L15 - Inheritance of simple and complex traits Flashcards
What are the early models of inheritance?
It has long been known that we can selectively breed traits. First models of hereditary e.g. Hippocrates predicted inhertiance of acquired characteristics. Problems: how can traits not yet expressed be inherited e.g. baldness. Parts of the body that are lost due to accident are still present in offspring. Aristotle proposed that the potential for producing traits is inherited, not the traits themselves.
What are transmission genetics?
They deal with the manner in which genetic differences among individuals are passed from generation to generation. The selection of favourable characteristics has existed for many years. Native americans selected corn with big ears and soft kernels and people in the Eurasian steppes selected horses for their temperament, making them suitable for riding or pulling.
What was Darwins early model of inheritance?
Predicted that ‘gemmules’ are particles of inheritance passed down to offspring. Predicted ‘blending inheritance’. Problems: pops become more homogenous over time, rare beneficial traits may be lost. Variants have no opportunity to increase in frequency even if they survive and reproduce more, because they gradually disappear over time.
What was Modern transmission genetics? Mendelian inheritance?
Gregor Mendel: experiments on peas 1856-64. Hybridised different pea varieties and characterised seven simple traits. Revolutionary discovery that proved blending inheritance was not the correct model: it is genes (not traits) that are transmitted in inheritance.
What experiment did Mendelian conduct?
Hybridisation (interbreeding) stduies involving two varieties of the pea plant, focusing on seven contrasting traits: colour of seeds, shape of seeds, colour of pods, shape of pods, colour of flower, position of flowers, and plant height. He studied strains of plants that were true breeding for each trait, meaning that the physical appearance of the offspring in each successive generation is identical to the previous one.
What is an crossing experiment example?
Trait: yellow vs green peas. P1 generation = parental generation. F1 generation = first generation of offspring. The trait appears in the F1 generation is dominant. The trait that does not appear is recessive. F2 generation = second generation of offspring.
What is mendelian inheritance at the genetic level?
P1 generation - AA produces gametes with ‘A’, aa produces gametes with ‘a’. F1 generation - Aa; 1/2 gametes have ‘A’, 1/2 have ‘a’. Use a punnett square to work out expected ratios of different genotypes in the F2 generation.
What is mendelian inheritance at the genetic level? - Chromosomes?
Segregation reflects the separation of homologous chromosomes during anaphase 1 of meiosis. Copies of A allele in replicated chromosome. Copies of a allelle in replicated homologous chromsome.
What is mendelian inheritance at the genetic level? What is the principles of transmission genetics?
They are statistical and are stated in terms of probabilities. E.g. AA x aa, probability of getting Aa = 1, probability of getting AA = 0. Additions rule: when possibilities are mutually exclusive, add the probabilities. E.g. Aa x Aa, probability of a single offspring being a homozygote (AA or aa) is 1/4 + 1/4 = 1/2. Multiplications rule: when outcomes can occur simultaneously, and the occurence of one doesn’t affect the other, multiply the probabilities. E.g. Aa x Aa, probability of two offspring both being aa is 1/4 x 1/4 = 1/16
What is independent assortment (Mendelian inheritance)?
Usually occurs: gametes segregate without being affected by other alleles. Hereditary transmission of either gene has no effect on the hereditary transmission of the other. Expected phenotypic ratio of F2 gen is 9:3:3:1
What does independent assortment reflect in gametes?
Independent assortment of genes in different chromosome reflects the fact that nonhomologous chromosomes can orient in either of two wats that are equally likely.
What happens with genes on the same chromosome in Mendelian inheritance?
Crossing over during meiosis cause recombination and means that genes can be inherited independently. If genes don’t assort independently, they show linkage disequilibrium. E.g. if they are close together on the same chromosome. Or if they are sex-linked (X & Y chromosomes have very little recombination)
What are the complications with Mendelian inheritance?
Most traits are not so simple. Incomplete dominance occurs e.g. colouration in snapdragons. Instead of C = dominant, c = recessive, we use superscripts R (red) and W (white) to indicate the alleles, as neither allele is dominant to the other. In this case the genotype ratio and the phenotype ratio are both 1:2:1, because each genotype has a distinct phenotype.
What are the complications with Mendelian inheritance? - What is Epistasis?
