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Flashcards in Variation and Inheritance Deck (48):

what are the four postulates of natural selection?

1) variation

2) inheritance

3) differential survival
in every generation more offspring are produced than can survive

4) extinction


what is variation?

variation is the raw material for evolution by natural selection

natural selection is a sorting process of differential survival and reproduction

without variation there can be no evolution!


what is discrete variation?

multiple forms within a species (polymorphism)

categories, you can put it into some kind of category

ex. different colored lizards each have their own behavior
orange: large territory
white: sneakers
green: small territory, well protected


what is continuous variation?

cannot put into categories

- height: because you're technically 5.4938457298 feet tall

- colors like skin or eye because there's a gradation


what are the two types of variation?

1) genetic
2) environmental


Darwin lacked a good mechanism of heredity. What idea did he favor?

pangenesis: all cell lines contribute to gametes

evidence favored Weismann's germplasm theory (germ vs. soma)


what is the mechanism of transmission? who thought of it?



Darwin needed Mendel --> his theory of natural selection almost required that heredity be Mendelian


what provides theory of heredity?

- genes are preserved during development

- genes are passed unaltered to offspring

- organism can "carry" gene without expressing phenotype

- phenotype may be intermediate but genes do not "blend"


blending vs. Mendelian

rare variants are blended out of the population


rare variants can persist and become established in the population


what is Mendel's Law of Dominance

are termed the dominant, and those which become latent [i.e. hidden] in the process recessive


what is simple dominance?

When 2 unlike unit factors (alleles) are present in an individual, one factor is dominant and its trait is preferentially expressed


what is segregation?

the two members of a gene pair segregate randomly and equally into the gametes, which then combine randomly and equally to form the next generation

each gamete receives one or the other of the alleles


what are the four parts of Mendel's Law of Segregation?

1) inheritance of traits is determined by "factors" (genes) that are passed on to descendants unchanges --> alternative versions of genes (alleles) account for the variation in inherited characters

2) an individual inherits one such unit from each parents for each trait

3) the two members of a gene pair segregate randomly and equally into the gametes which then combine randomly and equally to form the next generation

4) a trait may not show up in an individual but can still be passed on to the next generation


what are alleles?

alternative versions of genes


what is Hungtinton's Choea?

dominant gene

disrupts nerve function, loss of control over body and mind, then death

it's dominant because mutant gene with extra CAG is produced even if only one allele is present and the toxic protein disrupts function


what is Tay-Sachs disease?

revessive disease

neurological impairment often resulting in death due to the accumulation of gangliosides in neurons of the brain

it's a mutation of HEXA gene on chromosome 15 and the mutated form can't catalyze degradation of gangliosides --> even if only one normal gene is present, sufficient enzyme can be produced to catalyze ganglioside degredation


what is independent assortment?

characteristics are inherited independently of each other

dominant traits can appear in combination either with other dominant traits or with recessive traits

during gamete formation, segregating pairs of unit factors (alleles) assort independently of other pairs (e.g. color and shape are independent)


who is Punnett and what were his ideas?

the "father of genetics"

combined Mendel's laws with statistics

experimented with peas and poultry to verify Mendel's findings

creator of punnet sure as method to visualize allele combinations


who was Bateson?

vocal proponent of Mendel's laws of inheritance

coined the terms genetics, allele, heterozygote, and homozygote

established the Cambridge School of Genetics


how can you tell if a tall plant is either homozygous dominant (TT) or heterozygous (Tt)?

a cross of an unknown genotype with the recessive genotype

used to ascertain the identity of the unknown


what is mutation?

the ultimate source of all heritable variation


what are point mutations?

they alter single nucleotides

- synonymous
- transition
- transversion
- frameshift
- stop


what is sickle cell anemia?

substitution of thymine (T) for an adenine (A) in the gene for B-globin

if there's a thymine then the 6th amino acid is valine rather than glutamic acid

the change in PRIMARY protein structure has implications for quaternary structure and the RBC is misshapen


what are two sources of genetic variation?

