Week 1 - Mendelian genetics

Question 1

What is the difference between a gene and an allele?

Answer 1

A gene is a region of DNA that encodes a [function], where as an allele is a [variant] form of a gene, found at the same genetic [locus].

Question 2

What defines a diploid organism ?

Answer 2

An organism that has [two copies of each chromosome], one from each parent.

Question 3

What is the difference between homozygous and heterozygous?

Answer 3

Homozygous = has 2 [identical] alleles of a particular gene.
Heterozygous = has 2 [variant] alleles of a particular gene.

Question 4

What is blended inheritance and what was the issue with it?

Answer 4

It was an explanation for inherited characteristics (green + yellow = greeny yellow).
Didn’t account for:
1. Traits that skipped a gen
2. We aren’t a homogenous population

Question 5

What did Gregor Mendel do?

Answer 5

He did experiments on pea plants, he made true breeding lines for each trait he was studying - by self fertilising single traits to produce a homozygous line of offspring for that trait.

Question 6

Benefits of using Pea plants?

Answer 6

• Short generation time
• Many seeds per cross (larger sample size)
• easy to self and cross fertilise (by cutting
  anthers)
• discrete traits

Question 7

What are Mendel’s 3 laws?

Answer 7

Law of dominance
Law of segregation
Law of independent assortment

Question 8

How did Mendel demonstrate recessive traits can be masked?

Answer 8

By crossing 2 different true breeding lines and then self fertilising the 1st gen- producing 2nd gen (3:1) :
The first generations phenotype will be only 1 of the true breeding lines (dominant one).
The second generation will contain both true breeding phenotypes.
Second gens ratio = 3 (dominant) : 1 (recessive)
SUGGESTS CERTAIN TRAITS (RECESSIVE) CAN BE MASKED BY DOMINANT ONES.

Question 9

What is a Monohybrid cross? Which law does it prove?

Answer 9

The crossing of 2 organisms with different variants (alleles) of [1 gene]. E.g. Wrinkly X Round.
Producing 3:1 ratio (Seen in the 2nd gen).
Proves Mendel’s law of dominance.

Question 10

What is Mendel’s law of dominance?

Answer 10

When a dominant allele is present it determines the phenotype by masking the expression of a recessive allele.

Question 11

What is Mendel’s law of segregation ?

Answer 11

During gamete formation, each gamete will randomly receive 1 allele

Question 12

How did Mendel discover the law of independent assortment?

Answer 12

By following 2 traits at once in a cross (Dihybrid cross).

Question 13

What is a Dihybrid cross? Which law does it prove?

Answer 13

Occurs between 2 organisms with different variants (alleles) of [2 genes]. E.g. Wrinkly and green X Round and yellow.
Proves Mendel’s law of independent assortment.

Question 14

What is the law of independent assortment and what ratio does this produce?

Answer 14

During gamete formation genes on different chromosomes are inherited independently of each other. So alleles of different chromosomes have a 50% chance of ending up on the same gamete. 9:3:3:1

Question 15

When is the only time independent assortment is true?

Answer 15

If genes are not physically located on the same chromosome.

Question 16

What was the issue with Mendel’s research?

Answer 16

His laws [only considered assortment] (genes on different chromosomes) - so he considered traits to be completely independent.

Question 17

What did Morgan (& Sturtevant) discover?

Answer 17

Linkage (suggesting a “coupling” between certain alleles).

Question 18

What organism did Morgan use and why?

Answer 18

Fruit flies:
- low cost
- rapid generation
- genetic tools

Question 19

What are recombinants?

Answer 19

Gametes produced by the crossing over of parental chromosomes (non-parental phenotypes)

Question 20

How was linkage discovered?

Answer 20

Morgan crossed fruit flies and found observed ratio was not as expected (1:1:1:1) instead the parental phenotype was expressed more heavily. This is due to the closeness of the genes on the chromosome

Question 21

Can linked genes be separated?

Answer 21

Yes, during crossing over in meiosis (this may explain differences in ratios)

Question 22

What factor impacts the rate of gene recombination?

