3.08 Bio 1107 Midterm #3

Incomplete dominance vs codominance

Incomplete dominance: heterozygotes have their own phenotype (pink flowers). Codominance: heterozygotes display both phenotypes (AB blood) (p.86)

What are quantitative traits?

Continuous phenotypes controlled by additive alleles at multiple genes (e.g. height, skin color); polygenic inheritance = multiple genes (p.86)

What is multiple allelism?

More than 2 alleles determine a trait (e.g. ABO blood) (p.86)

Gene-by-gene interaction example

Phenotype at one allele depends on the allele at another gene (dog coat color with B locus and E locus) (p.86)

Pleiotropy

One gene controls multiple unrelated traits (p.86)

Gene-by-environment interaction

The environment determines the phenotype of alleles (p.86)

[Fig p.85 Pedigree symbols] In the pedigree, what do a filled symbol and a slashed symbol mean?

Filled square/circle = affected individual; slash through the symbol = deceased; empty square = male, empty circle = female (p.85)

What is Mendelian inheritance vs non-Mendelian inheritance?

Mendelian: one trait controlled by one gene with only two alleles, each either dominant or recessive. Non-Mendelian: anything else (Lecture pp.7-9)

What is the 'expression' vocabulary trap?

In molecular biology, 'expressed' means transcribed and translated (both the dominant AND recessive allele are usually expressed); in phenotype talk, 'expressed' means visible, so only the dominant allele is 'expressed'. In this class, use the molecular definition and say the dominant trait is 'visible' or 'displayed' (Lecture p.12)

Why does incomplete dominance happen mechanistically (four-o'clocks)?

The r allele produces no pigment and each R allele produces 1 'unit' of pigment, so 2R > 1R > 0R; the heterozygote shows an intermediate phenotype between the two homozygous phenotypes (Lecture p.14)

[LC] Rr x Rr in four-o'clocks (incomplete dominance): what is the phenotypic ratio?

C) 1/4 purple (RR), 1/2 lavender (Rr), 1/4 white (rr); the genotypic and phenotypic ratios match, which is the diagnostic for incomplete dominance (Lecture p.44)

What is co-dominance, and how is it different from incomplete dominance?

Co-dominance: the heterozygote displays BOTH phenotypes associated with each allele (no intermediate). Incomplete dominance: the heterozygote shows a single intermediate phenotype (Lecture pp.14-16)

Why is the ABO blood system co-dominant mechanistically?

All red blood cells carry a glycoprotein on their membrane; the IA allele adds alpha-N-acetylgalactosamine, the IB allele adds alpha-D-galactose, and i adds nothing. IA and IB both make their own sugar (no 'intermediate' sugar exists), so IAIB cells display both: type AB (Lecture pp.16-18)

ABO blood types and their genotypes

A: IAIA or IAi; B: IBIB or IBi; AB: IAIB; O: ii. The ABO locus is called 'I'; IA and IB are co-dominant; i is recessive. (The Rhesus factor locus 'R' is separate: R is dominant to r) (Lecture pp.17-19)

How many alleles for a gene can exist in the population vs in one diploid individual?

A population can have many (>2) alleles for a gene (multiple allelism), but each diploid INDIVIDUAL can have at most two different alleles in their genome (Lecture pp.11,21)

[IKE] Which statements are TRUE about non-Mendelian inheritance? (A-F)

B is true (polygenic inheritance produces continuous phenotypes) and… for A: FALSE, an individual has at most 2 alleles; C: FALSE (that is pleiotropy, not gene-by-gene interaction); D: FALSE (that is incomplete dominance, not co-dominance); E: FALSE (Mendelian chromosome behavior still explains non-Mendelian traits); F: FALSE (most traits are complex, not simple Mendelian) (Lecture p.22)

In the human skin-color model (AaBbCc x AaBbCc), what determines the phenotype?

Each 'capital' allele adds one unit of pigment; units are additive. What matters is the TOTAL number of capital alleles, not which genes carry them (so AaBbCc and AABbcc both have 3 units); each parent makes 8 gamete types at 1/8 each (Lecture pp.24-26)

In the AaBbCc x AaBbCc skin-color cross, what is the most common offspring phenotype?

The middle phenotype (3 pigment units, genotype AaBbCc), giving a bell-shaped distribution of phenotypes (like a normal distribution from the evolution module) (Lecture p.27)

[IKE] In the A,B,C skin-pigmentation model, which statements are true?

A is TRUE (AaBbCc and AABbcc both have 3 pigment units, same phenotype); B is FALSE (an AaBbCc individual makes 8 gamete types, not just ABC and abc); C is FALSE (most children are mid-range, extremes are rare); D is FALSE (selection acts on phenotypes, and even individuals with fewer pigment alleles have a phenotype) (Lecture p.48)

How do Labradors differ from human skin color in their gene control?

