HUMAN TRAI S EXERCISE
Analyses of human traits offer genetic examples including complete dominance. incomplete dominance, codominance, and sex linkage. In this exercise you will examine the complexities and modes of inheritance of many human genetic traits.
Analysis of human genetic characters is usually complex because many characters are influenced by multigene interactions and environmental factors. In this exercise we will concentrate on those characteristics determined by variations of a single gene without much environmental influence. Y bu should be able to determine the phenotype and make good inferences (if not precise) about the genotypes controlling the characteristic. This will be particularly important when you analyze dominant phenotypes because you will not know by certain whether the second allele is also dominant or recessi ve for that particular characteristic under study. Some of these characteristics are discussed below:
Ability to taste Phenylthiocarbamide (PTC)
Determined by a dominant allele T that seems to confer the ability to taste the chemical.
Homozygous recessives for the allele lack the ability to taste PFC. In order to asses your tasting abilities, place a piece of control paper on your tongue and keep it there for at least 10 seconds. Then. place a piece of paper in your mouth and keep it for 10 seconds. A distinct bitter taste will develop during this time if you are a taster. If you have to wonder if you taste or not, then you are a nontaster.
Ability to roll tongue
This characteristic is determined by a dominant gene R, that gives some people the ability to roll the tongue into a characteristic U shape.
Attached earlobes represent the homozygous recessive condition (gene f).
Studies suggest that placing the left finger over the right when you cross your hand is due to a dominant gene L.
Bent little finger
Caused by a dominant gene B that causes the last joint of the little finger to bend inward toward the fourth finger. Lay your hands flat on the bench while relaxing the muscles to observe this phenotype.
Due to a dominant gene W, this shows in some people as a drop in the hairline downward forming a distinct point at the center of the forehead. Examine your front hairline to determine if you have this phenotype.
Hitchhiker’s thumb (distal hyperextensibility of the thumb)
The expression of this trait is due to a recessive gene T with variable expressivity (only one thumb) with 5% reduction in penetrance.
Long palmar muscle
Homozygocity for the I gene determines the presence of long palmar muscle. This is detected by examining the tendons that run over the inside of the wrists. Close your fist tightly and flex your hand. If there are three tendons running into the wrist, you have the long palmar muscle.
Otherwise, if you see two tendons with the large middle one will be missing. then you do not have the muscle.
Mid digital hair
Presence of hair on the second (middle) joint of one or more fingers is due to a dominant allele M.
The following chart shows the inheritance pattern of a number of known traits in human beings that are single locus traits. Underline your phenotype and possible genotype for each trait. Then circle the phenotype that is the most frequent in the population (your class).
Trait Dominant Recessive Possible Genotype
2. Tongue rolling
3. Ea bes
4. Sw at Glands (sex-linked recessive)
SS Sa aa s_
5. Rh Factor
6. Blood Type
A, B. AB
AA. AO, BB, BO,
- Hair Color Dark Light HH Hh hh
10. Interlocking Fingers
Left over Right
Right over Left
LL Lr rr
I I. Bent Little Finger
12. Double joints in Fingers
14. Widow’s Peak
15. Curly Hair (incomplete dominance)
17. Nose Shape
19. Hitchhiker’s thumb
20. Color Blindness (sexlinked recessive)
cc Cc cc
- Hair Color Nonred Red Nn nn
21. Long Palmar Muscle
22. Mid Digital Hair
23. White Forelock (sexlinked recessive)
32. Inny-Outy Navel
Now, construct a pedigree of-your family that analyzes the inheritance of one of the traits shown in the previous table. Your pedigree should include your grandparents and their brothers and sisters in the first generation; your parents and their brothers and sisters in the second generation. and you and your brothers. sisters, and cousins in the third generation. Then, analyze your family pedigree and determine the mode of inheritance of the trait you selected.
Human Trait Analyzed