Quiz 2 - Mendelian/Non-Mendelian

Question 1

Why did Mendel choose to work with peas?

Answer 1

• They have a short generation time
  – They have different traits that can be observed
  – He could strictly control which pea plants mated with which
  – They are cheap & readily available
  – They can grow fast

Question 2

How do peas self-fertilize?

Answer 2

• Male organs (stamens) produce
  pollen grains, which make sperm
  – Female organs (carpels) produce
  eggs
  – A flower’s pollen falls on the
  female organ of that same flower

Question 3

How did Mendel control matings?

Answer 3

Cross-pollination
- Mendel could prevent self- pollination by removing male
organs from a flower
* He used pollen from one flower
and transferred it to the female organ of a flower on another plant whose male organs (stamen) have been removed
– Called a cross, or cross-pollination

Question 4

What was Mendel’s experiment?

Answer 4

Mendel cross-pollinated two
parental pea plants (represented
by P generation) to produce the
F1 generation (first filial)
o First, the stamens of a purple parent
flower are removed
o Next, pollen from stamens of a white parent flower are transferred to
carpel of the purple parent flower
o All F1 offspring are purple
Mendel allowed the F1 individuals (all were purple) to self-pollinate, thereby
producing the F2 generation (second filial)
– Many of the F2 plants had
purple flowers, but some had white
* Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation

Question 5

What did Mendel call what we now know is a gene?

Answer 5

heritable factor

Question 6

What was Mendel’s model to explain the 3:1 inheritance pattern observed in the F2 generation?

Answer 6

Four related concepts make up this model
- 1st concept: Alternative versions of genes account for variations in inherited characters (these alternative versions of a gene are now called alleles)
- 2nd concept: For each character an organism inherits two alleles, one from each parent (the two alleles at a locus on a chromosome may be identical, as in the true-breeding plants of Mendel’s P generation. Alternatively, the two alleles at a locus may differ, as in the F1
hybrids)
- 3rd concept: If two alleles at a locus differ, then the dominant allele determines the organism’s appearance, and the recessive allele has no noticeable effect on appearance (In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant)
- 4th concept: The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes (law of segregation); an egg or a sperm gets only one of the two alleles that
are present in the somatic cells
of an organism. This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis

Question 7

What is homozygous vs heterozygous?

Answer 7

• An organism with two
  identical alleles for a
  character is said to be
  homozygous for the gene
  controlling that character
• An organism that has two
  different alleles for a gene
  is said to be heterozygous
  for the gene controlling
  that character

Question 8

What is the law of independent assortment?

Answer 8

When Mendel considered two traits, each trait assorted
independently to gametes (he called this the law of independent
assortment of alleles)
- In meiosis I, the maternal and paternal chromosomes assort independently during cell division

Question 9

What is a monohybrid cross and what is it’s phenotypic and genotypic ratio?

Answer 9

Monohybrid cross: a genetic cross between parents that are both heterozygous for ONE trait
– For example = Rr x Rr
- Genotypic ratio is 1RR : 2Rr: 1rr
which translates to a 3 dominant trait: 1 recessive trait phenotypic ratio

Question 10

What is a dihybrid cross and what is. its phenotypic ratio?

Answer 10

Dihybrid cross – a genetic cross between parents that
are both heterozygous for TWO traits
– For example = RrYy x RrYy
– The result of a dihybrid cross would be a 9:3:3:1 phenotypic ratio (9 = both dominant traits, 3 = one dominant & one recessive trait, 1 = both recessive traits)

Question 11

What is complete dominance?

Answer 11

• Complete dominance occurs
  when phenotypes of the
  heterozygote and dominant
  homozygote are identical

Question 12

What is incomplete dominance?

Answer 12

• When two traits are not
  completely dominant over
  each other (instead there is
  blending of traits)
  —> in other words, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental
  varieties

Question 13

What is codominance and what are examples?

Answer 13

– When two traits are
BOTH equally dominant
(so both traits are
present at the same
time…NO blending)
—> two dominant alleles affect the
phenotype in separate, distinguishable ways
—> ex. roan cattle, ABO blood system, rhododendron

Question 14

What is an example of a trait that is controlled by more than 2 alleles?

Answer 14

Most genes exist in populations
in more than two allelic forms
- For example, the four
phenotypes of ABO blood group
in humans are determined by
three alleles for the enzyme that
attaches A or B carbohydrates to
red blood cells:
IA allele – encodes enzymes that
adds the A carbohydrate
– IB allele – encodes enzymes that
adds the B carbohydrate
– i allele – no enzyme encoded (so
no A or B carbohydrates)

Question 15

What is epistasis, epistatic and hypostatic gene?

