bio lec 14
What is incomplete dominance?
A form of inheritance where the heterozygote shows an intermediate phenotype between the two homozygotes.
🧠 Example:
FF = long flowering time
Ff = medium flowering time
ff = short flowering time
How does incomplete dominance differ from complete dominance?
Complete dominance: Heterozygote resembles one homozygote
Incomplete dominance: Heterozygote is intermediate
What phenotypic ratio results from incomplete dominance in a monohybrid cross?
1:2:1
1 homozygous dominant
2 heterozygous (intermediate)
1 homozygous recessive
compared to 3:1 in compelte dominance
What is penetrance?
The percentage of individuals with a particular genotype who actually express the associated phenotype.
🧠 Example: Extra fingers/toes found only
in P+PX (heterozygotes) but Not all cats or humans with the P+PX genotype for polydactyly have extra digits. this is incomplete pentrance
What is incomplete penetrance?
When not all individuals with the genotype show the trait.
What is expressivity?
The degree or severity to which a trait is expressed among individuals with the same genotype.
🧠 Example: In polydactyly, some individuals may have a fully formed extra finger, others just a small nub.
What causes variation in penetrance and expressivity?
Other genes (gene interactions)
Environmental factors
Regulatory mutations affecting gene expression (e.g., in enhancers)
What is a gene interaction?
When two or more genes at different loci influence the same phenotype, rather than acting independently.
What’s an example of gene interaction producing a novel phenotype?
Pepper color — controlled by two genes (Y and C):
Y+_ C+_ → red (double dominant)
Y+_ cc → peach
yy C+_ → orange
yy cc → white (double rec)
: What phenotypic ratio results from dihybrid gene interaction (like in pepper color)?
9:3:3:1 — but with four distinct colors instead of Mendel’s original seed shape/texture categories. Y locus and the
C locus interact to produce a single phenotype, the colour of
the pepper
How do you calculate the probability of a specific color in a testcross involving gene interaction?
try for peach Y+cc between a red Y+yC+c and cream yycc
se separate Punnett squares for each gene, then multiply the individual probabilities.
🧪 Example:
P(Y+)=1/2 P(cc)=1/2
P(Peach)=1/2×1/2=1/4
What is epistasis?
When one gene masks or hides the effect of another gene at a different locus.
🧠 Key Difference:
Dominance = interaction of alleles at same locus
Epistasis = interaction between genes at different loci
What is an epistatic gene? What is a hypostatic gene?
Epistatic gene = the gene doing the masking
Hypostatic gene = the gene being masked
What genes control coat color in Labrador retrievers? (recessive epistasis)
Gene B: pigment type
- B = black (dominant), b = brown (recessive)
Gene E: pigment deposition
- E = pigment deposited, e = no deposition → yellow
✅ ee = yellow regardless of B/b genotype → E is epistatic
In Labrador coat color, which gene is epistatic?
Gene E is epistatic to Gene B — it controls whether pigment is deposited at all.
In a biochemical pathway, which gene is epistatic?
The gene that acts upstream is usually epistatic to the downstream gene — it controls the availability of intermediate compounds.
Why is this important?
If the first gene blocks the pathway early, the final product is never made, regardless of downstream gene function.
recessive epistasis vs dominant epistasis
In recessive epistasis:
two recessive alleles for an upstream gene are
needed to mask downstream gene phenotypic
expression
In dominant epistasis:
only one allele is required in upstream gene for
masking effect
What causes dominant epistasis in summer squash color?
W locus (Enzyme I): W (dom) allele prevents compound B from being made → WHITE squash
Y locus (Enzyme II): yy (rec) prevents conversion to compound C → GREEN squash
W_ masks effect of Y/y, regardless of Y genotype
🧠 Phenotypes:
W_ → white
ww yy → green
ww Y_ → yellow
genomic imprinting
A phenomenon where the expression of an allele depends on whether it is inherited from the mother or the father. (autosomal genes)
🧠 Only one allele is expressed, the other is silenced.
What causes imprinting?
