Genetic Characteristics

Transcription

1 Genetic Characteristics...An Inventory and Explanation

2 Attached Earlobes Attached or Free

3 Tongue Rolling Roller or Non-roller

4 Dimples Absent or Present

5 Handedness Right-handed or Left-handed

6 Freckles Present or Absent

7 Curly Hair Present or Absent

8 Hand Clasping Left over or Right over

9 Hairline Shape Flat or Widow s Peak

10 Red-Green Colorblindness Can Differentiate or Can t

11 Attached Earlobes Although some sources say that this trait is controlled by a single gene, with unattached earlobes being dominant over attached earlobes, no published studies support this view. Earlobe attachment and shape are inherited, but it is likely that many genes contribute to this trait. As such, its pattern of inheritance is difficult to predict.

12 Tongue Rolling In 1940, Alfred Sturtevant observed that about 70% of people of European ancestry could roll their tongues and the remaining 30% could not. Many sources state that tongue rolling is controlled by a single gene. However, as Sturtevant observed, people can learn to roll their tongue as they get older, suggesting that environmental factors not just genes influence the trait. Consistent with this view, just 70% of identical twins share the trait (if tongue rolling were influenced only by genes, then 100% of identical traits would share the trait).

13 Dimples 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.

14 Handedness Overall, about 10% of people are lefthanded, but the number varies among cultures from 0.5% to 24%. Some have reported that handedness is controlled by just one or two genes, but this is not the case. 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.

15 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. Other genes and the environment influence freckle size, color, and pattern. For example, sun exposure can temporarily cause more freckles to appear. Freckles show a dominant inheritance pattern: parents who have freckles tend to have children with freckles.

16 Curly Hair Round hair follicles make straight hair, flattened or c-shaped hair follicles make curly hair, and oval hair follicles make wavy hair. Hair texture is a continuous trait, meaning that hair can be straight or curly or anywhere in between. Curly hair is influenced by genes much more than by the environment. While curly hair runs in families people with curly hair tend to have children with curly hair its inheritance patterns are often unpredictable. Multiple genes control hair texture, and different variations in these genes are found in different populations.

17 Hand Clasping 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.

18 Red-Green Colorblindness Red-green colorblindness is caused by a single gene located on the X- chromosome. You need at least one working copy of the gene to be able to see red and green. Since boys have just one X- chromosome, which they receive from their mother, inheriting one defective copy of the gene will render them colorblind. Girls have two X- chromosomes; to be colorblind they must inherit two defective copies, one from each parent. Consequently, redgreen colorblindness is much more frequent in boys (1 in 12) than in girls (1 in 250).

19 Hairline Shape While some sources say that widow s peak is a dominant trait controlled by one gene, no scientific study supports this claim. Complicating the question of heritability is the fact that the trait is continuous: some people have just a slight suggestion of a peak. Widow's peak is likely controlled by genes rather than the environment. But while hairline shape tends to run in families, its pattern of inheritance is usually unpredictable, suggesting that multiple genes are involved.