Genetics: Lecture 2

Question 1

What did Charles Darwin do?

Answer 1

Charles Darwin traveled for 5 years, making many observations about what he saw.

Example: he noticed that similar birds or even birds of the same species would have different features depending on which island they lived on. They had different beaks which made Darwin question if it was due to food resources or other things. Introduced idea of natural selection. His main area of interest was Galapagos.

Question 2

What did Darwin publish?

Answer 2

In 1859, Darwin published “The Origin of Species” which talked about the ideas of natural selection and how there must be variation for natural selection to occur.

Question 3

Theory of evolution by natural selection?

Answer 3

• A population could change (or evolve) only if variation in traits existed among its members.
• Variants that enhanced survival and reproduction would increase in frequency with each generation.
• Variants that did not would decrease in frequency and
  might eventually be eliminated from the population.

Question 4

When was this theory popular?

Answer 4

It was the dominant theory in biology at the end of the 19th century.

• it did not fit with how other people thought traits were moved/inherited from one generation to the next.

Question 5

How does this theory stand up?

Answer 5

The theory of evolution by natural selection proposed by Darwin is basically the origin of modern biology.

• everything we do now is through evolution and natural selection.

Question 6

“Blending” hypothesis?

Answer 6

The transmission of traits from parents to offspring was once hypothesized as traits “blending” not unlike paints. This was a hypothesis that was widely accepted in the 1800s. Blue + yellow = green.

• This idea works to a certain extent, as we see that TYPICALLY a tall parent and a short parent will have a child of medium height.

Question 7

“Particulate” hypothesis?

Answer 7

A hypothesis demonstrated by Mendel where different alleles were “dealt” to different offspring. Despite this one allele being dealt to offspring they would still have the whole deck containing the whole genome, and would carry it through further generations.

Question 8

When did Mendel begin experimenting?

Answer 8

Gregor Mendel began breeding garden peas in an Augustinian Abbey in Brno, Czech Republic in around 1957.

Question 9

Why were garden peas used?

Answer 9

• Easy to grow
• short lifespan
• many characters with easily distinguishable traits like flower colour, seed shape, and pod colour
• there are true-breeding varieties: with a particular trait being consistently inherited from one gen. to the next.
• It is possible for them to cross-fertilize or self-fertilize different varieties of peas.

Question 10

Technique used by Mendel?

Answer 10

• Stamens from one parental flower were cut off and the stamens from a white flower were used to pollinate the carpel of the purple parent flower.
• Carpel is female
• Stamen is male

Question 11

Results?

Answer 11

All of the resulting flowers were purple!

Question 12

Mendel’s question?

Answer 12

When F1 hybrid plants self-pollinate, which traits appear in the F2 generation?

Question 13

Full experiment?

Answer 13

• P generation: two true breeding parents crossed with the method mentioned before.
• F1 generation: All plants were purple hybrids and they self or cross-pollinated
• F2 generation: 705 purple-flowered and 224 white-flowered were counted

Question 14

What did Mendel find?

Answer 14

“heritable factor” (gene) for recessive trait (white flowers) is NOT destroyed,
deleted, or blended in the F1
generation BUT was merely
masked in the F1 generation.

Question 15

What is hybridization?

Answer 15

the crossing of two true-breeding
varieties

Question 16

Alleles are alternative versions of the same gene?

Answer 16

At one locus there can be two alleles: white and purple in this example

Allele for purple flowers: DNA with nucleotide sequence - this DNA sequence results in the production of an enzyme that helps to synthesize purple pigment.

Allele for white flowers: slightly different DNA sequence (1bp) - results in the absence of the functional enzyme and the absence of the pigment being synthesized.

Note: one purple flower allele results in sufficient pigment for a fully purple flower.

Question 17

Mendel’s 3 observations?

Answer 17

• Alternative versions of genes account for variations (alleles) in inherited characters.
• For each character, an organism inherits two copies (that is, two alleles) of a gene, one from each parent.
• If the two alleles at a locus differ:
  dominant allele determines the organism’s appearance recessive allele has no noticeable effect

Question 18

Dominant vs. Recessive

Answer 18

Dominant alleles mask recessive alleles

• dominant is not always better than recessive, they are simply different from one another and D masks M.

Question 19

Mendel’s first law observations?

Answer 19

The segregation of alleles during gamete formation.

• Use a punnet square (probability of genotypes/phenotypes) to figure out offspring genotypes and phenotypes

Question 20

Mendel’s first law?

