BIOL207 Lecture Notes - Lecture 15: Zw Sex-Determination System, Allosome, Mendelian Inheritance
5 May 2018
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Sexual Reproduction
1. Unicellular Eukaryotes → alpha and a.
2. Fission Yeast → P and M
Sex determination
1. Environmental → Sea turtles (based on egg temperature) → Warm = males, cool =
females.
a. Clown Fish → Male becomes female and undifferentiated fill the male role.
2. Single Autosomal Gene → Asparagus (M/m = male and m/m = female)
3. Haplodiploid System → Diploid females and Haploid males (queen and drones are
fertile) → bees, wasps and ants
a. Queen undergoes meiosis to become haploid and male undergoes mitosis to
produce gametes.
b. Eggs that are not fertilized are drones.
4. XY Sex chromosomes → Drosophila and Humans.
5. ZW Sex chromosomes → Birds, moths and butterflies → Z/Z = males and Z/W = males.
a. Sex-linked gene is on the Z chromosome.
Meiotic Recombination
1. Meiotic Recombination in single-celled Eukaryotes
a. Parental and recombinant gametes. (Meiotic recombination = production of
recombinants in offspring/gametes).
2. Meiotic Recombination in multicellular Eukaryotes → haploid cells that are parental or
recombinant.
Genes on Chromosomes
1. Different Chromosomes → Recombinant frequency is 50% independent assortment of
chromosomes occurs to produce recombinant gametes.
2. Same Chromosome → No crossovers between genes therefore no gametes are
recombinant → If cross-over occurs there are recombinants → Rf value is <50%.
3. Distances between genes were expressed in Genetic Map units (mu) or centimorgans
(cM).
Linkages of Genes
1. Linked → <50mu and on same chromosome.
2. Not Linked → on different chromosomes.
3. Not linked → >50mu.
How to determine relative location of two genes;
1. Do a test cross (male is homozygous recessive for all genes).
2. Calculate the Rf value
a. <50% the genes are linked: close on same chromosome.
b. RF = 50% the genes are not linked: far apart on same chromosome or different
chromosomes.
i. Cross-overs cancel each other out therefore maxes out at 50%
How are crossovers detected?
1. When one parent is heterozygous for two or more genes.
