In humans, the ability to bend the thumb back beyond vertical is called hitchhiker's thumb and is dominant to the inability to do so (OMIM 274200; see Problem 41). Also, the presence of attached earlobes is recessive to unattached earlobes (OMIM 128900).

Check your own phenotype and those of several friends or classmates.

Verified step by step guidance

1. Identify the phenotypes for hitchhiker's thumb and earlobe attachment in yourself and your friends or classmates.
2. Determine the genotype for each phenotype. Remember that a dominant phenotype can be either homozygous dominant or heterozygous, while a recessive phenotype must be homozygous recessive.
3. For hitchhiker's thumb, if a person can bend their thumb back beyond vertical, they have the dominant phenotype. If they cannot, they have the recessive phenotype.
4. For earlobe attachment, if a person has unattached earlobes, they have the dominant phenotype. If they have attached earlobes, they have the recessive phenotype.
5. Record the observed phenotypes and infer possible genotypes for each individual based on the dominant and recessive traits.

Key Concepts

Dominant and Recessive Traits

In genetics, traits can be classified as dominant or recessive based on their expression in an organism. A dominant trait, such as hitchhiker's thumb, will manifest in the phenotype even if only one copy of the allele is present. In contrast, a recessive trait, like attached earlobes, requires two copies of the allele to be expressed. Understanding this distinction is crucial for predicting inheritance patterns.

Phenotype vs. Genotype

The phenotype refers to the observable characteristics or traits of an organism, such as thumb flexibility or earlobe attachment, while the genotype is the genetic makeup that determines these traits. The relationship between genotype and phenotype is essential for understanding how traits are inherited and expressed in individuals. For example, a person with a dominant allele for hitchhiker's thumb will display this trait regardless of their other allele.

Punnett Squares

Punnett squares are a tool used in genetics to predict the probability of an offspring inheriting particular traits from its parents. By mapping out the possible combinations of alleles from each parent, one can visualize the likelihood of different phenotypes appearing in the offspring. This method is particularly useful for understanding inheritance patterns of traits like hitchhiker's thumb and attached earlobes.