The interaction of genes affecting the same trait. The white leghorn is white because the inhibitor allele/blocks expression of the pigment allele C. The white wyandotte is white because the pigment allele c does not produce pigment. Genotype of the form C-ii have coloured feathers, whereas all other genotypes have white feathers. The result is an F2 ratio of white: coloured of 13:3, which is a modified form of the expected 9:3:3:1.
What can incomplete penetrance and variable expression obscure?
Patterns of inheritance.
What is incomplete penetrance?
Trait not always expressed due to environmental effects or effects of other genes e.g. alleles that increase breast cancer risk don’t always lead to cancer.
What is variable expressivity?
The trait is always expressed but the severity varies e.g. alleles that reduce lung elasticity are likely to lead to lung disease if the person expressing them smokes.
What are pedigrees useful for?
Useful to understand inheritance of traits. Pedigree of a dominant allele for Brachydactyly (middle finger bone doesn’t grow). Pedigrees of families affected by brachydactyly suggest that the condition is a dominant trait. Equal likelyhood for males and females. One affected parent leads to half of offspring affected.
What does pedigree of a recessive allele for albinism show?
Albinism, is a condition that results in a reduced amount of melanin pigment produced in the skin, hair, and eyes. In a recessive trait pedigree: the trait may skip one or more generations, females and males are equally likely to be affected, individuals may have unaffected parents, affected individuals often result from mating between relatives, typically first cousins.
What is X-linked haemophilia in the European royal family?
X-linked traits might not skip generations, even if recessive (males only have one copy). Their inheritance can still be mapped out using a punnet square. Hemophilia results from a recessive mutation in a gene encoding a protein necessary for blood clotting. Affected individuals bleed excessively even from minor cuts. Affects 1 in 7000 males, famous due to its presence in many members of European royal family, from Queen Victoria, who was a heterozygous carrier of the gene.
What about more complex traits?
E.g. high blood pressure, obesity, diabetes, and depression. AKA quantitative traits - measure along a continuum - very common. Quantitative genetics tries to uncover the underpinnings of complex traits.
What application is there?
In many fields in biology
What about more complex traits - Many genes involved?
As many genes affect complex traits, the phenotype and genotype ratios observed by Mendel do not occur. No. of genes affecting complex traits is usually so large that different genotypes can have very similar phenotypes, making it difficult to see the effects of individual genes on a trait. In a few traits, the effects of the environment are minor and the number of genes is small, so the basis of the trait can be analysed. E.g. Seed casing in wheat, studied by Herman Nilsson-Ehle. His experiment demonstrated that complex traits are subject to the same laws that Mendel worked out for single-gene traits, but that the inheritance patterns are more difficult to see because of the number of genes involved. Here, three unlinked genes, each with two alleles, influence the intensity of red colouration of the seed casing in wheat.
What can be the difference in complex traits?
Some genes have relatively large effects, while others have small effects. For the majority of genes that contribute to a complex trait, the magnitude of their individual effects is typically quite small.
Which genes are involved?
Genomic-wide associations studies: genetic variants associated with critical illness in COVID-19. Through DNA sequence polymorphisms across the genome of many individuals, we can investigate which regions of the genome are associated with the trait we are interested in.
What often impacts quantitative traits?
The environment, variation can be created even when genes are identical. E.g. inadequate nutrition is linked to slow growth, salt intake is associated with an increased likelihood of high blood pressure, a junk-food diet high in fat and carbohydrates is a risk factor for obesity and diabetes. These enviro effects are seen in true-breeding (homozygous) strains, also called inbred lines.
How do genes interact with the environment?
Two inbred lines of mice, normal diet: both grow to be the same size, high-fat diet: A becomes obese but B doesn’t. Obesity in Line a results from a genotype-by-environment interaction. Genotype-by-environment interaction, implies interplay between genes and the enviro is difficult to predict.
What is an example of how genetic and environmental effects can interact in unpredictable ways?
Two strains of corn were introduced to two different nitrogen environments. A. strain 1 had little variation in its yield in the higher nitrogen environment. B. strain 2 increased its yield dramatically. This type of interaction implies that the effects of a genotype cannot be specified without knowing the environment, and vice versa. Therefore, no genotype is the best across a broad range of enviros.
What is Galton’s rule?
Regression towards the mean, occurs because of 1 recombination during meiosis breaks up gene combinations (2) environmental effects. Galton studied complex traits such as height, strength, and physical characteristics in humans. He concluded children of tall parents are taller than the average pop but shorter than their parents on average. Children from shorter parents have offspring that are, on average, taller than their parents but shorter than the average population.