Answer 15

• Epistasis is when two or more genes affect one phenotype because
  expression of one gene interferes with expression of another gene; the interaction between genes is antagonistic, such that one gene masks or interferes with the expression of another
• The epistatic gene does the masking (suppresses) while the hypostatic gene is
  masked
• So fewer than four phenotypes are observed and phenotypic ratio resulting from a dihybrid cross will deviate from 9:3:3:1 Mendelian ratio

Question 16

What is recessive epistasis?

Answer 16

• Recessive epistasis happens when the recessive allele controls another gene.
• Example: coat color of Labrador dogs, the E gene is epistatic while the B gene is hypostatic.
  – The E gene is responsible for developing fur pigmentation while the B gene determines the colour of the pigmentation
  – If the E gene is double
  recessive (ee), then the
  Labrador dog will be yellow
  no matter the genotype
  expressed at the B gene
  – If the E gene contains at least
  one dominant allele (Ee or
  EE), the fur pigmentation will
  occur
  – The B gene will give rise to the color of the fur with the dominant B allele representing black and the recessive b allele representing brown or chocolate
  – 9 dominant hypostatic (black): 3 recessive hypostatic (brown): 4 epistastic (yellow)

Question 17

What is dominant epistasis and give an example?

Answer 17

Dominant epistasis happens when the dominant allele of one gene
masks the expression of all alleles of another gene
* If an organism inherits one or two copies of the dominant allele,
they will have the trait
* EXAMPLE = squash color
– Alleles of a locus that we will call Y produce either yellow (Y_) or green (yy) fruit
- However, in the presence of a dominant allele at a second locus that we
call W, no pigment is produced at all, and fruit are white
– The W gene is therefore epistatic to the Y gene (thereby, making the Y gene
hypostatic)
– One possible biological interpretation of this segregation pattern is that the
function of the W allele somehow blocks an early stage of pigment
synthesis, before neither yellow or green pigments are produced
- 12 epistatic (white): 3 dominant hypostatic: (yellow): 1 recessive hypostatic (green)

Question 18

What is complementary epistasis and explain the example?

Answer 18

Also known as duplicate recessive epistasis
* Complementary gene interaction happens when both genes need
to work together to produce a phenotype
* Either gene when homozygous recessive is epistatic to the other
gene.
* EXAMPLE = sweet pea flower color
There are two genes that work in tandem to make the pigment
anthocyanin that imparts color in the flower
– One produces precursor 1 (allele C) and the other makes precursor 2 (allele P) – need both precursors to produce anthocyanin
– Gene C is responsible for producing chromogen (a colorless chemical
substance, which gives rise to white flower color) – this chemical substance
is a precursor to a biochemical pigment (which is anthocyanin)
– Gene P is responsible for producing an enzyme that converts chromogen
into a pigment called anthocyanin (which gives rise to purple flower color)
Homozygous recessive alleles at either of the two loci can mask the expression of dominant alleles at the two loci
– If Gene C is made up of both recessive alleles (cc), it is epistatic to the
dominant allele(s) found at the P gene – PP and Pp (so, P gene is
hypostatic). No chromogen will be synthesized – so, the flower color is white.
– If Gene P is made up of both recessive alleles (pp), it is epistatic to the dominant allele(s) found at the C gene – CC and Cc (so, C gene is hypostatic). No anthocyanin will be synthesized – so, the flower color will remain white
- No chromogen and no anthocyanin synthesis will take place when Gene C
and Gene P are both made up of homozygous recessive alleles – ccpp. The
flower color is white.
– Chromogen and anthocyanin synthesis will take place when dominant
allele(s) found in both genes – CcPP, CcPp, CCPp, CCPP. The flower color is purple
-9 purple : 7 white phenotypic ratio with
respect to flower color in sweet pea plants
[if both parents are heterozygous at the C
and P locus
- 9 dominant allele in both genes (purple) : 3 homozygous recessive in one, other or both genes (white)

Question 19

What is duplicate dominant epistasis?

Answer 19

• In duplicate dominant gene interaction, the dominant alleles at the two loci produce the same phenotype, but there is no additive or
  cumulative effect
• The genes in a redundant system are said to have duplicate gene
  action; they either encode the same gene product, or they encode
  gene products that have the same effect in a single pathway or
  compensatory pathways
• EXAMPLE = fruit shape of Capsella bursa pastoris (shepherd’s purse)
  – Two types of fruit shapes: triangular-shaped (dominant trait) and ovalshaped (or top-shaped) (recessive trait)
  – The dominant triangular fruit shape of Capsella bursa pastoris (shepherd’s purse) is determined by two pairs of genes, say A and B
• If any of these genes has at least one dominant allele, the fruit shape is
  triangular
• If both genes are homozygous recessive, the fruit shape is oval shaped
• 15 triangular : 1 oval-shaped
  phenotypic ratio with respect to fruit
  shape in shepherd purse plants [if
  both parents are heterozygous at the
  A and B locus – AaBb (triangular) x
  AaBb (triangular)]

Question 20

What is pleiotropy and what are two examples?

