#polygenic inheritance

Me and six other parents within my daughters dance team organized a carpool group. Yesterday was my day to provide shuttle for the girls. An hour long ride in a vehicle full of girls, ages ranging from 11 to 14, was pretty interesting. The girls weren’t even in the truck for 30 seconds before Hannah turns to me and says, “Mom, aren’t you my mom?!”

 I knew what this was about. So, I smiled, turned…

Multi-factorial traits with discontinuous distribution of phenotypes (result of many factors, genes and environment but phenotype is either present or not e.g blood group either A, B, AB or O)

e.g spina bifida

usually a threshold between a combination of genetic predisposition and accumulating environmental factors which once reached the trait shows

can have different threshold for males and females so more common in the sex with the lower threshold

  Quantitative traits with continuous distribution of phenotypes e.g height, volume of milk, thickness of wool etc

Polygenic inheritance

many gene loci contributing to the phenotype of the quantitative trait

each locus has two alleles eg., A1 & A2

no dominant phenotype

alleles are inherited in a similar way to any other trait

allele A1 adds to trait contributor

allele A2 does not add to trait non contributor

the effect of the alleles is cumulative

eg., If A1 adds 1cm to height then A1A1 = 2cms

looks like incomplete dominance from F1 but F2 shows difference  

As the number of genes controlling a phenotypic character increases the differences between phenotypic classes becomes less distinct.

To calculate number of polygenes: (1/4)^n = proportion of F2 that resemble the parents, where n = no. polygenes.  NB: Only use one of the groups that resembles the parents 

Heritability: A statistical measure of how strongly the phenotype of the offspring will resemble the phenotype of the parents.

Sandra Laing

Sandra was born a black baby to white Afrikaner parents in 1955. The implications of this in apartheid South Africa unravelled like a bad fairy tale. When apartheid ended, she became a curiosity, a minor celebrity, a remnant of South Africa’s shame.

She is a “throwback”, or, to be precise, “of polygenic inheritance”. Her parents were pro-National Party and members of the Dutch Reformed Church. What happened to Sandra Laing is such an example of apartheid at its most cruel that her story is taught as a chapter in the history of South Africa at matriculation level, the final year of senior school. Her story has now been made into a mesmerising film, Skin, in which Sophie Okonedo, herself of mixed race, shimmers with empathy in the part of Laing.

The implications of looking dark-skinned and fuzzy-haired when your parents moved in an all-white society were huge. Questions were asked of her mother. It was illegal under the Immorality Act for a man and woman of different races to have sex, and Sannie Laing, Laing’s mother, couldn’t explain her child’s appearance.