Genetics Flashcards
Describe Mendel’s experiment and his observations.
○Mendel crossed a tall pea plant with a dwarf pea plant (Klug, et al., 2020, pp.74). The offspring from this cross (F1 generation) were all tall plants, and the dwarf trait had disappeared (Klug, et al., 2020, pp.74).
○He then self-fertilised the F1 generation, and found that the dwarf trait which had disappeared in the F1 generation had reappeared in the F2 generation (Klug, et al., 2020, pp.74).
○The ratio of tall plants to dwarf plants was 3:1 (Klug, et al., 2020, pp.74).
○He repeated this experiment while observing six other traits and found that the results were all the same (Klug, et al., 2020, pp.74).
○He crossed two plants with contrasting traits and the F1 generation all had one parents trait while the trait from the other parent had disappeared (Klug et al., 2020, pp.74).
○Self-fertilisation of the F1 generation revealed that the contrasting trait which had disappeared in the F1 generation had reappeared in the F2 generation, and the ratio of the trait observed in the F1 generation to the contrasting trait which had disappeared in the F1 generation was 3:1 (Klug, et al., 2020, pp.74).
○From this he hypothesised that certain units of heredity existed which controlled the expression of these traits and were passed from generation to generation without being changed (Klug, et al., 2020, pp.74).
○Furthermore, these findings were not sex dependant as it didn’t matter which P1 acted as the source of pollen and which acted as the ovum source, the results were still the same (Klug, et al., 2020, pp.74).
References: Klug, W.S., Cummings, M.R., Spencer, C.A., Palladino, M.A., Killian, D.J. (2020) Concepts of Genetics. 12th rev. edn. Harlow: Pearson Education.
Describe Mendel’s four postulates.
○Mendel’s first law states that genetic traits are determined by certain unit factors which exist in pairs in individual organisms (Klug, et al., 2020, pp.74-75).
○An individual receives one unit factor from each parent (Klug, et al., 2020, pp.74-75).
○His second law states that in individuals who have two contrasting unit factors for a single trait, one acts dominant while the other acts recessively (Klug, et al., 2020, pp.74-75).
○The trait which is expressed in the F1 generation is the dominant trait while the trait which disappeared in the F1 generation acts recessively (Klug, et al., 2020, pp.74-75).
○His third law states that during gamete formation, the unit factor pairs randomly separate so that each gamete receives one of the pair, with equal chances of receiving either pair (Klug, et al., 2020, pp.74-75).
○His fourth law states that pairs of unit factors segregate independently of each other, meaning one pair doesn’t influence another pair (Klug, et al., 2020, pp.77).
References: Klug, W.S., Cummings, M.R., Spencer, C.A., Palladino, M.A., Killian, D.J. (2020) Concepts of Genetics. 12th rev. edn. Harlow: Pearson Education.
Incomplete dominance
○Heterozygote has an intermediate phenotype as the gene is unable to fully express (Klug, et al., 2020, pp.100).
○This arises due to an absent/non-functional enzyme, caused by a loss-of-function mutation (Klug, et al., 2020, pp.100).
○The expected phenotypic ratio of 3:1 changes to 1:2:1 due to the lack of dominance (Klug, et al., 2020, pp.100).
References: Klug, W.S., Cummings, M.R., Spencer, C.A., Palladino, M.A., Killian, D.J. (2020) Concepts of Genetics. 12th rev. edn. Harlow: Pearson Education.
Describe a genetic disease caused by incomplete dominance.
○A genetic disease caused by lack of dominance is Tay-Sachs disease (Klug, et al., 2020, pp.101).
○Homozygous recessive individuals lack hexosaminidase A, which plays a role in lipid metabolism, causing fatal lipid-storage disorder (Klug, et al., 2020, pp.101).
○Heterozygotes have a single copy of the mutant gene and thus only have fifty percent of enzyme function (Klug, et al., 2020, pp.101). As this is enough for normal biochemical function, heterozygotes appear phenotypically normal (Klug, et al., 2020, pp.101).
References: Klug, W.S., Cummings, M.R., Spencer, C.A., Palladino, M.A., Killian, D.J. (2020) Concepts of Genetics. 12th rev. edn. Harlow: Pearson Education.
Codominance
○Heterozygote expresses both phenotypes simultaneously.
○For example, in the ABO blood group system, IA and IB alleles are dominant to each other and produce an AB phenotype.
Multiple alleles
○When more than two alleles of a gene are present in a population (Klug, et al., 2020, pp.102).
○Individuals still only have two alleles (Klug, et al., 2020, pp.102).
○For example, there are three alleles which determine blood type: allele A, allele B and allele O (Klug, et al., 2020, pp.102).
○These give four different blood types: type A (IA IA), type B (IB IB), type AB (IA IB) or type O (ii).
References: Klug, W.S., Cummings, M.R., Spencer, C.A., Palladino, M.A., Killian, D.J. (2020) Concepts of Genetics. 12th rev. edn. Harlow: Pearson Education.
Describe the ABO blood group system.
○The H substance on erythrocytes is composed of N-acetylglucosamine, galactose and fucose (Klug, et al., 2020, pp.102).
