Lecture 2: Mendelian Genetics

Question 1

What were the two hypotheses to explain inheritance in 1837?

Answer 1

1. one parent contributes more to an offspring’s inherited traits (eg Aristotle contended that it was the male and that a fully formed homunuculus was inside the sperm)
2. blended inheritance - the traits of the parents are blended in their offspring (like blue and yellow to make green) - explained single offspring, but not siblings, or the next generation

Question 2

5 characteristics of a model organism:

Answer 2

1. short generation time
2. can be inbred (self-fertilise)
3. simple reproductive biology
4. small size
5. large numbers of progeny for robust statistical analysis

Question 3

why was Pisum sativum a good choice for Mendel’s experiments?

Answer 3

• Well characterized, cultivated plant, grew well in Brno
• Could be self-fertilized (selfed) - pollen from the plant could be used to
  pollinate its own flowers - allows inbreeding
• Could obtain and maintain pure-breeding lines - these always bred
  true producing the same trait generation upon generation
• Could be readily cross-fertilized to create hybrids between pure-breeding
  lines - could have carefully controlled matings and reciprocal crosses - to
  rule out the effect of one parent versus the other
• Could examine clear-cut (qualitative/discrete) traits where there were 2 forms of the trait- “either-or” choices - unambiguously distinguish forms of the trait
• Could have a large number of plants and progeny, so could subject the
  data to statistical analysis - Mendel did quantitative analyses that produced
  robust results and aided interpretation

Question 4

The August Krogh Principle

Answer 4

For many problems there is an animal on which it can be most conveniently studied

Question 5

what is a reciprocal cross?

Answer 5

a breeding experiment where two different parental strains are crossed twice, with the sex of the parents switched in the second cross

Question 6

process of cross-pollination of pea plants

Answer 6

1. pollen transferred from one pea plant to the stigma of recipient pea plant (with anthers previously removed) with brush
2. seed forms and germinates

Question 7

process of selfing of a pea plant

Answer 7

transfer of pollen onto stigma of same plant

Question 8

examples of antagonistic pairs that Mendel investigated

Answer 8

• seed colour (yellow/green)
• seed shape (round/wrinkled)
• flower colour (purple/white)
• unripe pod colour (yellow/green)
• ripe pod shape (round/pinched)
• stem length (long/short)
• flower position (axial - along stem/terminal - at tip of stem)

Question 9

broad overview of process of Mendel’s investigation

Answer 9

1. isolated pure forms of each trait
2. crossed 2 pure breeding lines that differed at one trait only
3. looked at progeny (F1 and F2)

Question 10

how did dominance manifest itself in Mendel’s experiments?

Answer 10

one of the two traits in an antagonistic pair was dominant and would always be manifested in the F1 hybrid

Question 11

dominant antagonistic traits in Mendel’s experiments

Answer 11

• yellow
• round
• purple
• green
• round
• long
• along stem - axial

Question 12

how did Mendel disprove the theory of uniparental inheritance and demonstrate that both contribute equally?

Answer 12

reciprocal crosses revealed that not only were traits dominant but also that this was independent of the parent

‘it is immaterial to the form of the hybrid which of the parental types are used in the cross’

Question 13

define a locus

Answer 13

a genetically defined location - strictly speaking, we don’t know if it is only one gene or not - but it behaves like a single gene

Question 14

define an allele

Answer 14

alternative form at a given locus

Question 15

define dominant

Answer 15

the allele that manifests itself regardless of the other allele that is present - indicated by an upper-case letter (e.g. A) - the trait that is manifest in a hybrid

Question 16

define recessive

Answer 16

an allele whose effect is “masked” when the dominant allele is present - all alleles at a locus must be recessive in order for the recessive allele to manifest itself - indicated by a lower-case letter (e.g. a)

Question 17

define homozygous

Answer 17

when both alleles at a given diploid locus
are the same – i.e. AA or aa

Question 18

define heterozygous

Answer 18

when there is one dominant and one
recessive allele present at a diploid locus– i.e. Aa

