SB 6: Ch 17, Ch 5.1 - Organellar Inheritance, Gene Linkage & Recombination

If genes on different chromosomes assort independently because nonhomologous chromosomes align independently during Meiosis 1, how do genes on the same chromosome assort?

What is genetic linkage?

• Where particular alleles of genes tend to travel together during vertical or horizontal gene transfer

What affects the probability of recombination?

• The farther apart two genes are on a chromosome, the greater is the probability of separation through recombination

How can we see a gene’s relative location on a chromosome?

• Geneticists can use data about how often genes separate during transmission to map the gene’s relative locations on a chromosome

What does it mean for two genes to be "syntenic"?

• They are located on the same chromosome

• Example: Two X-linked genes that determine a fruit fly’s eye color and body color

In the white gene example, what does the dominant allele (w⁺) specify, and what does the recessive allele (w) specify?

In the yellow gene example, what does the dominant allele (y⁺) specify, and what does the recessive allele (y) specify?

• w⁺ = red eyes (wild-type); w = white eyes (mutant)

• y⁺ = brown body (wild-type); y = yellow body (mutant)

Distinguish lowercase y/y⁺ from uppercase Y, and the slash symbol.

• Lowercase y and y⁺ refer to alleles of the yellow gene (body color). Uppercase Y refers to the Y chromosome, which does not carry genes for eye or body color.

• The slash symbol separates the genes found on the two chromosomes of a pair — either an X and Y chromosome, a pair of X chromosomes, or a pair of homologous autosomes.

A male fly has the genotype w y / Y. What does this genotype mean structurally, and what is the resulting phenotype?

• Structurally: his X chromosome carries the w and y mutant alleles, and his Y chromosome carries neither gene. (The Y chromosome is because he’s a male).

• Phenotype: white eyes and a yellow body

In the cross w y⁺/w y⁺ female (white eyes, brown body) × w⁺y/Y male (red eyes, yellow body), what are the F1 genotypes/phenotypes, and why do F1 males resemble their mother?

• F1 = brown-bodied red-eyed females (w y⁺/w⁺y) and brown-bodied white-eyed males (w y⁺/Y).

• Males resemble their mother because their phenotype directly reflects the single X they inherited from her — they got no X from their father.

• The F1 females are dihybrids because they carry two different alleles for each X-linked gene (one from each parent): w and y⁺ from their mother's X, w⁺ and y from their father's X.

If the white and yellow genes assorted independently (Mendel's 2nd law), what 4 gamete types would F1 dihybrid females produce, and in what ratio?

• w y⁺, w⁺y, w⁺y⁺, and w y — all in a 1:1:1:1 ratio

• ½ Parental = w y⁺ and w⁺y (match the original P generation combinations).

• ½ Recombinant = w⁺y⁺ and w y (new combinations produced by reshuffling).

When genes assort independently (Mendel’s), what is true of the parental vs. recombinant F2 progeny numbers and the gamete ratios?

• Parental and recombinant numbers are equal, because a doubly heterozygous F1 individual produces an equal number of all four gamete types (1:1:1:1)

What defines two genes as "linked"?

• The F2 progeny number of parental genotypes outnumbers the F2 progeny number of recombinant genotypes

• Alleles behave as if they are connected to each other most of the time (instead of assorting independently)

How does this linkage work on the yellow and white genes on the Drosophila X chromosome?

• Only 1 in 100 gametes is recombinant, the parental allele combinations (w y⁺ & w⁺y, or w⁺y⁺ & w y) are inherited together 99 times out of 100.

Why don't linked autosomal genes follow the classic 9:3:3:1 F2 ratio?

• The 9:3:3:1 ratio assumes the four gamete types (AB, Ab, aB, ab) are produced in equal frequency (independent assortment)

• This makes each of the 16 Punnett square boxes equally likely.

• Linkage means parental gametes are produced more often than recombinant gametes, so the boxes are no longer equally likely.

If F1 dihybrids are configured A B / a b (linked), how does the F2 phenotypic ratio shift from 9:3:3:1?

• The 9/16 and 1/16 classes increase at the expense of the two 3/16 classes.

If F1 dihybrids are configured A b / a B (linked), how does the F2 ratio shift instead?

• The two 3/16 classes increase at the expense of the 9/16 and 1/16 classes.

How do testcrosses simplify detecting linkage?

