Genetics
Lesson 5.3
Monohybrid Cross
5.3.2 Simple Dominance Traits
In this lesson, we 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 trait 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 can be seen in figure 5.15
Trait | Dominant | Recessive |
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 |
Figure 5.15[1]
As we start to do talk more about genetic traits, there is some vocabulary we must become familiar with. 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. Let’s practice!
Practice Problem:
(remember to use the 4 part method)
Sam and Edna recently found out they are expecting a baby. They are curious to see if their child will have dimples. Sam is heterozygous for the trait and Edna does not have dimples. What is the probability that their child will have dimple?
1) label the alleles
*If a letter is not called out in the problem any letter can be used. Which allele is dominant or recessive with either be named in the problem or use can use Figure 5.15 above.
D = Have dimples
d = Do not have dimples
2) Label the cross
Sam (heterozygous) X Edna (recessive) (the “X” represents “the cross”)
Dd X dd
3) Do the Punnett square
4) List all possible results
50% Dd, have dimples
50% dd, do not have dimples
In the case of Sam and Edna, they have 50% chance of having a child with dimples and a 50% of having a child without.
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