Mendelian traits in humans

Autosomal dominant

A 50/50 chance of inheritance.

Sickle-cell disease is inherited in the autosomal recessive pattern. When both parents have sickle-cell trait (carrier), a child has a 25% chance of sickle-cell disease (red icon), 25% do not carry any sickle-cell alleles (blue icon), and 50% have the heterozygous (carrier) condition.^[1]

If one parent has sickle-cell anaemia and the other has sickle-cell trait, then the child has a 50% chance of having sickle-cell disease and a 50% chance of having sickle-cell trait.^[1]

Heredity of phenotypic traits: Father and son with prominent ears and hair whorl.

An example of the codominant inheritance of some of the four blood groups.

Mendelian traits in humans concerns how, in Mendelian inheritance, a child receiving a dominant allele from either parent will have the dominant form of the phenotypic trait or characteristic. Only those that received the recessive allele from both parents, known as zygosity, will have the recessive phenotype. Those that receive a dominant allele from one parent and a recessive allele from the other parent will have the dominant form of the trait. Purely Mendelian traits are a tiny minority of all traits, since most phenotypic traits exhibit incomplete dominance, codominance, and contributions from many genes.

The recessive phenotype may theoretically skip any number of generations, lying dormant in heterozygous "carrier" individuals until they have children with someone who also has the recessive allele and both pass it on to their child.

The human Y chromosome is composed of about 59 million base pairs and is passed virtually unchanged from father to son. The mitochondrial DNA (mtDNA) comes only from the mother and is given to both male and female children.

Up to eighteen percent(18%) of children have a different father than what is believed and is called Non-paternity event.^[2] This is discovered when a child develops an illness and the parents genomes are tested for carrier potential.^[3][4][5]

If there are any defective alleles in a human that follow the Mendelian pattern, they may be corrected using the new technology of CRISPR.^{[6] [7] [8] [9] [10] [11]}

Examples

These traits include:

• Wet (dominant) or dry (recessive) earwax - dry is found mostly in Asians and Native Americans^[12]
• Albinism (recessive)
• Brachydactyly (shortness of fingers and toes)
• Blood type
• Cystic fibrosis
• Ectrodactyly
• Hereditary breast–ovarian cancer syndrome
• Hereditary nonpolyposis colorectal cancer
• HFE hereditary haemochromatosis
• Huntington's disease
• Lactase persistence (dominant)
• Leber's hereditary optic neuropathy
• Marfan syndrome
• Retinoblastoma
• Sickle-cell disease

Questionable traits

May be Mendelian but there is conflicting evidence:

• Ability to smell (bitter almond-like) hydrogen cyanide (recessive) ^[13]

Traits previously believed to be Mendelian

Some traits were previously believed to be Mendelian, but their inheritance is likely based on more complex genetic models, possibly involving more than one gene. These include:^[14]

• Eye color
• Hair color
• Morton's toe
• Tongue rolling
• Ability to taste phenylthiocarbamide (dominant) - largely determined by a single gene, TAS2R38, with two common alleles, though there are 8 possible haplotypes^[15] Because it is not a trait where the dominant tastes and the recessive cannot, but rather a continuous gradient in ability to detect PTC, it is not a real example of a simple mendelian trait. This is best exemplified by the fact that two non-tasters (recessive trait) can, in fact, have a child that can taste PTC (dominant trait).^[16]
• Widow's peak (allele)
• Detached (dominant) or attached (recessive) earlobes

Blood group inheritance

Blood groups that children may inherit from their parents.^[17][18]

See also

• Blood type distribution by country
• Consanguinity
• CRISPR
• DNA repair
• Electrophoresis
• Genetic disorder
• Heritability
• Human chromosomes
• Human genetic variation
• Human genetic clustering
• Human inbreeding in royal blood lines
• Punnett square

References

1. 1 2 "Inheritance of Sickle Cell Anaemia - Sickle Cell Society". 
2. ↑ Sussman LN, Schatkin SB (1957) "Blood-grouping tests in undisputed paternity proceedings". JAMA 164:249–250
3. ↑ "Mommy's little secret" The Globe and Mail. Author Carolyn Abraham. December 14, 2002
4. ↑ "Paternity testing: analysis of six blood groups and HLA markers" November 1982
5. ↑ "Sperm Wars: The Science of Sex, biologist Robin Baker, PhD"
6. ↑ "CRISPR gene-editing tool has scientists thrilled — but nervous" CBC news. Author Kelly Crowe. November 30, 2015.
7. ↑ "First Human Embryos Edited in U.S." author Steve Connor. July 26, 2017
8. ↑ Gene editing used to correct sickle cell disease, human trials planned
9. ↑ "Teenager's sickle cell reversed with world-first therapy" March 2, 2017
10. ↑ U.S. scientists are fixing genetic defects in human embryos.
11. ↑ In Breakthrough, Scientists Edit a Dangerous Mutation From Genes in Human Embryos
12. ↑ "Myths of Human Genetics: Earwax". 
13. ↑ "OMIM Entry - 304300 - CYANIDE, INABILITY TO SMELL". 
14. ↑ "Myths of Human Genetics: Introduction". 
15. ↑ Kim, U. K., E. Jorgenson, H. Coon, M. Leppert, N. Risch, and D. Drayna. 2003. Positional cloning of the human quantitative trait locus underlying taste sensitivity to phenylthiocarbamide. Science 299: 1221-1225
16. ↑ McDonald, J.H. 2011. Myths of Human Genetics. University of Delaware. http://udel.edu/~mcdonald/mythptc.html
17. ↑ "ABO inheritance patterns". Inheritance patterns of blood groups. Australian Red Cross Blood Service. Retrieved 30 October 2013. 
18. ↑ "ABO blood group system". Abobloodtypes.webnode.com. Retrieved 2015-02-02. 

Further reading

• Mange, Elaine J.; Mange, Arthur R. (1999). Basic Human Genetics (second ed.). Sunderland (MA): Sinauer. ISBN 0-87893-497-9. Lay summary (16 October 2010). 
• Speicher, Michael R.; Antonarakis, Stylianos E.; Motulsky, Arno G., eds. (2010). Vogel and Motulsky's Human Genetics: Problems and Approaches. Heidelberg: Springer Scientific. ISBN 978-3-540-37653-8. doi:10.1007/978-3-540-37654-5. Lay summary (4 September 2010). 

External links

• OMIM Online Mendelian Inheritance in Man