• By crossing the doubly heterozygous parent to a parent homozygous recessive for both genes, geneticists can directly read off the gamete types (and thus parental vs. recombinant frequencies) transmitted by the heterozygous parent from the offspring phenotypes.

Why does maternal inheritance count as non-Mendelian inheritance?

• It challenges Mendel's assumption that maternal and paternal gametes contribute equally to inheritance — here only the mother's contribution determines offspring phenotype.

What do mitochondria do, and what's notable about their genome?

• They convert energy from glucose/nutrients into ATP

• They have their own DNA (separate from nuclear DNA); the mitochondrial genome encodes some but not all needed gene products — the rest are nuclear-encoded and imported

How do mitochondria behave during the cell cycle, and what's the consequence for daughter cells?

• They double in size, replicate their mtDNA, and divide in half each generation

• When the cell divides, mitochondria are distributed randomly/passively, causing large variation in mitochondria number between individual cells.

What are the two stages of ATP production in mitochondria, and what does each do?

• (1) Krebs cycle (in the matrix) metabolizes pyruvate to generate NADH and FADH2. (

• 2) Oxidative phosphorylation (inner membrane enzyme complexes) uses NADH/FADH2 to drive an electron transport chain, transferring electrons ultimately to oxygen.

What is RNA editing?

• The process that converts pre-mRNAs to mature mRNAs by changing the base sequence

What do chloroplasts do?

• They capture solar energy and store it in carbohydrate bonds via photosynthesis.

What structures inside chloroplasts hold chlorophyll and the photosynthetic electron transport proteins, and what happens during the light-trapping phase?

• Thylakoids.

• During the light-trapping phase, light photons boost chlorophyll electrons to higher energy levels

• Those energized electrons drive an electron transport system that converts water into oxygen and protons.

What two products does photosynthetic electron transport generate, and what are they used for?

• NADPH (electron donor) and ATP (via an ATP synthase similar to mitochondria's)

• In the Calvin cycle (sugar-building phase), enzymes use that ATP and NADPH to convert CO2 into carbohydrates.

How do chloroplast genomes compare to mitochondrial genomes in size uniformity?

• Chloroplast genomes are much more uniform in size across plants/algae than mitochondrial genomes are.

What is the endosymbiont theory's key claim?

• It proposes mitochondria and chloroplasts descended from once free-living bacteria engulfed by an ancestral eukaryotic cell.

What 4 lines of molecular evidence support the endosymbiont?

• (1) both organelles have their own DNA replicating independently of the nuclear genome

• (2) mtDNA/cpDNA lack histones/nucleosomes, like bacterial DNA

• (3) mitochondrial translation uses N-formyl methionine and tRNAfMet, as bacteria do

• (4) bacterial translation inhibitors (chloramphenicol, erythromycin) block mitochondrial/chloroplast translation but not eukaryotic cytoplasmic translation.

What's the difference between biparental and uniparental organelle inheritance, and what are the two forms of uniparental?

• Biparental = progeny get organelles from both parents.

• Uniparental = organelles come from just one parent — either maternal (all from mother) or paternal (all from father).

What's the most common uniparental pattern in animals, and what are two exceptions worth remembering?

• Maternal inheritance is most common.

• Exceptions: in bananas, chloroplast DNA is maternal but mitochondrial DNA is paternal; in sequoia trees, both chloroplast and mitochondrial DNA are paternally inherited.

Why does the "poky" trait in Neurospora exhibit non-Mendelian inheritance?

• Because maternal and paternal gametes don't contribute equally to progeny phenotypes (organelle-based, not nuclear).

Besides zygote size differences, what are 3 mechanisms that can cause maternal-only organelle inheritance?

• (1) In some plants, early zygote divisions shunt paternal organelle genomes into cells that won't become the embryo.

• (2) In some animals (e.g., tunicates), fertilization physically excludes paternal organelles (only the sperm nucleus enters the egg).

• (3) In many animals, the zygote actively destroys paternal organelles after fertilization.

What does it mean for a cell/organism to be heteroplasmic vs. homoplasmic?

• Heteroplasmic = contains more than one genotype of an organellar genome (e.g., both wild-type and mutant cpDNA).

• Homoplasmic = contains only one type of organellar DNA.

Why is yeast called an isogamous species?

• Because its gametes (haploid cells of opposite mating type) are similar in size and morphology to each other.