Answer 20

Two alleles for a heritable character segregate (separate from each other) during gamete formation and end up in different gametes. This is a RANDOM process.

• two alleles that can be different for any single heritable character.

Example: either the P and p allele could be on a pair of homologous chromosomes

Question 21

Allele?

Answer 21

alternate versions of a gene

Question 22

Genotype _____?

Answer 22

determines phenotype

Question 23

Heterozygous?

Answer 23

an organism with two different alleles for a character

Question 24

Mendel’s background?

Answer 24

• Monk in the Czech Republic
• Failed exams to become a teacher
• Went to university in Vienna and was mentored (influenced) by physicist Doppler
• Doppler taught him the experimental method: asking questions, making hypotheses, counting results, using large sample sizes, writing things down, using statistical measures.

Question 25

Homozygous

Answer 25

an organism with a pair of identical alleles

Question 26

Magic Ratios? (hybrid crosses)

Answer 26

3:1 phenotypic ratio
1:2:1 genotypic ratio

Question 27

The test cross?

Answer 27

We know the results of the phenotype (phenotypic ratio) and one parent and we need to determine the other parents’ genotype.

Question 28

Why did Mendel succeed?

Answer 28

Mendel succeeded because he used math, observing a 3:1 ratio.

Question 29

What did Mendel look at?

Answer 29

He looked at 7 characters and always observed a ratio close to 3:1. This worked because he used very large samples and counted all of them.

• shape and colour were the closest to the ratio, because he counted more of them than the others

Question 30

What happens with two genes?

Answer 30

A dihybrid cross (two genes that are both heterozygous in F1)

P generation: cross between two individuals who are true breeding for two characters.

F1: YyRr self pollinates to give F2

Question 31

Hypothesis of dependent assortment?

Answer 31

certain characters stay attached to one another through generations

example: YR and yr will always stay together

Question 32

Hypothesis of independent assortment?

Answer 32

characters are independent of one another

example: results from YyRr are YR, Yr, yR, yr

Question 33

Independence?

Answer 33

Genes on different chromosomes means independence

dihybrid phenotypic ratio is 9:3:3:1 due to independent assortment

Question 34

Mendel’s second law?

Answer 34

Independent Assortment of Genes

During gamete formation, a pair of alleles for one gene will segregate independently of a pair of alleles for another gene.

Question 35

Mendelian ratio/heirarchy?

Answer 35

Gene transmission that works with both of Mendel’s laws.

Question 36

Deviations from Mendel’s laws? Mitochondrial DNA?

Answer 36

• Some traits are not on nuclear chromosomes. This is due to the endosymbiotic theory.
  -Mitochondrial DNA is from the mother only as a sperm head is the only part that enters the egg and the body/tail falls off.

Example: mitochondrial or chloroplast chromosomes

Question 37

Deviations from Mendel’s laws? Same chromosome?

Answer 37

Traits that are on the same chromosome are linked.

This deviates from Mendel’s Second Law. When genes are located near each other on the same chromosome, they tend to be inherited together and they are said to be linked.

• It is not a given but with crossing over they can change from time to time and move places between homologous pairs.

Question 38

How do traits move on the same chromosome?

Answer 38

• Alleles that are close together on the same chromosome have a higher chance of segregating together than alleles that are far apart.
• This is due to the fact that the possibility of a cross-over event occurring between gene alleles increases the further away they are from each other on a chromosome.
• A and C have higher chance being separated than C and B

Question 39

Deviations from Mendel’s laws? X-linked?

Answer 39

Traits carried on sex-chromosomes.

Examples: mammals XX female, XY male; birds, ZW females, ZZ males.

Question 40

Autosomes?

Answer 40

Chromosomes that are not sex chromosomes. We have 44!

Question 41

Effect of X-linked?

Answer 41

We have two homologous chromosomes, so if something goes wrong with one the other is a backup. This is different for males who only have one of each, they have no backup. If there is an x-linked trait then a female with it is a carrier whilst a male is going to have no functional counterpart.

Question 42

X inactivation?

Answer 42

Occurs in females:

• after fertilization and a few zygotic devisions one of the x chromosomes is inactivated.
• it is random in each of the cells
• females with this are called mosaics as it occurs at random in each cell.
• this is advantageous for females as if there is an issue with one or a defect, a nearby cell can counter it as they have a functional version.

Question 43

Incomplete dominance?

Answer 43

An allele of a certain gene creates a gene product (a protein) which has an effect that will make the effect of another allele.

Example: Red flower and a white flower creates a pink flower