○A antigen is produced when IA expresses an enzyme which adds N-acetylglucosamine to the H substance (Klug, et al., 2020, pp.102).
○B antigen is produced when IB expresses an enzyme which adds galactose to the H substance (Klug, et al., 2020, pp.102).
○IAIB heterozygotes add either N-acetylglucosamine or galactose to available sites on erythrocyte (Klug, et al., 2020, pp.102).
○ii doesn’t add any sugar to the H substance and the H substance protrudes from the erythrocyte (Klug, et al., 2020, pp.102).
References: Klug, W.S., Cummings, M.R., Spencer, C.A., Palladino, M.A., Killian, D.J. (2020) Concepts of Genetics. 12th rev. edn. Harlow: Pearson Education.
Describe the Bombay phenotype
○The Bombay phenotype arises when the H substance isn’t synthesised correctly due to a mutation in FUT1, which codes for fucosyl transferase (Klug, et al., 2020, pp.103-104).
○Mutation causes fucose to not be added to the carbohydrate chain, preventing the addition of terminal N-acetylglucosamine or galactose by IA or IB as they don’t recognise the incomplete H substance as a substrate (Klug, et al., 2020, pp.103-104).
○Therefore, individuals are phenotypically type O as the H substance has no antigens on its surface (Klug, et al., 2020, pp.103-104).
References: Klug, W.S., Cummings, M.R., Spencer, C.A., Palladino, M.A., Killian, D.J. (2020) Concepts of Genetics. 12th rev. edn. Harlow: Pearson Education.
Lethal alleles
○Mutations in essential genes which causes death as it causes production of non-functional gene products (Klug, et al., 2020, pp.105).
○Heterozygotes can survive if they produce sufficient amounts of the gene product needed for survival (Klug, et al., 2020, pp.105).
○In recessive lethal alleles, homozygous recessive individuals won’t survive (Klug, et al., 2020, pp.105).
○In dominant lethal alleles, one copy of the allele is enough to cause death (Klug, et al., 2020, 106).
It is rarely observed as the individual must reproduce before expression of the lethal allele (Klug, et al., 2020, pp.106).
○For example, Huntington disease is caused by a dominant autosomal allele (Klug, et al., 2020, pp.106).
Heterozygotes can survive up to adulthood due to delaying the onset of the disease, but will die after gradual motor and nervous degeneration (Klug, et al., 2020, pp.106).
References: Klug, W.S., Cummings, M.R., Spencer, C.A., Palladino, M.A., Killian, D.J. (2020) Concepts of Genetics. 12th rev. edn. Harlow: Pearson Education.
Describe the Agouti gene mutation.
○The Agouti gene shows how a lethal mutation acts recessive for one phenotype (embryonic development) but dominant for another (hair colour) (Klug, et al., 2020, pp.106).
○The regulatory region of the AY allele is deleted, therefore the expression of the AY allele is unregulated (Klug, et al., 2020, pp.106).
○This mutation is a gain-of-function mutation (Klug, et al., 2020, pp.106).
○The Agouti phenotype is expressed when wild-types deposit yellow pigment as a band on the black hair shaft (Klug, et al., 2020, pp.106).
○Heterozygotes always have one copy of the AY allele turned on, thus they deposit the yellow pigment throughout the entire length of the hair shaft, causing yellow mice (Klug et al., 2020, pp.106).
○The lethal effect is caused by the deletion of a coding region of an adjacent gene, making it non-functional (Klug, et al., 2020, pp.106).
○Heterozygotes can survive as they produce enough gene product but homozygous recessive individuals die (Klug, et al., 2020, pp.106).
References: Klug, W.S., Cummings, M.R., Spencer, C.A., Palladino, M.A., Killian, D.J. (2020) Concepts of Genetics. 12th rev. edn. Harlow: Pearson Education.
Pleiotropy
○When one gene affects more than one phenotypes.
○Marfan syndrome is caused by an autosomal dominant mutation of the gene which codes for fibrillin (Klug, et al., 2020, pp.114-115).
○Fibrillin is present in many tissues in the body, therefore mutation causes multiple phenotypic effects, such as: lens dislocation of eye; long bones in limbs; and increased risk of aortic aneurysm (Klug, et al., 2020, pp.114-115).
○Porphyria variegata is caused by an autosomal dominant mutation which causes inability to metabolise porphyrin (Klug, et al., 2020, pp.114-115).
This causes toxic build-up of porphyrins (Klug, et al., 2020, pp.114-115).
This has several effects, such as: deep red urine; abdominal pain; muscular weakness; headaches; delirium; insomnia; convulsions; vision problems (can lead to blindness); fever; and a racing pulse (Klug, et al., 2020, pp.114-115).
References: Klug, W.S., Cummings, M.R., Spencer, C.A., Palladino, M.A., Killian, D.J. (2020) Concepts of Genetics. 12th rev. edn. Harlow: Pearson Education.
Linkage
The failure of two genes to assort independently.
Crossing over
Crossing over is when DNA is exchanged between homologous chromosomes.
This occurs during synapsis, when non-sister chromatids overlap.
Crossing over gives rise to different combinations of alleles and thus causes genetic diversity.
