dominant allele, or homozygous recessive if it carries two copies of the same recessive allele. Heterozygous means that an organism has two different alleles of a gene. Statistically, the most frequent distribution is the inheritance from one parent to half of the children (in case of complete penetrance).
An allele is one of the possible forms of a gene. Most genes have two alleles, a dominant allele, and a recessive allele. If an organism is heterozygous for that trait or possesses one of each allele, then the dominant trait is expressed.
Probability for children to express the dominant trait: 50 – 100 %.
Examples of Combination
- One parent is heterozygous for the trait: Aa x aa = 50 % Aa + 50 % aa
- One parent is homozygous for the trait: AA x aa = 100 % Aa
If the trait is the trigger for disease, homozygous individuals are often affected more severely than heterozygous ones. In the pedigree, autosomal-dominant diseases can be tracked over generations since, in every generation, persons are diseased. (Exceptions are, e.g., achondroplasia and Marfan’s syndrome due to de novo mutations.)
Autosomal Dominant Diseases
- Polydactyly (more fingers than usual)
- Brachydactyl (unusually short fingers)
- Ectrodactyly (lobster-claw hand or split foot)
- Achondroplasia (disproportioned dwarfism): Disturbed cartilage formation. The absence of growth plates in the bones results in disproportioned dwarfism with a normal length of the torso and markedly short extremities. In 80 % of the cases of achondroplasia, a de novo mutation is the cause.
- Marfan’s syndrome: Mutation in the gene for fibrillin with the disturbed synthesis of connective tissue.
- Huntington’s disease: Causes are triplet (CAG) repeat expansions, which have destructive consequences for the neurons of the basal ganglia in the brain. Penetrance depends on the number of triplet repeats: With > 60 CAG triplets, Huntington’s disease already occurs in the juvenile age.
In autosomal-recessive inheritance, the carrier has to be homozygous for the trait to be inherited. Heterozygous people act as carriers, but they do not express the respective phenotype. A phenotypically diseased person has to have the recessive gene in both parents.
Examples of Combination
- If a parent is homozygous and healthy and the other one is heterozygous for the trait, all children will be phenotypically healthy, but half of them will be carriers. Aa x AA = 50 % Aa + 50 % AA
- If both parents are heterozygous for the trait, 25 % of the children will be diseased, 50 % will be heterozygous carriers, and 25 % are homozygous and healthy. Aa x Aa = 25 % aa + 50 % Aa + 25 % AA
In pedigrees, autosomal-recessive diseases are not as easy to track as autosomal-dominant diseases. Several generations can be without the trait before someone becomes diseased again. As you can imagine, the offsprings from marriages between related persons are affected more often since the chances that both parents carry the recessive gene is increased.
- Albinism: Disturbed biosynthesis of melanin due to absent tyrosine hydroxylase. Results in lack of pigments: very white skin and hair and high sensitivity to UV light.
- Phenylketonuria: Phenylalanine cannot be degraded due to lack of phenylalanine hydroxylase and it accumulates in the organism. Consequences are very severe psychomotor retardations. Treatment: a diet low in phenylalanine!
- Cystic fibrosis: Trinucleotid deletions on chromosome 7, which codes for the CFRT transporter (chloride transporter). Chloride ions cannot be transported and added to secretions, which remain viscous with severe clinical consequences for the affected patients (among other things, recurrent respiratory infections, and malabsorption disorders). All exocrine glands are affected.
An autosomal-recessive trait that does not represent a disease is blood type 0.
Gonosomes: Important Facts on Chromosomes X and Y
In the human set of chromosomes, there is a pair of chromosomes that is not homologous: the gonosomes X and Y. They determine the genetic gender.
The following table provides an overview of the properties and functions of the Y-chromosome and X-chromosome.
|Few information (location of very few genes)||Rich in information (regulation of many functions of the organism)|
|Most important gene of the Y-chromosome determines gender (sex determining region of Y SRY)||Double dosage of X-chromosomal localized genes|
|SRY induces the development of the testes, production of testosterone||Dosage compensation mechanism: Compensation through irreversible inactivation of one of the two X-chromosomes >> Barr bodies (sex chromatin) at the edge of the nucleus|
|Sexual maldevelopments: XX man: During meiosis, the SRY gene is translocated to other chromosomes >> male phenotype despite missing Y-chromosome, sterility XY woman: SRY inactivation due to mutation >> secondary gender development does not occur, possible infertility||Genetic mosaics: In 50 %, the maternal X-chromosome is active in the female organism; in 50 %, the paternal X-chromosome.|
|Testicular feminization: a defective receptor for testosterone >> XY person with female phenotype, sterility||Reversion of the inactivity in the germline|
Gonosomal Modes of Inheritance
Traits bound to the gonosomes are present if the respective trait is located on one of the two gender chromosomes.
X-Linked Dominant Inheritance
The X-linked dominant inheritance trait is a very rare inheritance mode.
Examples of Combination
- Father is a carrier of the mutated allele: All sons are healthy, all daughters are diseased. Xy x xx = 50 % xy + 50 % Xx
- Mother is a carrier of the mutated allele: 50 % of the sons, 50 % of the daughters are diseased. xy x Xx = 25 % Xy + 25 % xy + 25 % Xx + 25 % xx
X-Linked Dominant Diseases
- Hypophosphatemia (vitamin D-resistant rickets)
- Rett’s syndrome
- Alport’s syndrome
X-Linked Recessive Inheritance
By far more frequent is the X-linked recessive inheritance modus. Men are considerably more frequently affected than women concerning the presentation of the trait.