Methylation (addition of CH₃ to DNA)
Involves imprinting control regions. Controls
inactivation of clusters of genes (3-12 genes) on
specific Chromosomes
Often uses long noncoding RNAs (like X-inactivation)
What gene is a well-known example of imprinting? (ex. in humans/mice)
Igf2 (Insulin-like growth factor 2)
Paternal allele is expressed
Maternal allele is silenced
📌 If that paternal allele is amorphic or hypomorphic,
offspring have low fetal size and growth (regardless of allele
inherited from mother)
What about the Igf2 receptor (Igf2r)?
Opposite imprinting:
Maternal Igf2r is expressed
Paternal Igf2r is silenced
→ Loss of maternal Igf2r → overgrowth
Is genomic imprinting Mendelian?
No — it is non-Mendelian because expression depends on parental origin, not just genotype.
genomic imprinting is a type of
epigenetics – some traits are determined by
modifications to DNA or chromatin (like DNA
methylation)
nuclear genes vs mitochondrial genes
Nuclear genes regulate characteristics (Human - 20,000 Genes)
Mitochondrial genes can regulate important
characteristics!! (Human – 37 genes)
What is cytoplasmic inheritance?
Inheritance of traits encoded by non-nuclear genes, especially in mitochondria or chloroplasts.
How are cytoplasmic genes inherited?
From the mother only
Passed to both sons and daughters
Fathers do not pass on mitochondrial genes
diploid receives maternal mito DNA
How do reciprocal crosses differ with cytoplasmic inheritance?
They give different results — only the mother’s genotype affects the offspring’s phenotype.
Why is phenotypic variation high in mitochondrial traits?
Because mitochondria are randomly distributed to daughter cells during cell division — no mechanism ensures even distribution. even though it might start even with time large variations created (some with almost all one type or the other)
What is an example of cytoplasmic inheritance in plants?
Variegation in Four-o’clocks — some branches are green, white, or variegated depending on the mitochondria/chloroplasts inherited from the mother cell.
who studied snail chirality
Arthur Boycott (1920s)
tried to study snail
chirality using mendelian
principles of heredity
Why is snail shell spiral direction (chirality) not inherited like pea color?
Because the phenotype of the offspring is determined by the genotype of the mother, not the offspring’s own genotype.
✅ This is known as the genetic maternal effect.
What determines whether a snail will be right-handed or left-handed?
The mother’s nuclear genotype determines the direction of the offspring's shell spiral — due to maternal mRNA and proteins deposited into the egg during oogenesis.
Why is this important in development?
It shows that the first instructions for how to build a body (like body axes or symmetry) come from the mother's gene products, not from the embryo’s DNA.
🧠 Key takeaway: This is a powerful example of non-Mendelian inheritance where developmental fate is pre-set by the maternal environment.
What is the genetic maternal effect?
A trait where the **offspring’s phenotype is determined by the genotype of the mother, not the offspring’s own genotype.
How is this different from cytoplasmic inheritance?
Genetic maternal effect = nuclear genes from the mother, but the phenotype is shaped by maternal mRNAs/proteins/ organelles deposited into the egg. The offspring inherits genetic material
from both the mother and father BUT the
phenotype of the offspring is determined
by the genotype of the mother
Cytoplasmic inheritance = actual organelle genes (e.g., mitochondrial) passed from mother only
In cytoplasmic inheritance, the offspring
inherit the genetic material only from the
mother
What is an example of genetic maternal effect?
Snail shell chirality (direction of spiral)
Right-handed (S⁺) = dominant
Left-handed (S) = recessive
Progeny phenotype depends on mother’s genotype, not their own
Why does the maternal effect occur?
Because the egg contributes more than DNA — it also provides:
Organelles
mRNA
Proteins that control early development
What alleles control snail shell chirality and what do they do?
S⁺ (dominant) → right-handed shell
→ produces a substance that causes rightward rotation
S (recessive) → left-handed shell
→ does not produce the substance
Why can an S⁺S snail still have a left-handed shell?
Because its mother was SS and did not deposit the right-rotating substance into the egg → the embryo develops a left-handed shell despite having an S⁺ allele.
he egg cytoplasm (from mom) contains the early molecular instructions, so the offspring’s phenotype reflects the mother's genotype, not its own.