Download
1 / 52
E N D
Mendelian traits in humans • In Mendelian inheritance, a child receiving a dominant allele from either parent will have the dominant form of the phenotypic trait or characteristic. Only those that received the recessive allele from both parents, known as zygosity, will have the recessive phenotype. Those that receive a dominant allele from one parent and a recessive allele from the other parent will have the dominant form of the trait.
Purely Mendelian traits are a tiny minority of all traits, since most phenotypic traits exhibit incomplete dominance, codominance, and contributions from many genes. • The recessive phenotype may theoretically skip any number of generations, lying dormant in heterozygous "carrier" individuals until they have children with someone who also has the recessive allele and both pass it on to their child. • The human Y chromosome is composed of about 59 million base pairs and is passed virtually unchanged from father to son. The mitochondrial DNA (mtDNA) comes only from the mother and is given to both male and female children.
Attached ear lobes are attached directly to the side of the head Earlobe Attachment
Some scientists have reported that this trait is due to a single gene for which unattached earlobes is dominant and an attached earlobe is recessive. Other scientists have reported that this trait is probably due to several genes.
Forelock White forelock (dominant trait) vs. No white forelock (recessive trait) A white forelock is a patch of white hair, usually located at the hairline. Widow's Peak (below) is dominant over no widow's peak hairline.
Broad NoseNarrow Nose Broad Nose - nostrils extend beyond the space between the eyesNarrow Nose - nostril span restricted to the space between the eyes
This trait is reportedly due to a single gene .Straight thumb (dominant trait) vs. Curved thumb (recessive trait) When viewed from the side as in the illustration below, curved thumbs can be seen as part of a circle.
Bent Little Finger A dominant allele causes the last joint of the little finger to dramatically bend inward toward the 4th finger. Lay both hands flat on a table relax your muscles, and note whether you have a bent or straight little finger.
Mid-digital Hair Some people have hair on the middle segment of one or more of their fingers, while others don't. Having any hair there at all means that you have the dominant phenotype. Complete absence of hair is recessive.
Pinky: Straight pinky (recessive trait) vs. Bent pinky (dominant trait)
Longer 2nd toe is dominant over 2nd toe shorter than big toe.
Tongue Rolling 70% of people of European ancestry could roll their tongues and the remaining 30% could not.
Tongue rolling ability may be due to a single gene with the ability to roll the tongue a dominant trait and the lack of tongue rolling ability a recessive trait.However, many twins do not share the trait, so it may not be inherited.
Dimples are reportedly due to a single gene with dimples dominant (people may exibit a dimple on only one side of the face) and a lack of dimples is recessive.Cleft chin is dominant over no cleft
Dimples are highly heritable, meaning that people who have dimples tend to have children with dimples—but not always. Because their inheritance isn't completely predictable, dimples are considered an “irregular” dominant trait. Having dimples is probably controlled mainly by one gene but also influenced by other genes.
Overall, about 10% of people are left-handed, but the number varies among cultures from 0.5% to 24%. Some scientists have reported that handedness is due to a single gene with right handedness dominant and left handedness recessive. Multiple studies present evidence that handedness is controlled by many genes—at least 30 and as many as 100—each with a small effect; many are linked to brain development. Environment also plays an important role: some cultures actively discourage left-handedness.
Freckles Freckles are small, concentrated spots of a skin pigment called melanin. Most fair-skinned, red-haired people have them.
Freckles are controlled primarily by the MC1R gene. Freckles show a dominant inheritance pattern: parents who have freckles tend to have children with freckles.This trait is reportedly due to a single gene’ the presence of freckles is dominant, the absence of freckles is recessive.
Early geneticists reported that curly hair was dominant and strait hair was recessive. More recent scientists believe that more than one gene may be involved.
Multiple genes control hair texture, and different variations in these genes are found in different populations. For instance, curly hair is common in African populations, rare in Asian populations, and in-between in Europeans. Straight hair in Asians is mostly caused by variations in two genes—different genes from the ones that influence hair texture in Europeans. And different genetic variations make hair curly in African and European populations.
One study found that 55% of people place their left thumb on top, 45% place their right thumb on top, and 1% have no preference. A study of identical twins concluded that hand clasping has a strong genetic basis (most twins share the trait), but it doesn’t fit a predictable inheritance pattern. It is likely affected by multiple genes as well as environmental factors.
While allergic reactions are induced by things a person comes in contact with, such as dust, particular foods, and pollen, the tendency to have allergies is inherited. If a parent has allergies, there is a one in four (25%) chance that their child will also have allergy problems. The risk increases if both parents have allergies.
Eye Color Eye color, as well as hair and skin, is a complex trait; not a case of simple inheritance. The main pigment is melanin, and the more melanin, the darker the color. Although the genetics of eye color is complex, alleles for production of melanin dominate those for lack of melanin. So if we evaluate eye color as brown (dominant) or non-brown (recessive), it can be treated as a characteristic of simple inheritance.
Color blindness is due to a recessive allele located on the X chromosome. Women have two X chromosomes, one of which usually carries the allele for normal color vision. Therefore, few women are colorblind. Men only have one X chromosome, so if they carry the allele for color blindness, they will exhibit this trait. Thus, color blindness is seen more frequently in men than in women.
Red-green color blindness follows a very predictable recessive, sex-linked inheritance pattern. A woman with one defective copy of the gene and one functional copy, even though she is not colorblind herself, is known as a "carrier." She has a 50% chance of passing the defective copy to each of her children. Half of her sons will be colorblind, and half of her daughters will be carriers.
The differences between identical twins don't come from DNA—they all come from external factors.
Identical twins: same DNA, different environmentIdentical twins have exactly the same DNA, but they are not exactly alike. Each twin has his or her own personality, talents, likes, and dislikes. There are even diseases that appear in one twin but not the other, including arthritis, diabetes, autism, schizophrenia, cancer, and many others. The differences between identical twins don't come from DNA—they all come from external factors.
To about 75% of us, the chemical PTC (phenylthiocarbamide) tastes very bitter. For the other 25%, it is tasteless. The ability to taste PTC is controlled mainly by a single gene that codes for a bitter-taste receptor on the tongue. Different variations, or alleles, of this gene control whether PTC tastes bitter or not. PTC tasting follows a very predictable pattern of inheritance. Tasting is dominant, meaning that if you have at least one copy of the tasting version of the gene, you can taste PTC. Non-tasters have two copies of the non-tasting allele.
Achoo Syndrome This dominant trait is also called the photo sneeze reflex. If, when suddenly exposed to light, you sneeze (usually two or three times) you have the genes for achoo syndrome. Next time you go to a movie, exit the dark theater through a door that leads directly outside. It's fun to wait outside and watch the people emerge from the movie. Some will sneeze as soon as they are exposed to light.