Question 19

define homozygote

Answer 19

an individual who is homozygous at the
locus in question

Question 20

define heterozygote

Answer 20

an individual who is heterozygous at the locus in question

Question 21

define hybrid

Answer 21

derived from two different parents

Question 22

define monohybrid

Answer 22

one hybrid locus

Question 23

define dihybrid

Answer 23

two hybrid loci

Question 24

define true-breeding

Answer 24

homozygous at the loci/locus in question

Question 25

define P

Answer 25

parental generation

Question 26

define F1

Answer 26

first filial generation - the offspring derived from the parental generation

Question 27

define F2

Answer 27

second filial generation - the offspring derived from the F1 generation

Question 28

define self

Answer 28

an inbreeding cross that involves individuals that are genetically identical (eg a plant with itself, or between full siblings derived from true breeding plants)

Question 29

Mendel's experiments - monohybrid cross

Answer 29

- green peas (yy) crossed with yellow peas (YY) - F1 all yellow (Yy) - self-fertilisation of F1 - F2 yellow:green = 3:1 - the reappearance of the recessive trait completely disproves 'blending' and uni parental inheritance

Question 30

how did Mendel infer the law of segregation (Mendel's First Law)?

Answer 30

F3 - diagram on slide 26

Question 31

define and explain the law of segregation (Mendel's First Law)

Answer 31

- two members of a gene pair segregate from each other into the gametes, so that one-half of the gametes carry one member of the pair and the other one-half of the gametes carry the other member of the pair - the alleles unite at random, one from each parent, at fertilisation

Question 32

Mendel's first law incorporates the fact that his results reflected

Answer 32

basic rules of probability

Question 33

define genotype

Answer 33

pair of alleles present in an individual

Question 34

define phenotype

Answer 34

observable characteristic of an organism

Question 35

give an example of how genotype and phenotype differed in Mendel's experiments

Answer 35

F2 generation phenotype ratio was 3:1 = yellow:green genotype ratio was 1:2:1 = YY:Yy:yy

Question 36

how can we discriminate between dominant homozygotes (eg YY) and heterozygotes (Yy)?

Answer 36

Test crosses reveal unknown genotypes: unknown genotype x homozygous recessive diagram on 34

Question 37

Mendel's experiments: dihybrid crosses

Answer 37

- crossed YYRR (yellow and round) with yyrr (green and wrinkled) - F1 were all identical (YyRr) - in F2, there were 4 different phenotypic combinations, including ones that weren't originally present in the parents. - 16 possible allelic combinations, 9 unique genotypes, 4 different phenotypes - this suggested that there had been a shuffling of alleles, proposed in the law of independent assortment

Question 38

Mendel's second law - Law of independent assortment

Answer 38

- during gamete formation, the segregation of alleles at one locus is independent of the segregation of alleles at another locus - results in predictable ratios of phenotypes in the F2 generation as shown by a Punnett square - follows basic laws of probability

Question 39

how can we calculate the number of possible allele combinations in gametes?

Answer 39

2^n n= number of gene loci where the organism has two different alleles

Question 40

use of product rule

Answer 40

Product Rule – "AND" Situations Use the product rule when you're calculating the probability of two or more independent events happening together.

Question 41

use of sum rule

Answer 41

Sum Rule – "OR" Situations Use the sum rule when calculating the probability of either of two mutually exclusive events occurring.

Question 42

Law of probability for multiple genes

Answer 42

loci assort independently - so we look at each locus independently to get the answer (eg on slide 43)

Question 43

summary of Mendel's 1865 paper

Answer 43

1. inheritance is particulate - not blending 2. there are two copies of each trait in a germ cell (before meiosis) 3. gametes contain one copy of the trait 4. alleles segregate randomly 5. alleles are dominant or recessive - thus the difference between genotype and phenotype 6. different traits assort independently