BSC 315 Study Guide - Comprehensive Midterm Guide: Gregor Mendel, Punnett Square, Mendelian Inheritance

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BSC 315
MIDTERM EXAM
STUDY GUIDE
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BSC 315
1. Segregation and Assortment
Major Topics
Mendel’s Law of Segregation
o Genetic nomenclature: phenotype, genotype, trait, diploid, haploid, allele,
heterozygous, homozygous, dominant, recessive, pure-breeding, hybrid,
monohybrid, dihybrid, self-cross, test-cross
o Scientific theory
o Results of monohybrid crosses
Independent assortment
Phenotype
Trait: characteristic of the organism, e.g., seed color, flower color, height, etc.
Phenotype: appearance, form, physiological state, e.g., yellow/green peas, purple/white
flowers, tall/short people, striped/solid cats, etc.
Understanding that physical appearance is inherent to individual is ancient
Genotype
Genotype: allele composition
o Each individual has two copies of every gene
o A gene may have two or more variant (gene variant = allele)
o Gene “A” with two alleles, A and a: three possible genotypes: AA, Aa and aa
o Homozygous genotype: AA and aa
o Heterozygous genotype: Aa
Genotype/Phenotype
Many different genotype/phenotype relationships are possible
Strict dominance/recessiveness
o Heterozygotes have the same phenotype as one of the homozygotes
Example: Aa has the same phenotype as AA
o The A allele is dominant; the a allele is recessive
Example
o Seed color (trait) in peas
o Phenotypes: yellow and green
Yellow: genotypes YY and Yy
Green: genotype yy
o Yellow is the dominant phenotype
o The Y allele is dominant and the y allele is recessive
Mendel
Genotype: Phenotype relationships recognized ~150 years ago by Gregor Mendel
o Mendel: Austrian monk. Published studies on pea genetics in 1866
Mendel’s Observations
Started with pure-breeding strains of peas
o Pure-breeding: strain that produces the same phenotype generation after
generation. AKA “true-breeding”
Mendel’s observations:
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o In a cross between two pure-breeding strains with different phenotypes, all of the
progeny have one of the two phenotypes
o When the progeny are self-rosses, both phenotypes are present in the second
generation progeny in a 3:1 ratio, i.e., ¾ of one and ¼ of the other
Self-cross: self-mating, or cross to a genetically-identical individual
Mendel’s Theory
Genotype/phenotype
o Peas have two copies of each gene (diploid)
o Genes come in variants (alleles)
o The Yy and YY genotypes have the same phenotype (complete dominance), but
are different from the yy phenotype
Segregation
o Gametes (egg/sperm) have only one copy of each gene (haploid)
o Fertilization is random and restores the diploid gene number
What is a Scientific “Theory?”
Theory: an explanation for observed or experimental data, that proposes a natural and
universal underlying mechanism
A theory explains all of the observations in a unified way, generally involves mechanisms
that cannot be observed directly, and makes predictions of future results
These are not scientific theories:
o I think John likes Mary
o An invisible gremlin in the seeds determines whether they become yellow or
green
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