Independent Study
Exceptions to Simple Dominant (Mendelian) Inheritance
Name
Due Monday, January 19, 2016
For this independent study, you will be working with your existing knowledge and skills of using phenotypes and
genotypes to solve genetics probability problems and interpreting the results. Not all inheritance cases follow the simple
dominance rules (where the dominant allele completely masks the recessive in a heterozygote), so this activity is
designed to help you understand some of these exceptions and to teach you how genetics problems are solved in those
cases. To do this, work on each section by completing the introductory tasks first (reading and interpreting the
description, reading the textbook excerpt if applicable, and watching an online video), and then looking at the example.
Then, complete the practice problems. Highlight, note-take, and work with peers!
Simple (or "Complete") Dominance aka the "usual" way we do inheritance problems. You will sometimes see this as
Simple Mendelian Inheritance (giving credit to Gregor Mendel, a monk who was a pioneer in inheritance studies).
Description: Simple dominance occurs when an inherited trait is coded for by a single gene and that gene has two versions, or
alleles: the dominant version and the recessive version. In a heterozygote, the dominant allele of the gene masks the effect of
recessive allele, and what you see is the same phenotype as a homozygous dominant.
Example: In mice, the ability to run normally is a dominant trait. Mice with this trait are called running mice (R). The recessive trait
causes mice to run in circles only. They are called waltzing mice (r). Cross a heterozygous mouse with a waltzing mouse.
Problem: In mice, the ability to run normally is a dominant trait. Mice with this trait are called running mice (R). The recessive trait
causes mice to run in circles only. They are called waltzing mice (r). Two running mice, both whose mothers were waltzers, were
crossed. How many of their 20 offspring would you expect to be waltzing like their grandma mouse? Explain why.
Incomplete Dominance
Description: Incomplete dominance occurs when the dominant allele is not completely expressed over its paired allele, the
recessive. In other words, the phenotype expressed by the heterozygote (Aa) is a blend or combination of the phenotypes of both
alleles. That means the expression of AA would appear differently than Aa and differently from aa. This is different than in complete
dominance inheritance, where the dominant allele completely masks the recessive allele in the heterozygote, where the phenotype
for AA = Aa. In this case, the phenotype for AA is different than Aa, and aa.
Watch: https://www.youtube.com/watch?v=YJHGfbW55l0 (ONLY the first 1:57 of it)
Read: Essay in our textbook, Incomplete Dominance, pages 510-511
Example: A brown bird was crossed with a white one and all the offspring produced were tan. If these offspring were crossed and produced 20
new individuals, what colors and ratios would be expected?
Problems:
1.
In a certain type of plant, flower color shows incomplete dominance. When the red allele combines with the white alleles the result is a
pink flower. If a pink flowering plant is crossed with a white flowering plant, what is the probability of getting a plant with white flowers?
Red flowers? Pink flowers?
2.
Oompahs can have red, blue, or purple hair. The allele that controls this trait is INCOMPLETELY DOMINANT, where purple hair is caused
by the heterozygous condition. Show a "key" for the genotypes and phenotypes of hair color. Olga Oompah has red hair and marries
Oliver Oompah who has blue hair. They have 32 children. What colors of hair do their children have, and how many of each?
Sex Linked Inheritance
Description: Sex-linked inheritance occurs when the alleles of interest are found on the X chromosome. Because females have XX,
they have 2 alleles in their genotype, but since males have Xy, they only contain one allele. Inheritance of particular traits varies
depending on the sex of the child in a cross of traits found on the 23rd pair of chromosomes, or the X and y.
Here are the genotypes for:
A homozygous dominant female = XAXA
A heterozygous female = XAXa
A homozygous recessive female = XaXa
A homozygous dominant male = XAy
A heterozygous male = not possible, since only one allele
A homozygous recessive male = Xay
How to write alleles: Write the sex chromosomes first (mom is XX, dad is Xy), and then fill in the alleles as "exponents" (Hh, HH, etc)
but nothing is placed on the y chromosome.
Watch: https://www.youtube.com/watch?v=h2xufrHWG3E or "Punnett Squares and Sex Linked Traits"
Read: Textbook excerpt attached, "Sex-linked genes exhibit a unique pattern of inheritance"
Example: In humans, hemophilia is a sex linked trait. Females can be normal, carriers, or have the disease. Males will either have the disease or
not (but they won’t ever be carriers). Here are the genotypes.
X H X H = female, normal
X H X h = female, carrier
X h X h = female, hemophiliac
X H Y = male, normal
X h Y= male, hemophiliac
Show the cross of a man who has hemophilia with a woman who is a carrier. What is the probability that their children will have the disease?
A woman who has hemophilia marries a normal man. How many of their 8 children will be predicted to have hemophilia, and what is their sex?
Problems:
3.
In humans, red-green color blindness is a recessive x-linked train. Show the percent of girls and boys with r/g color blindness that result
from a cross between a colorblind man a woman who is not a carrier for that trait.
4.
The gene for hemophilia is known to be x-linked. Cross a female hemophiliac with a normal man with no history of hemophilia in his
family.
Codominance of Multiple Alleles
Codominance occurs when there may be 2 or more dominant alleles for a given trait, and 1 recessive allele. If both dominant alleles
are present in the genotype, both traits are expressed. If only 1 dominant and 1 recessive allele are present, the dominant allele will
mask the recessive. If both recessive alleles are present, the recessive trait is expressed.
How to write alleles: If you know the problem involves multiple alleles and codominance, you can write both dominant alleles as is
(like A and B, which is a classic example for blood types) and the recessive allele as is (O in the case of blood types). Another way is
to write the dominant alleles as a capital letter with an "exponent" denoting the allele and the recessive allele as an undercase
letter. An example will help the best, so...
Homozygous dominant =
Heterozygous =
Homozygous recessive =
AA or IAIA
AB
AB or I I
OR
BB or IBIB
OR
IAi or IBi
Because both A and B alleles are equally dominant
ii
Watch: https://www.youtube.com/watch?v=9O5JQqlngFY or "Amoeba Sisters Multiple Alleles and Codominance"
Read: textbook excerpt attached, "Many genes have more than two alleles in a population"
Example:
The A and B antigen molecules on the surface of red blood cells are produced by two different enzymes. These two enzymes are encoded by
different versions, or alleles, of the same gene: A and B. The A and B alleles code for enzymes that produce the type A and B antigens respectively.
A third version of this gene, the O allele, codes for a protein that is not functional and does not produce surface molecules. Two copies of the gene
are inherited, one from each parent. The possible combinations of alleles produce blood types in the following way:
A man with type O blood and a woman with type AB blood have children. Give the genotypes and phenotypes of their offspring.
Problem:
5.
A type AB woman marries a man with type B whose mother was type O. What percent of their offspring would be expected to be type A?
Textbook Excerpts