Answer 22

The distance of genes on the same chromosome - the further apart 2 genes are the higher the chance segregation will occur.

Question 23

What do recombination frequencies show?

Answer 23

Where genes are relative to each other on chromosomes.
Closer = lower recombination frequency
Further = higher recombination frequency

Question 24

What is a centimorgan and what are they used for?

Answer 24

A unit of probability- how likely one gene is to be separated from another due to a crossover event. 1 cM = 1% chance
Bigger cM means genes are further away from each other.

Question 25

What is the equation for calculating centimorgans?

Answer 25

Recombination frequency (cM) = Numb of recombinant progeny / total numb of progeny X 100 Answer will be a percentage

Question 26

Which genes cannot be mapped?

Answer 26

Those that are located more than 50 cM apart as their recombination frequency will be 50 %- meaning they cannot be distinguished from being located on a separate chromosome

Question 27

What factors affect the calculations of centimorgans ?

Answer 27

1. Multiple crossing over events 2. Small samples size

Question 28

What are Mendel's 4 assumptions?

Answer 28

1. Genes are not physically linked 2. Each trait is controlled by a single gene 3. Each gene has only 2 alleles 4. There is a clear dominant-recessive relationship between the alleles MOST GENES DON'T MEET THESE CRITERIA

Question 29

What is incomplete/semi dominance? Give an example and the ratio produced

Answer 29

Where offspring are a blend of both parents E.g. Red, pink and white snap dragons 1:2:1

Question 30

What is Co-dominance? Give example

Answer 30

Where both alleles are expressed E.g. Black and white patches on a cow

Question 31

What are multiple alleles? Give example

Answer 31

A gene with multiple alleles E.g. Blood groups

Question 32

What is Epistasis? Give example and the ratio produced

Answer 32

Where alleles at different loci interact with each other (potentially masking the expression of the other). E.g. The expression of gene B influences the expression of gene A - gene B is epistatic to gene A 9:4:3

Question 33

Which 2 ways can the environment influence gene expression?

Answer 33

1. Phenotypic plasticity 2. Continuous variation

Question 34

What is phenotypic plasticity?

Answer 34

Refers to different phenotypes occurring from the same genotype due to environmental conditions. E.g. changing coat colour in cats due to enzymes for temperature

Question 35

What is continuous variation?

Answer 35

Phenotype being the result of an accumulation of contributions by multiple genes (more common than traits due to single genes).

Question 36

What is the difference between Autosomes and Allosomes ?

Answer 36

Autosomes determine an organisms physical characteristics (22 pairs), Allosomes are sex chromosomes (1 pair).

Question 37

How does the Y chromosome in males differ from the X ?

Answer 37

The Y chromosome is very reduced It includes the SRY (sex-determining region Y) sequence

Question 38

What is the SRY responsible for ?

Answer 38

It is a transcription factor responsible for initiating male sex determination - testes development and inhibits female anatomical structural growth.

Question 39

How did Morgan discover sex linkage ?

Answer 39

When studying fruit flies he found there were no females with white eyes so the white eyed gene is located on the sex chromosome

Question 40

What are X-linked genes?

Answer 40

Genes located on the X chromosome. Females (XX) and males (XY) are affected differently. If males have a mutated (recessive) allele it will be expressed as they don't have a another X chromosome to dominate it (unlike females).

Question 41

What is X- inactivation and why does it happen?

Answer 41

X inactivation occurs in females during embryonic development to one of her X chromosomes to prevent a "double dose" of X-linked genes. One X chromosome condenses into a Barr body (a compact object)

Question 42

Why does X-inactivation not mean more women are colour blind?

Answer 42

A different X chromosome will inactivate in each cell. Therefore in female carriers (1X mutated, 1 X normal), Half the eye cells should be normal and half colourblind. It only takes a portion of functioning cones (in the retina) with the normal gene to allow for normal colour vision. But if she inherits two mutated X chromosomes then she will be colourblind.

Question 43

Are X-linked traits more common in males or females?

Answer 43

Males (XY)

Question 44

What ratio in a dihybrid cross indicates the occurrence of epistasis?

Answer 44

9:4:3