Answer 20

The ability of a single gene to have multiple phenotypic effects
* One gene can also influence a combination of seemingly unrelated
characteristics
* Example 1 – in tigers and Siamese cats, the gene that controls
fur pigmentation also influences the connections between a
cat’s eyes and the brain
* Example 2 – sickle cell anemia can cause:
o rapid destruction of sickle cells leading to anemia
o clumping of cells causing interference with blood
circulation
o accumulation of red blood cells in spleen leading to
enlargement

Question 21

How can phenotype affect genotype and what is the norm of reaction and give an example ?

Answer 21

Norm of reaction is the phenotypic range of a genotype influenced by the environment
* For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity
– For true blue flowers, the hydrangeas need to be grown in acidic soil (pH 5.5 and lower)
– For pink flowers, the plants need neutral to alkaline soils (pH 6.5 and higher)
– For purple blooms (or a mix of blue and pink flowers on the same plant), the pH of the soil must be 5.5 and pH 6.5.

Question 22

How is Marfan syndrome an example of pleiotropy?

Answer 22

Marfan syndrome:
o Unusually tall height
o Thin fingers/toes
o Dislocation of the lens of the eye
o Heart problems (aorta bulges/ruptures)
These symptoms don’t appear directly related to one another, but as it turns out, they can all be traced back to the mutation of a single gene
- This gene encodes a protein used in
creating elastic fibrils that give strength and flexibility to the body’s connective tissues
– Mutation of this gene results in fewer fibrils
– The eye and aorta normally contain many fibrils that help maintain structure, explaining why these two organs are strongly affected
- In addition, the fibrils serve as “storage shelves” for growth factors. When there are fewer of them in Marfan syndrome, the growth factors cannot be shelved and thus cause excess growth (leading to the characteristic tall, thin Marfan build)

Question 23

What is polygenic inheritance and what are examples?

Answer 23

Polygenic inheritance, in simple terms, implies a character or
phenotypic trait is regulated by more than one gene
- In biology, it refers to the quantitative inheritance wherein two or more independent genes additively affect a single phenotypic trait
- Human traits such as hair color, eye color, height, skin color, and
blood pressure are some of the examples of polygenic inheritance
– Accordingly, traits, such as height or skin color, cannot be categorized as
‘tall’ or ‘short’ / ‘light’ or ‘dark’
– These traits exhibit multiple and continuous variations of the phenotype
– EXAMPLE = Although there are two major eye color genes, there are at least 14 additional genes that play roles in determining a person’s exact eye color
- EXAMPLE = involving the color of wheat kernels to see how
Mendelian inheritance of multiple genes (plus a little incomplete
dominance of alleles) can produce a broad spectrum of phenotypes
– In this example, there are three genes that make reddish pigment in wheat
kernels, which we’ll call A, B, and C
– Each comes in two alleles, one of which makes a unit of pigment (dominant
allele) and one of which does not make any pigment (recessive allele)
– The aa genotype would contribute zero units of pigment, the Aa genotype
would contribute one unit, and the AA genotype would contribute two—
basically, a form of incomplete dominance

Question 24

How is polygenic inheritance different from mendelian inheritance?

Answer 24

polygenic inheritance is different from Mendelian inheritance in terms of pattern and expression
– In Mendelian Inheritance, each phenotypic trait is monogenic, meaning the expression of this trait is either expressed or masked based on it being ‘a dominant’ or ‘a recessive’ gene
– In polygenic inheritance, the expression of the phenotypic trait does not display complete dominance wherein the phenotypic expression of a trait in an offspring is a mixture or additive of traits displayed in parents

Question 25

What is intragenic interaction and what are examples?

Answer 25

Intragenic interaction = Two alleles of a gene, which are present on
same gene locus on the two homologous chromosomes, react (or
interact) to produce modified phenotype that deviates from
Mendelian inheritance patterns
– Examples include incomplete dominance, codominance, and multiple alleles

Question 26

What is intergenic interaction?

Answer 26

Intergenic interaction = The gene interaction in which the gene
present in one gene locus interacts with the another gene present in
another gene locus of the same or different chromosomes for the
expression of phenotypic character
– Examples include epistasis and polygenic inheritance

Question 27

What is polymeric gene interaction epistasis?

