T3 - Genetics and modern synthesis

Question 1

What is blending inheritance ?

Answer 1

Theory that pehnotypes in offspring are an average (blend) of their 2 parents

Darwins version of this was called pangenesis - all parts of te body produce particles of hereditary information (gemmules) which accumulate in the gonads and are transferred to offspring

• Environmental impacts could alter gemmule production so this theory was not further developped

Impacts > phenotype > gemmules > gonads > offspring

Question 2

What does a blending theory eventually lead to?

Answer 2

A population of the exact same phenotypes allowing for no variation

Question 3

What is the germ plasm theory? Who developped this idea?

Answer 3

August Weismann developped the theory for multi cellular organisms proposing that heritable information was transmitted only by the germ cells in the gonads (reproductive cells/gametes)

All other body cells (somatic cells) did not transmit such info. and only serve to carry out the body functions necessary for the transmission of the germ cells

Question 4

What is a germ cell vs a somatic cell?

Answer 4

Germ cell = pass on hereditary info to offspring

Somatic cell = bodily functions and supporting of germ cells

Question 5

How are somatic cells produced?

Answer 5

Germ cells produce somatic cells (soma) anew each generation

Info flows from germ cells to somatic cells but not the other way around (one way flow of info)

The soma is disposable; the germ plasm is (potentially) immortal

Question 6

How do germ cells and somatic cells differ in plants, corals and sponges?

Answer 6

In these taxa, germ cells are produced by somatic cells and changes in those somatic cells can affect subsequent germ cells derived from them and hence can be transmitted across generations

Somatic cells can also carry on mutations that occur to germ cells to offspring

Question 7

Why did Mendel succeed in his experiments with peas?

Answer 7

• he worked with discrete (binary) characters rather than quantitive ones
• he used true breeding plants (plants made self-fertile that only get 1 type of offspring - no variation)

Question 8

Mendel proposed a model in which heredity was controlled by ‘factors’, what are these factors now called today?

Answer 8

These factors are called genes - a specfic sequence of DNA (sometimes RNA in viruses) that encoded the synthesis of a gene product (RNA/protein)

Question 9

What is a locus?

Answer 9

(Plural loci) = broader term meaning a specific location on a chromosome; locus may or may not contain an actual gene (could be a promoter region with no coding genes)

Question 10

What are alleles?

Answer 10

Unique variants of a gene that differ in nucleotide sequence

Question 11

What are Mendel’s laws?

Answer 11

Law of dominance; if 2 alleles at a locus differ, then 1 (the dominant allele) determines the phenotype and the other (recessive allele) has no noticeable effect

Law of segregation; alleles segregate independently during meiosis such that a heterozygote produces 50% of their gametes carrying one allele and 50% carrying the other allele

Law of independent assortment; during gamete formation, alleles at a given locus segregate independently of those at other loci (the allele a gamete receives for one gene does not influence the allele it receives for another gene)

Question 12

What is a phenotype vs a genotype?

Answer 12

Phenotype = any quantifiable character/trait of an organism (morphologcal, behavioural, physiological)

Genotype = the genetic makeup of an individual in terms of the identity of the alleles it carries at one or more loci

Question 13

What is the multiplication (product) rule in Mendelian inheritance?

Answer 13

The probability that 2 or more independent events will both occur is the product of their indiv. probabilities

Pr(A and B) = Pr(A) x Pr(B)

Question 14

What is the addition (sum) rule in Mendelian inheritance?

Answer 14

For outcomes that are mutually exclusive (only one can occur) the probability that any one of them occurs is the sum of their independent probabilities

Pr(A or B) = Pr(A) + Pr(B)

Question 15

What are some reasons that Mendel’s laws were not 100% correct? Why are inheritance patterns more complex that he hypothesized?

Answer 15

1. Mendel’s law of (complete) dominance is far from universal

• dominant-recessive is a continuum and all possible intermediate phenotypes can occur (incomplete dominance/partial dominance) - the offspring becomes a mixture or blend of the 2 parents (red+white=pink)
• co-dominance can also occur (contribution of both alleles are distinctly visible in the phenotype) - partially red and partially white flower

2. Alleles do not always segregate independently within a locus (not always fair in a heterozygote)

• Meiotic drive
• Ex. in maize; Rr x rr test crosses produced 70% rr offspring

3. Addition complication for single genes

• Pleiotropy
• Dominance has nothing to do with the frequency of an allele
• there can be more than 2 alleles at a gene/locus

4. Assortment of alleles at one locus is often not independent of those at another locus

• physical linkage

5. Recombination
6. Many traits are quantitative - they vary in a continuous manner (height, BP, hair colour)

• quantitative variation
• height is not only either tall/short, there exists heights in between (variation)

Question 16

What is recombination?

Answer 16

Cross over events during meiosis - this can produce the missing gamete tyes

Probability of this event between 2 loci on a chromosome increase as the distance between them increases
i.e. if 2 loci are too close, recombination is less likely to occur

Question 17

What is a physical linkage?

Answer 17

Physical linkage = arises because genes are physically close together on a chromosome, these genes tend to be inherited together

Question 18

What is pleiotropy?

Answer 18

Pleiotropy = one gene affects multiple phenotypes (extremely common)

Question 19

What is meiotic drive?

Answer 19

Meiotic drive = when a locus mipulates meiosis to favor the transmission of one allele over another (overrepresentation vs under-representation)

Question 20

Why does quantitative variation arise?

Answer 20

i) multiple (often many) genes affect the trait (polygenic)

ii) an indivs. environment can also affect expression of the trait

Question 21

What are polygenic traits?

Answer 21

When multiple genes affect the trait

As the number of loci increases, phenotypic variation becomes more finely graded (continuous)

Truly continuous environmental variation further obscures discrete genetic segregation (someone could have many ‘tall’ genes but is malnourished during childhood)

Question 22

What is saltationism?

Answer 22

The thought that evolution occured from selection acting on mutations of large effect

Question 23

What is the Hardy-Weinberg equation? Why was it created?

Answer 23

Was created from the criticism that if Mendelian genetics were true we should always see a 3:1 ratio of dominant:recessive phenotypes which is not true

p^(2)+2pq+q^(2)=1

Question 24

What is a population?

Answer 24

Group of individuals of the same species that live in the same area and that interbreed

A species can consist of more than 1 population

Question 25

What is a gene pool?

Answer 25

Consists of all copies of each allele at a given locus

Question 26

What is the math behind obserevd genotype frequencies within a gene pool?

Answer 26

• freq. CRCR = # CRCR indiv. / total # of indiv.
• freq. CRCW = # CRCW indiv. / total # of indiv.
• freq. CWCW = # CWCW indiv. / total # of indiv.

These all must sum to 1 - if it does not equal 1, there is no population

Question 27

What are the HW assumptions?

Answer 27

1. Diploid locus that reproduce sexually (2 alleles)
2. Random mating wrt the lcus (random union of gametes)
3. No NS at the locus
4. No mutation at the locus
5. Indivs. do not move in or out of the population (no migration)
6. Population is infinitely large (no genetic drift)

Question 28

What are the expected frequencies if HW assumptions are met?

Answer 28

p^2= expected freq. CRCR
2pq = expected freq. CRCW
q^2= expected freq. CWCW

recall the sum must = 1