Labrador coat color: two genes controlling ONE discrete trait (B determines black vs brown pigment; E controls whether pigment is deposited into fur, with ee blocking deposition, producing yellow dogs from B- ee or bb ee). Human skin color: three genes with ADDITIVE alleles making a continuous quantitative trait (Lecture pp.31-32)

Labrador coat color genotypes and phenotypes

B- E- = black; bb E- = brown (chocolate); B- ee = yellow; bb ee = yellow (the ee genotype blocks melanin deposition regardless of the B locus) (Lecture p.29 fig)

[IKE] The A,B,C skin-pigmentation model is an example of what? (select all)

A) A quantitative trait and B) Polygenic inheritance (not a gene-by-gene interaction: there is no masking, the alleles are additive; not gene-by-environment in this model) (Lecture p.33)

What is the frizzle chicken example of?

Pleiotropy: one gene contributes to multiple traits. The dominant frizzle allele causes curly feathers, abnormal body temperature, higher metabolic rate, and fewer eggs (Lecture p.35)

What human examples illustrate each non-Mendelian pattern?

Gene-by-gene: baldness (an allele at another locus prevents hair production); quantitative: skin pigmentation (up to ~400 additive genes); multiple allelism: HLA (over 1000 alleles for some genes); pleiotropy: PKU (one gene affects intellectual disability, hair and skin pigmentation, posture, gait); gene-by-environment: height (genetics plus childhood nutrition) (Lecture p.38)

What are the gene-by-environment examples from the slides?

Hydrangeas: same genotype, different flower colors depending on soil acidity and aluminum content; Himalayan rabbits: same genotype, different pigment patterns at different rearing temperatures; human height: about 20-40% of variation is attributed to environment, mainly nutrition (Lecture p.37)

[LC] An IAi individual mates with an IBi individual: what fraction of offspring are type O?

1/4: each parent passes i with probability 1/2, so ii (type O) = 1/2 x 1/2 = 1/4 (Lecture p.45)

[LC] Why is IA dominant over i?

B) The IA allele encodes an enzyme that produces a particular sugar molecule, while the i allele encodes an inactive version of the enzyme (dominance is about enzyme function, not allele 'strength', fitness, or transmission) (Lecture p.46)

[LC] A type A-positive mother has a B-negative son and an O-negative daughter. What are the son's and mom's genotypes?

Son: IBirr; Mom: IAiRr (daughter is iirr). Mom must be IAi (she made an O child), Rr (she made an rh-negative child), and the son got IB from dad and i, r from mom (Lecture p.47)

[LC] Two double heterozygotes cross where dominant C INHIBITS kernel color and dominant B gives blue vs red. What ratio?

D) 12:3:1 (dominant epistasis: any C- genotype masks the B locus, giving colorless kernels; 12 'inhibited' : 3 blue : 1 red). The standard 9:3:3:1 no longer applies because one gene's phenotype depends on another locus (Lecture pp.50-51)

[LC] The maize kernel-color example (12:3:1 instead of 9:3:3:1) is best described as:

C) A gene-by-gene interaction (the phenotype at the B locus depends on the allele at the C locus) (Lecture p.51)

[LC] Which class examples are cases of multiple allelism?

D) Human ABO blood type and E) the HLA locus (Labrador coat color is gene-by-gene; human skin color is polygenic; Himalayan rabbit coat color is gene-by-environment; hydrangea color is gene-by-environment; four-o'clocks is incomplete dominance with two alleles; frizzle is pleiotropy) (Lecture p.52)

[LC] In a species with two genes each having alleles 0, 1, 2 units of pigment, which statement is NOT true?

D) Coat color is a pleiotropic trait is NOT true (it is quantitative, polygenic, and multiple allelic; pleiotropy is one gene affecting multiple traits, not additive units); A0A0B2B2 (4 units) does equal A1A1B1B1 (4 units) in phenotype (Lecture p.53)

[LC] Are dominant alleles more common than recessive alleles in a population?

F) The statement is false; dominance does not make an allele more common (dominance is about phenotype in the heterozygote, not about fitness, transmission, or frequency) (Lecture p.54)

What is a discrete trait vs a quantitative trait (lecture definitions)?

Discrete trait: only a few possible phenotypes falling into distinct classes. Quantitative trait: a continuous phenotype controlled by additive alleles at multiple genes (the result of polygenic inheritance) (Lecture p.42)

Do Mendel's rules still apply to non-Mendelian traits?

Yes: MOST traits are complex and quantitative, not one-gene-two-allele, BUT the Mendelian rules (independent assortment and segregation, based on chromosome behavior) still apply to these 'non-Mendelian' traits (Lecture p.41)

What is gene-by-gene interaction (epistasis), defined strictly?

Two (or more) genes controlling one DISCRETE trait such that the phenotype associated with one allele depends on alleles at another gene locus (NOT additive alleles or a continuous phenotype as in quantitative traits) (Lecture pp.31,42)