Part 1: Simple Dominance Traits
Introduction:
In this assignment, you will explore how simple dominance traits work, and also practice predicting which allele will be shown in a given scenario. Simple Dominance traits are traits that consist of a dominant and recessive allele. The dominant allele is the more powerful allele and will be expressed (shown) even if only one dominant allele is present in the genotype. Recessive alleles are only expressed when two recessive alleles are present.
In doing Punnett squares for simple dominance traits, we use a capital letter to signify dominant traits and a lowercase letter for recessive traits. Some common simple dominance traits in human can be seen in the table below. Whether or not these traits are a result of the express of simply one gene or not is not completely known or agreed on in the scientific community.
Trait Dominant Allele Recessive Allele
Earlobes free attached
Tongue Roller present absent
Mid-digital Hair(hairs on middle portion of finger) present absent
chin cleft present absent
facial dimples present absent
Bent little finger(pinkie finger bent side to side) present absent
Hand clasp(clasp hands, which thumb is on top?) left right
Hitch Hiker’s Thumb(thumb tip is able to bent upwards) absent present
A person can either be homozygous or heterozygous for a trait. A person that is homozygous contains two of the same alleles in their genotype. This would be shown in a Punnett square by two capital letters, TT, or two lower case letters, tt. To be more descriptive homozygous traits are often referred to as homozygous dominant, two capital letters, or homozygous recessive, two lower case letters.
A person can also be heterozygous for a trait. This means that they person has both a dominant and recessive allele for a trait (remember, we get one allele from our biological mother and one allele from our biological father, so we have two alleles for each trait.) Because dominant alleles are more powerful, the dominant trait will be expressed in this case. We often say that this type of person has the dominant trait, but carries the recessive trait. Because of this, they could pass either allele onto their children. We represent heterozygous traits with both a capital and lower case letter, Tt. To keep things uniform we always place the capital letter before the lower case letter.
Using Punnett squares you can predict how common a trait will be.
Complete the following problems using the “4 part method.”
1. Predict the outcome of a cross between a person who is a homozygous tongue roller and a heterozygous tongue roller. The ability to roll your tongue is a dominant trait.
2. Predict the outcome of a cross between two people who have no freckles. Freckles are a dominant trait.
3. Unattached/free earlobes are a dominant trait. Predict the outcome of a cross between a person who is homozygous recessive for attached earlobes with a person who is heterozygous for unattached earlobes.
4. Predict the outcome of a cross between a person who is homozygous dominant for a widow’s peak with someone who is homozygous recessive. A widow’s peak is a dominant trait.
In plants, height is a simple dominance trait. Plants can either have the dominant tall trait (T) or the recessive dwarf(short) (t).
5. What is the predicted outcome of a cross between a homozygous tall plant and a homozygous dwarf plant.
6. A tall and dwarf plant are crossed ½ of their offspring are tall and ½ are dwarf. What are the genotypes of the parent plants?
In humans, polydactylism, having an extra finger is rare but due to a dominant trait.
7. When one parent is normal and the other parent has an extra finger, but is heterozygous for the trait, what is the chance they will have a child without polydactylism?
○ What is the chance that their second child will be born without polydactylism?
○ What is the chance that their third child will be born without polydactylism?
Part 2: Incomplete Dominance
Watch the video below.
Link: https://youtu.be/7WoEFjjmQrs
Solve the problems below and state the answers. Attach a drawing to show your work.
1. In tulips flower color is determined by an incomplete dominant gene. Both red and blue tulips are homozygous and combine to create a purple heterozygous genotype. What is the predicted outcome of a red and purple tulip?
2. In horses, the color of coat shows incomplete dominance. A brown and a white coat are equally dominant to each other. A cross between these two types produces a golden coat (called a palomino). Predict the genotypes and phenotypes of the offspring in the following crosses.
○ A brown and white horse
○ A golden and white horse
○ A brown and golden horse
Part 3: Introduction: Codominance Traits
A codominance trait is one in which two traits combine to make a third trait in which both traits are still expressed or present. This type of trait can be easily seen using fur color. For example a white (WW) and black (BB) rabbit will combine to create a white and black spotted rabbit. (BW)
Watch the following video.
Link: https://youtu.be/P-BlTannKOw
Then answer the following questions
1. In cattle, hair color is determined by an codominant gene. Both brown and white cows are homozygous and combine to create a white with brown spotted (roan) heterozygous genotype. What is the predicted outcome of a roan and roan cow?
2. In a some daisy flower species, flowers can have 6 or 8 petals. If a 6 petal daisy is crossed with a 8 petal daisy, their offspring will have some flowers with 6 petals and some flowers with 8 petals
○ Is this an example of codominance or incomplete dominance? Why?
○ What is the expected outcome of a mixed petal daisy and a 6 petal daisy?
3. In this same daisy species, if you cross a plant that has red flowers to one that has white flowers, you produce a plant that has pink flowers?
○ Is this codominance or incomplete dominance? Why?
○ Show the cross of a white flowered plant to a pink flowered plant. What are your expected results?