BSC 315 Study Guide - Midterm Guide: Lethal Allele, Red Blood Cell, Mount Everest

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BSC 315 Test 1 Study Guide
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
Mendel’s Crosses
Hybrid: offspring of two true-breeding strains
Monohybrid cross: one trait analyzed
Dihybrid: two traits
Trihybrid: three traits
Mendel’s Theory of Genetics
When was Mendel’s Theory proven?
o Theories are never proven
The scientific method:
o A theory is proposed to explain a set of observations
o The theory makes predictions
o The predictions are tested, and the theory is rejected (or modified) if the
predictions are not met
o Multiple successful tests suggest the theory is “true”
Law of Segregation
The two alleles of a gene segregate from each other during gamete formation, and unite at
random at fertilization, restoring the diploid chromosome number
The team “law” is applied arbitrarily and informally
Punnett Square
A simple method to portray a cross (post-Mendel):
o X and Y axes contain the gametes
o Each box is a fertilization event
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Mating to heterozygotes produces:
o 3:1 phenotypic ratio (yellow:green)
o 1:2:1 genotypic ratio (YY:Yy:yy)
Mendel’s Test of His Theory
Prediction: the yellow F2s contain both YY and Yy in a 1:2 ratio
Test: determine the genotype of yellow F2s
o Test cross: cross to determine the genotype of an unknown individual
o Procedure: unknown x homozygous recessive (homozygous recessive = tester
strain)
Yellow F2s have two different genotypes, YY and Yy, abbreviated as “Y-
o (“-“ = unknown allele, either Y or y_
Test cross Y- x yy. Predict progeny if Y- is actually:
o YY YY x yy 100% yellow
o Yy Yy x yy 50% yellow (Yy) and 50% green (yy)
Mendel tested many yellow F2 plants and found:
o 1/3 were YY (gave 100% yellow progeny in a test cross)
o 2/3 were Yy (gave 50:50 ratio of yellow:green in a test cross)
Mendel’s theory has passed a test!
Independent Assortment
Dihybrid crosses
The product rule
Working backwards to find genotypes
Dihybrid Crosses
Two traits
P: Yellow round x green wrinkled YYRR x yyrr
F1: yellow round x (self) YyRr x (self)
F2: ? ?
Independent Assortment
What gametes are produced by a YyRr individual?
Law of independent assortment: gametes carrying “Y” contain either “R’” or “r” with
equal probability.
Four types of gametes in equal numbers are produced by an individual of genotype YyRr:
o ¼ YR
o ¼ Yr
o ¼ yR
o ¼ yr
Calculating Ratios from a Punnett Square
P: Yellow round x green wrinkled YYRR x yyrr
F1: yellow round x (self) YyRr x (self)
F2: 9 different genotypes 9/16 yellow, round (Y-R-)
4 different phenotypes 3/16 green, round (yyR-)
(9:3:3:1 ratio) 3/16 yellow, wrinkled (Y-rr)
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