BISC 102 Lecture Notes - Lecture 7: Synapsis, Internal Fertilization, Sister Chromatids

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Fungi/Animals share a common ancestor In cladograms, actual ancestors are never
inferred (presumed unknown) unlike many older trees
In cladograms, actual ancestors are never inferred (presumed unknown) unlike many older
trees
From Eukaryotes to sex and meiosis
Asexual Reproduction:
Some organisms regularly reproduce without sex (Example without fertilization of
eggs/sperm or conjugation)
The parent passes 100% genes to its offspring
Very common in plants: Many do both kinds of reproduction (Example:
Strawberry)
Selfing (Self-pollination): Not truly asexual but produce offspring genetically similar to
parent (Example: Peanuts)
Sexual Reproduction: Occurs through fertilization of gametes to produce a genetically
novel individuals
Offspring similar but still genetically distinct from parents/siblings
Each parent passes 50% of its genes to each offspring
Seed plants: Angiosperm {internal fertilization)
Ovary becomes fruit
2 Sperm
3 celled gametophyte
Paradox of Sex:
By breeding with a male, a female halves her genetic contribution to her offspring
Only half the population (female) is capable of bearing offspring (asxual competitors
would win by out reproducing sexual species
Assumptions of the model on the next slide:
All females give birth to the same number of offspring
The sexual population is 1:1 (male to female)
Cost of sex:
Off-spring producing by each female in each generation
Benefits of sex:
Genetic recombination: Shuffling/new combinations of alleles
Can adapt more quickly to changing environment (speeds up evolution)
○ Parasites: Major danger to all species
Variations by meiosis can cause immune genotypes are present in the
population may avoid extinction of population
Harmful mutations (most are recessive) can be masked by a second copy of
a normal gene on a second chromosome (not in haploid prokaryotes)
Generalized Eucaryotes life cycle:
Meiosis accomplishes: A reduction from diploid (2n) to haploid state
This happens through separation of each chromosomes from its homolog
Genetic recombination: Crossing over and independent assortment
Human Diploid (2n) Karyotype
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