If a trait is simple and genetically inherited, then: -The trait is not determin

Question

If a trait is simple and genetically inherited, then:

-The trait is not determined by enviornmental factors

-Two distinct phenotypes are observed

-One allele is dominant and one allele is recessive. Two parents with recessive phenotypes will produce offspring with the recessive phenotype

-Identical twins express the same phenotype

Do these traits follow simple Mendelian Laws?

1. Bitter tasting

Claim: The ability to taste PTC is a simple, two-allele inherited trait, and the allele for tasting (T) is dominant over the nontasting allele (t).

Analyze the following information to support or refute the claim.

a) Approximately 70% of people tested are tasters of PTC (7/12 people tasted PTC in my class)

b) Sensitivity to PTC can vary by five orders of magnitude, meaning that some people can detect it at very low concentrations and others can only detect it at high concentrations. Most people fall into one of two categories: those who taste PTC at low doses and those who cannot taste it at all.  

c) Chimpanzees, like humans, show variable sensitivity to PTC biterness

Explanation / Answer

1. Yes, those above mentioned traits follow simple Mendelian law.

According to Mendel, a simple inheritance pattern is found in sexually reproducing organisms. The presence of observable phenotype is solely based on the genetic make up of the organism (genotype). A trait is characterised by presence of alternate forms of genes (alleles) on the homologous pair of chromosomes. Two variants of allele is present, one of which is able to express itself as functional protein always (Dominant) and other is not able to express itself in presence of dominant one (Recessive). These two alleles corresponds to three sets of genotypes (AA: Homozygous dominant, Aa: Heterozygous dominant and aa: Homozygous recessive) that gives rise to two sets of phenotype (dominant and recessive). (LAW OF DOMINANCE).

An individual with recessive phenotype will have homozygous recessive genotype condition, so it will produce gametes of only one kind, having recessive allele. So, parent with recessive phenotype will give rise to only recessive offsprings. (LAW OF SEGREGATION).

As identical twins share same sets of genotype, so the corresponding phenotypes will be same too.

Although these traits follow simple Mendelian inheritance law, Neo-Mendelism (Extensions of Mendel's law) advocates the influence of environmental factors on the phenotype of the organisms. A phenotype observed is the interaction between genotype and environment in which it is being exposed.

So, identical twins may share same genotype, but they are mostly non-identical in their phenotype depending upon the environmental factors.

2. The ability to taste PTC (Phenylthiocarbamide) is used as a genetic marker in studying inheritance. It is inherited from one generation to another, so it is a two allelic trait. In the study, the data presented is the frequency of occurrence of trait in given population. It does not corresponds to the dominancy of the trait. For analysing dominance of an allele, we must have pedigree data to study the inheritance pattern of the trait.

As the gene product is related to taste buds, the combination of allele present in the individual induces different sets of taste buds, corresponding to range of phenotypes.

Presence of the trait in Chimpanzees shows the ancestral link with human along with the selective advantage during natural selection.

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