Leader: Kirsten Karkow
Course: Biol 211 - Wilsey
Date: 3/2/2012
Worksheet 17
Supplemental Instruction
Iowa State University
Define the following:
Mendel’s First Law: Law of Segregation – allelic pairs separate during gamete formation. (Alleles
are discrete units that recombine at fertilization.)
*We now know this occurs through the process of meiosis.
Mendel’s Second Law: Law of Independent Assortment – when an allelic pair separates, they do
it independently of how other allelic pairs separate.
*We now know this is only true for the non-linked alleles (usually on separate
chromosomes). Linked alleles don’t display Mendelian genetics.
Monohybrid – organisms that are heterozygous for one character. (Aa)
Dihybrid – organisms that are heterozygous for two characters. (AaBb)
Testcross – when one organism with an unknown genotype (AA or Aa?) is crossed with a
homozygous recessive organism.
If all resulting plants display the dominant trait, the original organism was AA.
If half display the recessive trait, the original organism was Aa.
Complete Dominance – relationship in which a heterozygote is indistinguishable from a
homozygous dominant organism. Phenotypic ratio of 3:1. Uses capital/lowercase letters.
Incomplete Dominance – relationship in which a heterozygote has a phenotype that is
intermediate of the two homozygous parents. Phenotypic ratio of 1:2:1. Superscripts.
Codominance – relationship in which both alleles are displayed by the heterozygote. Phenotypic
ratio of 1:2:1 if not multiple alleles. Also uses superscripts.
Multiple Alleles – term used when there are more than 2 possible alleles. (Ex. – blood type is
determined by IA, IB and i [three possible alleles].)
Pleiotropy – when an individual allele affects more than one character. (Ex. – sickle cell anemia
also provides resistance to malaria.)
Epistasis – when a gene at one locus determines the expression of a gene at a second locus.
Phenotypic ratio of 9:3:4. (Ex. – fur color is determined by one gene, whether you even express
that fur color or are albino occurs at another gene.)
Supplemental Instruction
1060 Hixson-Lied Student Success Center  294-6624  www.si.iastate.edu
Give short answers to the following concept questions:
What experimental results led Gregor Mendel to the particulate hypothesis? What would have
happened if the blending hypothesis were true?
When Gregor Mendel self-crossed his heterozygous F1 generation, he obtained plants that
displayed the previously masked recessive trait. Thus he concluded alleles must be discrete
units that don’t change – they’re just reshuffled.
If the blending hypothesis were true, any future plants would have only displayed the
“blended” trait.
When does the phenotypic ratio equal the genotypic ratio and when are they different?
The only times in which the phenotypic ratio equals the genotypic ratio is when alleles display
incomplete dominance (or with codominance for a gene with only two allele options).
Is the phenotype of an organism always determined by genetics?
No, the environment is a contributor to many characters. Ex. – skin color, weight.
Pretend that for some organism, brown eyes are dominant to blue eyes, black fur is dominant
to gray fur, and having five toes per foot is dominant to having four toes per foot. All of these
characters display Mendelian genetics. What is the probability that any given organism will
have brown eyes, gray fur, and five toes per foot?
Brown eyes: blue eyes = 3:1 → .75 chance for brown eyes.
Black fur: gray fur = 3:1 → .25 chance for gray fur.
Five toes: four toes = 3:1 → .75 chance for five toes.
.75 x .25 x .75 = .14
Draw out the result of a dihybrid cross of an organism with the genotype RrTt, where R = red, r
= white, T = thorns, t = no thorns. Assume Mendelian genetics.
RT
RT RRTT
Rt
RRTt
rT
RrTT
rt
RrTt
Rt RRTt
RRtt
RrTt
Rrtt
rT RrTT
RrTt
rrTT
rrTt
rt RrTt
Rrtt
rrTt
rrtt