Examples of Combination :
- If a man has the mutated allele, he is phenotypically diseased and gives this allele to his daughters. If the mother is homozygous and healthy in this case, her daughters will only become carriers. The father’s allele is not given to the sons, who are neither diseased nor carriers. xY x XX = 50 % xX + 50 % Xy
- If the mother is a heterozygous conductor, 50 % of her sons will be diseased. 50 % of her daughters will become carriers. XY x Xx = 25 % xY + 25 % XY + 25 % xX + 25 % XX
- If the man is diseased and the woman is a carrier, 50 % of the sons and 50 % of the daughters will become diseased. The other half of the daughters will become carriers. xY x Xx = 25 % xY + 25 % XY + 25 % Xx + 25 % xx
X-Linked Recessive Diseases
- Hemophilia A (factor VIII deficiency) and hemophilia B (factor IX deficiency)
- Dyschromatopsia (heterogonia)
- Duchenne muscular dystrophy
A very famous genealogical tree showing an X-linked recessive disease is the family tree of Queen Victoria of England. The royal family suffered from hemophilia.
Inheritance of Blood Types
20 different blood type systems are currently existent. The most important ones are the AB0 system, the MN system, and the Rh system.
ABO Blood Type System
The AB0 blood type system is an example for multiple alleles (genetic polymorphism >> read everything about formal genetics). In multiple alleles, there are two alleles of one gene within one population. In each individual, these alleles behave dominantly/recessively or co-dominantly. In the AB0 system, the following alleles exist.
- A for antigen A
- B for antigen B
- 0 for no antigen
A and B are co-dominant concerning each other and dominant concerning 0. They carry antigens in their glycocalyx. Concerning the recessive allele, 0 is concealed and can only be present in homozygosity. The 0 allele does not carry any antigen in the glycocalyx.
MN Blood Type System
In co-dominance, both alleles simultaneously appear in the phenotype (at heterozygosity). In pure co-dominant inheritance, one can always directly deduct the genotype from the phenotype: no recessive gene that could hide is present. An example of this is the MN blood type system. There are two different alleles of a protein in the glycocalyx of the erythrocytes in the MN system.
The following table summarizes the genotypes and resulting phenotypes of the blood type inheritance in the MN system and the AB0 system. These rules of inheritance can be used for an intermittent paternity test. If the blood type constellation between father and child does not fit, paternity can be excluded and further genetic tests are not needed.
|AB0 system||A||AA, A0|
Rh System: the Rhesus Factor
The Rhesus factor is a protein and is inherited dominantly. Rh-negative persons can produce antibodies against the Rh-factor, but only after contact with Rh-positive blood. This leads to problems in pregnancy of an Rh-negative mother with an Rh-positive father. During birth, the Rh-positive blood of the child has contact with the maternal blood.
The mother then produces antibodies against Rh. They are able to pass the placenta in the next pregnancy and agglutinate the Rh-positive erythrocytes in the fetal organism. The consequence is severe jaundice of the child at birth with severe cerebral damages.
|Heterozygous Father Rhrh||Homozygous Father RhRh|
|Complications develop in 50 % of the offspring||Complications develop in 100 % of the offspring|
After the birth of the first child, the mother is injected with a high dose of Rh-antibodies. They mask the child-Rh-antigens and make for a quick elimination and, as a result, no maternal Rh-antibodies are produced.
Genomic Imprinting: Marking According to Paternal Origin
Everything you have learned so far shows that most genes are present twice: one of the mothers and one of the father. According to Mendel’s rules, dominant alleles determine the phenotype. For some sorts of inheritance, however, it has been observed that it is crucial from which parent the affected allele was inherited.
The activity of one of the two genes is significantly decreased. This marking of paternal genes is referred to as genomic imprinting. The imprinting occurs in the development in the germ cells. In about 20 genes, genomic imprinting could be verified.
Genomic Imprinting occurs over again in each generation, the inherited imprints are deleted.
Deletion in the Proximal Section of the q-Arm of Chromosome 15
In this example, the genomic imprinting is illustrated. If the child inherits the defect chromosome from the father, the child has Prader-Willi syndrome (among others with mental retardation, dwarfism, obesity). If the child inherits the defect chromosome from the mother, the child develops the so-called Angelman syndrome (among others disproportion of the skull and the face, uncontrolled laughing attacks).
Exercise for Risk Calculation in Human Genetics
Question: Two brothers suffer from X-chromosomal-recessively inherited the disease. How great is the risk for their niece (the daughter of the sister) to be a heterozygous conductor?
Solution: Both brothers must have inherited the mutated allele from their mother. As a healthy conductor (Xx), she gives the allele to her daughter with a probability of 50 % (p = 1/2). She then gives the mutated allele with a probability of once again 50 % (p = 1/2) to her children (her husband is healthy and is, thus, not involved in the calculation). The probability for her niece to be affected is calculated via the product of the individual probabilities: ½ x ½ = ¼ = 25 %
Source: G. Poeggel (2009): Kurzlehrbuch Biologie, p.99. Thieme Verlag.