1) mutation

2) recombination


what is recombination?

a source of genetic variation!

recombination shuffles existing variation into new combinations

creating new combinations leads to greater levels of phenotypic variation


what is mutation?

mutation is the ultimate source of all heritable variation!

ex. unequal crossing over alters larger sections of DNA


are mutation and recombination random or directional?


new mutations/recombinations have no inherent tendency to result in adaptation --> they are random with respect to adaptation*****

given that organisms are adapted to begin with, these changes are more likely to be nonadaptive or even maladaptive

but...mutations and recombinations can be nonrandom at the molecular or chromosomal level because certain nucleotide substitutions are more likely than others


what are the limits of mutation?

mutation acts on existing genes, modifying or generating new alleles

impact can be large or small:
- mutation in regulatory gene can cause early death
- redundancy in genetic code means not every substitution specifies a new codon (silent mutations)


what controls mutation rate, μ?


mutations happen because of sloppy copying, DNA degradation, and random processes

although mutations are generated through random processes, they may not be propagated/inherited randomly


is mutation random?

although mutations are generated through random processes, they may not be propagated/inherited randomly

The mutation that provides resistance to antibiotics in bacteria is generated randomly. Its spread through the population is NOT random, though—it is a predictable response to a set of circumstances

same goes for pesticides


what is co-dominance?

both alleles are expressed, and the heterozygote manifests a third phenotype with properties of both alleles

ex. blood types!!!!

NOT blending - heterozygotes lets the A allele and the B allele exist simultaneously in your blood


what is incomplete dominance?

Heterozygote results in a third unique phenotype. Alleles are “blended”together

ex. Red and white flowers produce a heterozygous pink flower

looks like blending but really what's happening is that the heterozygous is expressed in a third phenotype rather than one or the other


what are causes of complications for understanding genetic variation?

1) pleiotropy

2) epistasis


what is pleiotropy?

when a single gene impacts two or more seemingly unrelated traits

one gene mutation effects two alleles!

ex. Holt-Oram: defective allele for TBX5 causes malformation of upper limbs and abnormal septa in heart


what is epistasis?

when one gene interferes with the expression of another gene

Many genes interact to make skin color, eye color, hair color  if you’re recessive at a specific gene (albino gene) then all color producing genes are shut off

this happens because albinism alleles disrupt production of melanin pigment and other phenotypes are not distinguishable without melanin


what is the difference between pleiotropy and epistasis?

pleiotropy: one gene wit multiple phenotypic effects

epistasis: two genes contribute to one phenotype


what are polygenic traits?

several genes may make products that contribute to a phenotype --> MULTIPLE genes each with multiple alleles all interacting to produce a phenotype

ex. at least 3 genes determine eye color

Traits that display a continuous distribution, such as height or skin color, are polygenic. The inheritance of polygenic traits does not show the phenotypic ratios characteristic of Mendelian inheritance, though each of the genes contributing to the trait is inherited as described by Gregor Mendel.


what can complicate gene expression?

non-genetic factors

genotype is expressed in the context of its bearer --> this is what is meant by environmental cause of variation!

nature vs. nurture


what are environmental effects?

anything in the environment that induces variation in the population

the environment in which organisms grow and mature determines the degree to which genetic "potential" is realized

ex. temperature within a turtle nest determines sex

ex. you have the genes to be that tall but your environment limited you to being not as tall


what is a type of environmental effects?

maternal effects

anything mom is doing can have effects on the embryo and can have post-birth effects too  mom is an environmental effect!!!!!!! She is in no way altering the genome of the baby

ex. environment of womb/egg, amount of yolk in egg, post birth maternal care


is variation due to environmental effects a factor in evolution?

Variation due ONLY to environmental effects is NOT a factor in evolution


how do you know if a phenotype is environmentally or genetically determined?

1) controlled crosses

2) common garden experiments

3) cross fostering


what are controlled crosses?

determine whether phenotypes are inherited according to expectations of Mendelian genetics


what are common garden experiments?

raise offspring under identical conditions and run several "gardens" to test effect of particular variables


what is cross fostering?

common gardens for animals but the offspring are raised by parents other their own


what is the relation between phenotype, genotype, and environment? what's the equation?

Vp = Vg + Ve

Vp = phenotypic variation
Vg = genotypic variation
Ve = environmental variation


what is heritability? what's the formula for it?

heritability is the proportion of variation that can be attributed to genotype

*h^2 = Vg/Vp*

heritability is high when Ve is low

indicated fidelity of trait transmission and thus how readily evolution can act on an attribute


what is the relationship between heritability and natural selection?

h^2 determines the response to selection

higher h^@ means stronger response to selection!

if a trait is perfectly heritable, h^2 = 1, then ALL of the impact of selection will be observed