Introduction The combination of alleles inherited from your parents is called a genotype. When a person exhibits a dominant trait, he/she probably does not know his/her genotype for that trait. Take, for example, the genetic trait of tongue rolling. A tongue rolling allele (R) is dominant; a non-tongue rolling allele (r) is recessive. The alleles you have for that trait could be identical. For example, you may have received a tongue rolling allele (R) from both you mom and dad; in that case, you would also be a tongue roller, with a genotype of “RR”. Or you may have received a non-tongue rolling allele (r) from both your mom and dad; in that case you would not be able to roll you tongue and your genotype would be “rr”. In either case, when both alleles for a trait are identical, they are referred to as homozygous, from the Greek words “homo” (meaning “same”) and “zygote” (from “zygoun”, meaning “to join”). A zygote is a cell that is formed at conception, by the union of two gametes (egg and sperm).
Sometimes the combination of alleles inherited from your parents is different. For example, you may have received a tongue rolling allele from you mom, but not your dad. In this case, your genotype would be “Rr”. This combination of alleles is referred to as heterozygous, from the Greek words “hetero” (meaning “different”) and “zygote”.
The important thing to remember is that both “RR” and “Rr” will result in a tongue rolling kid. In a simple inheritance pattern involving two alleles, it is only necessary to receive one copy of the dominant allele to exhibit the dominant trait. A person who exhibits a dominant trait will probably not know his/her genotype for that trait. On the other hand, a person who exhibits a recessive trait always knows what his/her genotype is because no dominant allele is present.
Follow the directions indicated on the chart on Worksheet #3 to determine if you are dominant or recessive for certain traits. Once this has been established, predict what your possible genotype(s) is/are.
Dragon Genetics Worksheet #3:
“Do Your Ears Hang Low?”
Activity 3: Are you dominant or recessive for this trait? Based on your phenotype, try to determine whether you are dominant or recessive for each of the traits listed below. For each trait, read the statement, perform the test or task if necessary, and then, in the first column, write whether you are “dominant” or “recessive” for this trait, and try to predictg your genotype in the second column.
Are you dominant or recessive for this trait?
My possible genotype(s) for this trait is/are:
SBT Paper – Taste the paper your teacher gave you. Sodium benzoate taste paper is used as a diagnostic tool in medicine. The ability to taste the chemical is a dominant trait. People who cannot taste this chemical are recessive for the trait. (T or t)
TONGUE ROLLING - Can you roll your tongue into a U shape (like a taco shell)? If so, you are dominant for this trait. If you cannot roll your tongue into a U shape, you are recessive for this trait. (R or r)
EARLOBES – It is a dominant trait to have earlobes that hang down. A person who has earlobes attached directly to the side of his/her head is recessive for this trait. (L or l)
EARBUMP – Some people have a bump, called a Darwin’s ear point, on the inside rim of the upper part of the ear ridge. If you have the bump it is due to the presence of a dominant allele. If you are lacking the bump you are recessive for the trait. (B or b)
WIDOW’S PEAK – If your hairline forms a downward “V” in the middle of your forehead, you have a widow’s peak. (Good examples of widow’s peaks can be found on TV character Eddie Munster and most depictions of vampires.) A widow’s peak is caused by a dominant allele. A straight or even hairline indicates you are recessive for this trait. (V or v)
HITCHHIKER’S THUMB – People who can bend the last joint of their thumbs back to a 90 degree angle are dominant for hitchhiker’s thum. Those who lack this ability are recessive for this trait. (H or h)
POLYDACTYLY – If you were born with more than five fingers or toes on either of your hands or feet, you possess a dominant trait for polydactyly. Having five fingers and toes is a recessive trait. NOTE: In the US, most peole with extra toes and fingers usually have them removed shortly after birth. (P or p)
SYNDACTYLY – Having webbed fngers and toes is a dominant trait. Sometimes only a pair of fingers or toes is webbed. If you have fingers and toes that are separated from one another, then you are recessive for this trait. (S or s)
CLEFT CHIN – If you have an indentation in the middle of your chin you have a cleft chin. (Actor John Travolta has a prominent cleft chin.) A cleft chin is a recessive trait. The absence of a cleft chin is a dominant trait.(C or c)
HAIR WHORL – Ask a friend to look at the back of your head to determine if your hair whorls to the right (clockwise) or to the left (counterclockwise). (If you use hairspray or gels on your hair this may be hard to determine.) A clockwise whorl is a dominant trait; a counterclockwise whirl is a recessive trait. (W or w)
Activity adapted from Science Kit® & Boreal® Laboratories “Dragon Genetics”