Andrew's Content Flashcards
What are some key similarities in genetic structure and processes across different organisms?
All organisms on Earth have similar genetic components and carry out the same basic processes of DNA replication, transcription and translation, as well as mitosis and meiosis. Almost all organisms also have recombination due to crossing over and gene conversion, although the mechanisms may differ.
What are some of the model organisms that are used to identify genes and genetic mechanisms? And why are they chosen for this purpose?
Fruit flies (D. melonagaster), Mice (M. musculus), E. coli, C. elegans
- They are chosen due to their short life span, ability to reproduce rapidly, and the low cost and ease with which they can be genetically engineered. They also present far less complex genetic or physiological systems than humans.
What is phenotypic variance?
The degree to which a phenotype differs between individuals within a population e.g. height in humans has high phenotypic variance
What is heritability?
The degree to which phenotypic variance can be attributed to genetic variance
What is broad-sense heritability? How is it catagorised?
Broad-sense heritability takes into account multiple types of genetic variation that affect the variation of a phenotype. These sources of genetic variance are categorised based on how different alleles, or versions of a gene, interact with one another:
- Additive genetic variance occurs when different alleles of the same gene (a heterozygous genotype) produce an intermediate phenotype when compared to identical alleles (a homozygous genotype)
- Dominant/recessive genetic variance occurs when heterozygous genotypes produce the same (or a very similar) phenotype as the homozygous genotype containing dominant alleles
- Epistatic genetic variance occurs when one gene’s alleles mask the expression of a different gene’s alleles
What is additive genetic variance?
Additive genetic variance occurs when different alleles of the same gene (a heterozygous genotype) produce an intermediate phenotype when compared to identical alleles (a homozygous genotype)
What is dominant/recessive genetic variance?
Dominant/recessive genetic variance occurs when heterozygous genotypes produce the same (or a very similar) phenotype as the homozygous genotype containing dominant alleles
What is epistatic genetic variance?
Epistatic genetic variance occurs when one gene’s alleles mask the expression of a different gene’s alleles
What is narrow-sense heritability? And why is it more practical to calculate compared to broad-sense heritability?
Narrow-sense heritability only takes into account how additive genetic variation affects phenotype. Narrow-sense heritability is usually more practical than broad-sense heritability because it is easier to calculate and often provides a very good estimate of broad-sense heritability.
What is the difference between broad-sense and narrow sense heritability?
Broad sense refers to the amount of phenotypic variance that is attributed to the genetic variance, whereas narrow sense is the proportion of phenotypic variance that results only from additive genetic variance (does not include confounders of dominance, epistasis and genetic interactions)
After completing a punnet square to see whether a trait is sex-linked, how would one fully determine whether the trait is truly sex-linked not autosomal?
Via a reciprocal cross, which swaps the male and female traits (e.g. white-eyed male –> white-eyed female, and red-eyed female –> red-eyed male), which would produce a different result than in the original punnet square. If it produced the same results, this would indicate it is controlled by a gene on an autosomal chromosome.
What does hemizygous mean as Morgan proposed it in his fruit fly experiments?
That eye colour was hemizygous because the gene was present only once in the organism and there was no homologous gene on the Y chromosome
What is crisscross inheritance as Morgan proposed in his fruit fly experiements?
Gene transmission from a male parent to a female offspring and then from that female offspring to a male offspring
What is a pedigree? And what does it show/determine?
A chart showing the inheritance of a trait or health condition through generations of a family. It shows the relationships among family members and indicates which individuals have a trait(s) of interest. This can help determine how a trait/condition might be passed down through the generations and what might accompany it.
What are some of the key assumptions for pedigree analysis?
- Complete penetrance: an individual in the pedigree will be affected when the individual carries at least one dominant allele of a dominant trait or two recessive alleles of a recessive trait.
- Rare-in-Population: the trait in question is rare in the general population.
- Not-Y-Linked: the causative genes in these problems may be autosomal or X-linked, but are not Y-linked.
How is recessive inheritance identified in pedigree analysis? What is autosomal recessive inheritance and X-linked recessive inheritance? And how are they different?
- If any affected individual has two unaffected parents, the disease must be recessive. This means each parent contributes one disease allele while still having a dominant allele that masks the effects of the recessive trait.
- Recessive traits often skip generations
- Autosomal recessive inheritance is a recessive trait located on autosomes (non-sex chromosomes) and affects both sexes equally. For an individual to be affected, they must inherit two recessive alleles.
- X-linked recessive inheritance are recessive traits located on the X-chromosome. When a trait is X-linked, a single recessive allele is sufficient for a male to be affected. This is because the male is hemizygous, having only one allele of an X-linked trait. - A father transmits his allele of X-linked genes to his daughters but not his sons. A mother transmits an allele of X-linked genes to both her daughters and her sons.
What is the phenotypic ratio for a monohybrid (one trait) cross between two heterozygotes?
3:1
What is the phenotypic ratio for a dihybrid (two traits) cross between two heterozygotes?
9:3:3:1
What does Mendel’s First Law (The Principle of Segregation) entail? And how is this supported through test crosses?
The two members of a factor (gene) segregate from each other during the formation of gametes (in meiosis), and the offspring receives one allele from each parent. This is demonstrated by crossing an individual expressing a dominant phenotype with a homozygous recessive individual.
What does Mendel’s Second Law (The Principle of Independent Assortment) entail? And how is this supported through test crosses?
The factors (alleles) for different pairs of traits assort independently of one another. Support for this law was a result of dihybrid crosses monitoring seed shape (smooth or wrinkled) and seed colour (yellow or green).
How many genotypes and phenotypes do you expect for Mendel’s heterozygote crosses (given the assumption of complete dominance)?
Based on the number of segregating pairs, the number of phenotypic classes would be 2 to the power of n (number of segregating pairs), and the number of genotypic classes would be 3 to the power of n
What are Mendelian diseases?
Single gene defects that cause a certain disease
What are null mutations?
Mutations that result in complete absence of gene product
What are loss of function mutations?
Mutations caused by the introduction of a stop codon (nonsense mutation), Insertion/deletion of one to a few bases (frameshift mutation), or a larger deletion (e.g. loss of first exon or >50% of total exons)
What are gain-of-function mutations?
Mutations where the altered gene product results in a new phenotype, the majority of which are inherited in a dominant manner.
What is complementation?
Complementation refers to a relationship between two different strains of an organism that both have homozygous recessive mutations that produce the same phenotype (for example, a change in wing structure in flies) but which do not reside on the same (homologous) gene. These strains are true breeding for their mutation. If, when these strains are crossed with each other, some offspring show recovery of the wild-type phenotype, they are said to show “genetic complementation”. When this occurs, each strain’s haploid supplies a wild-type allele to “complement” the mutated allele of the other strain’s haploid, causing the offspring to have heterozygous mutations in all related genes. Since the mutations are recessive, the offspring will display the wild-type phenotype.
Deviations from expected Mendelian rations suggest…
Interactions between alleles of different genes