Answer 27

• Involves two independent dominant genes that control the same phenotype
• If one gene is heterozygous or homozygous dominant (while the
  other gene is homozygous recessive), this will generate a specific phenotype
• However, when both genes are either heterozygous or homozygous
  dominant, the phenotypic effect is enhanced (or another phenotype is generated)
• EXAMPLE = fruit shape in squash
  There are three types of fruit shape in this plant: disc, spherical and
  long
  – The disc shape is controlled by two dominant genes (A and B), the spherical shape is produced by either dominant allele (A or B) and long shaped fruits develop in double recessive (aabb) plants
• 9 disc-shape : 6 spherical : 1 long
  phenotypic ratio with respect to fruit
  shape in summer squash [if both
  parents are heterozygous at the A and B locus – AaBb (disc) x AaBb (disc)]
• Here plants with A-B- (9/16) genotypes produce disc shape fruits, those with Abb- (3/16) and aaB-
  (3/16) genotypes produce spherical
  fruits, and plants with aabb (1/16) genotype produce long fruits

Question 28

What is the allelic series for rabbit fur color?

Answer 28

Full color (C) > Chinchilla (c^ch) > Himalayan (c^h) > Albino (c)

Question 29

What is the difference between wild-type vs mutant alleles?

Answer 29

• wild-type: when many alleles exist for the same gene, this is the most common phenotype or genotype among wild animals (considered the standard/norm)
• All other phenotypes or genotypes are considered variants (or
  mutants) of this standard, meaning that they deviate from the wild
  type – the variant may be recessive or dominant to the wild-type
  allele

Question 30

What are examples of age-dependent gene expression?

Answer 30

• Human male pattern baldness (androgenetic alopecia) is usually not
  expressed until an individual reaches the age of 20 or older
• A dominant mutation of the Grey gene in horses causes progressive
  hair depigmentation, resulting in a completely white coat by the age
  of six to ten years
• A dominant allele of the huntingtin (HTT) gene causes adult-onset
  Huntington’s disease – it is not expressed until an individual is 30 to
  50 years of age

Question 31

What is sex-dependent gene expression and what are the three types?

Answer 31

• Expression of sex-dependent traits varies with the sex of the
  individual
• This is usually due to the influence of sex hormones on gene
  expression
• Sex traits can be categorized into three types of inheritance: sexlimited, sex-linked, and sex-influenced

Question 32

What is sex-limited traits and what is an example?

Answer 32

Sex-limited traits are traits that are visible only within one sex
– EXAMPLE = Feathering morphology in chickens
o Hen feathering in roosters is characteristic of certain domestic chicken breeds
o Males expressing this condition develop plumage characteristic of a hen, but
are genetically and behaviorally male
o The gene controlling this trait is autosomal and expressed only by males
§ Genotype = hh (wild-type) will produce hen-feathering in females and roosterfeathering in males
§ Genotype = h+h (heterozygous) will produce hen-feathering in females and henfeathering in males
§ Genotype = h+h+ will produce hen-feathering in females and hen-feathering in
males

Question 33

What are sex-linked traits and what’s an example?

Answer 33

Sex-linked traits are generally influenced by genes on the X
chromosome
– This is because the X chromosome is large and contains many more genes
than the smaller Y chromosome
– In a sex-linked disease, it is usually males who are affected because they
have a single copy of X chromosome that carries the mutation
– In females, the effect of the mutation may be masked by
the second healthy copy of the X chromosome
– EXAMPLE = color blindness

Question 34

What are sex-influenced traits and what is an example?

Answer 34

Sex-influenced traits are influenced by genes on autosomes, hence
such genes are present in both the sexes but their expression
varies in different sexes
– EXAMPLE = male pattern baldness (B is the dominant allele for baldness, b is the normal allele)

Question 35

What is temperature-dependent gene-expression and what is an example?

Answer 35

Temperature is known to influence phenotypes (the protein
products of specific alleles are functional at a certain temperature)
* EXAMPLE = Himalayan rabbits carry the C gene (mainly the ch
allele), which is required for the development of pigments in the
fur, skin, and eyes, and whose expression is regulated by
temperature
–- Specifically, the ch allele is inactive above 35°C and it is maximally active
from 15°C to 25°C
– This temperature regulation of gene expression produces rabbits with a
distinctive coat coloring
–- In the warm central parts of the rabbit’s body, the gene is inactive, and no pigments are produced (causing the fur color to be white)
–- Meanwhile, in the rabbit’s extremities (i.e., the ears, tip of the nose, and feet), where the temperature is much lower than 35°C, the C gene actively produces pigment, making these parts of the animal black
- Similar temperature-sensitive alleles
are found in other animals (Siamese
cats and Himalayan guinea pigs)
- If fur is removed from a patch on the rabbit’s back and and ice pack is
placed there (creating a cold environment), the new fur that grows
back is black