Mendelian Genetics

Question 1

What was the conclusion about gametes before microscopy was invented? What was another incorrect hypothesis?

Answer 1

that sperm carry little people (humunculus)

that traits of a parent are equally ‘blended’ in an offspring

Question 2

What was the correct hypothesis about inheritance?

Answer 2

The particulate law of inheritance - individual ‘particles’ or units of inheritance deliver specific traits and are maintained in offspring - we now call these particles genes

Question 3

Who discovered the basic principles of heredity? Who determined that genes are encoded by DNA? Who determined the structure of DNA?

Answer 3

Gregor Mendel in 1867

Griffith in 1944

Watson and Crick and Franklin in 1953

** crazy that Mendel’s discovery came first

Question 4

What is hybridization?

Answer 4

Cross pollinating two contrasting true-breeding varieties (ex. purple flowers x white flowers)

Question 5

What are the two fundamental principles of genetics?

Answer 5

1. Law of segregation
2. Law of independent assortment

Question 6

What were Mendel’s 4 conclusions from his experiments?

Answer 6

1. There are alternative versions of ‘heritable units’ that can be passed on to progeny - called alleles
2. For each character, an organism receives two copies (two alleles) of any given gene, one from each parent
3. If alleles differ at a locus, then one must be dominant
4. The law of segregation: the two alleles for a heritable character segregate (separate from each other) during gamete formation (ex. meiosis in animals) and end up in different gametes

Question 7

Describe the law of segregation.

Answer 7

You start with a set of homologous chromosomes

They duplicate their DNA

Anaphase I separates the homologous chromosomes

Anaphase II separates the sister chromatids

Half the gametes get one allele of a gene, the other half gets the other

** the two alleles for a heritable character end up in different gametes

Question 8

What is the phenotype and what is the genotype

Answer 8

Phenotype: the observed trait of a characteristic

Genotype: the genetic makeup that produces the phenotype

Question 9

Due to the law of segregation, each gamete produced in the F1 generation has a ________ % change of having either the dominant P or recessive p allele

Answer 9

50

Question 10

What is a dihybrid cross. What would happen in a dihybrid cross if genes didn’t sort independently?- how did this relate to mendel’s discoveries?

Answer 10

Follows two characters at once
- each parent as to be true-breeding for both characters (homozygous for both traits)

The F1 progeny will be heterozygous for each gene (dihyprids)

There are now 4 possible combinations of alleles in the gametes (law of independent assortment), therefore each gamete has a 25% change of carrying one of the four genotypes

Makes a phenotypic ratio of 9:3:3:1 in the F2 generation

If genes didn’t sort independently, the parental genotypes would be passed on to the F1 generation together (YR would always go together and yr would always go together)
- it would result in a 3:1 phenotypic ratio, but Mendel observed a 9:3:3:1 ratio

Question 11

What is the law of independent assortment

Answer 11

Two or more genes will assort independently into gametes - each pair of alleles segregates independently of any other pair of alleles during gamete formation

Question 12

In what scenario would two independent genes be passed onto progeny in a dependent fashion?

Answer 12

Linked genes -> if genes are really close on a chromosome they are “linked”. Alleles of these genes will be packaged into chromosomes together during meiosis

Question 13

When do you use the multiplication rule and the addition rule?

Answer 13

Multiplication rule: used to determine the probability of two or more independent events happening together in some specific combination
- what’s the probability of getting an
RR genotype? (2 Rr parents) -> probability of sperm and egg getting an R are both 1/2 and because they are independent of each other we would multiply them to get 1/4

Addition rule: used to determine the probability of two or more mutually exclusive events will occur together in some specific combination
- What’s the probability of getting a heterozygote (here there are two options, either the sperm is Rr or the egg is Rr) so you add those probabilities together - 1/4+1/4 = 1/2

Question 14

What is incomplete dominance?

What is complete dominance?

Answer 14

Incomplete dominance - In heterozygotes, the phenotype is a blend of both traits (ex. pink flower when red and white are crossed)

Complete dominance - the phenotype is dominant in heterozygotes

Question 15

What is co-dominance?

Answer 15

Results in the heterozygotes having a unique phenotype that is different from the homozygous phenotypes - ex. blood type IA, IB = AB blood type
* blood type is also an example of having more than two alleles for the same gene in a population of individuals (multiple alleles)

Question 16

What is Pleiotropy?

Answer 16

Occurs when one gene (or the alleles of one gene) affect multiple phenotypes
ex. sickle-cell disease
ex. albanism causes lack of melanin in skin but can also cause light sensitivity in eyes

Question 17

Describe penetrance

Answer 17

A homozygous genotype doesn’t ALWAYS produce a specific phenotype in all members of the group.

We refer to this as penetrance

ex. women homozygous for a mutant allele of BRCA1 gene are likely to develop breast cancer but it doesn’t mean they for sure will (65% of them do, so this gene is 65% penetrant)

Question 18

Explain expressivity

Answer 18

When a in a group of individuals with the genotype for a specific gene, a range of traits is displayed
- expressivity is the degree to which a particular genotype is expressed in the phenotype
ex. when every dog has the same genotype for the whiteness gene but each one expresses the trait to a different degree so we get a range of phenotypes

Question 19

What is epistasis?

Answer 19

Occurs where the phenotypic expression of a gene at one locus affects the phenotypic expression at a second locus *the phenotypes are dependent on each other

ex. Labrador retrievers
E = can deposit pigments in hair
e = can’t deposit pigments in hair
B = black fur
b = brown fur

The alleles segregate as normal but if the dog is homozygous ee, it doesn’t matter what genotype of gene B it is since the dog can’t deposit hair pigments anyways - it’ll be a golden lab

Question 20

Explain what multifactorial means

Answer 20

When the phenotype depends on the genotype as well as the environment

ex. red hydrangeas result from growing in basic soil and the same hydrangeas would turn blue in acidic soils

*also how twins can end up looking different based on their experiences and environments

Question 21

What is polygenic inheritance

Answer 21

occurs when many genes contribute to a single phenotype
ex. height or skin colour
* phenotype is quantitative instead of “either-or”

these traits usually follow a normal distribution across a population of individuals

many polygenic traits are also affected by environment (ex. skin colour affected by sunlight exposure)

Question 22

In a pedigree, what are the two signs for if a trait is dominant or if it is recessive?

Answer 22

Dominant:
- it will appear more often in the progeny
- an offspring displaying that trait will always have a parent with the trait

Recessive:
- it will appear less often in the progeny
- an offspring displaying that trait can have parents that do not display the trait