Ptc Genetics Lab Student Worksheet (Page 9 of 10) in pdf

PTC

Appendix A - Mendelian Genetics - A Brief

Overview

Background

During the 1860’s, an Austrian monk by the name of Gregor Mendel was studying pea plants. As a result of his

experiments, he developed and introduced a new theory of genetic inheritance. The typical notion of inheritance at the

time was that traits were a result of parental “essences” mixing together, rather like blending yellow and red paint to get

orange. Mendel believed heredity was the result of distinct units of inheritance, and every individual unit (gene) acts

independently in one’s genome. Mendel thought that all traits are controlled by single genes, however we now know that

many traits are determined by multiple different genes, as well as environmental factors.

Simple Mendelian Inheritance Pattern

According to Mendel’s theory, the inheritance of a specific trait or characteristic depends on these units being passed

on from parent to offspring, and he developed two laws to describe the process. There are different forms of genes,

and these different forms are called alleles. Mendel’s Law of Segregation states that the genes of a parent split into their

constituent alleles and that a person inherits one allele from each parent for any given gene, resulting in a complete

gene. It also states that the allele that gets passed on is completely up to chance. The other law is the Law of Independent

Assortment which states that alleles are passed to the offspring independently of one another. This means that

inheritance of a gene or genes at one location in the genome will not affect the inheritance of other genes elsewhere. If

the alleles inherited from both parents are the same, the person is considered to have a homozygous genotype for that

trait. If the alleles are different, their genotype is heterozygous.

Genotype vs Phenotype

Your genotype is the actual genetic code you possess, whereas your phenotype is the expression of your genotype in an

observable way, like your hair or eye color, your ability to taste certain chemicals, etc. Your phenotype is affected by

the alleles in your genotype. Alleles have different classifications based on how they interact. They can be considered

dominant, co-dominant, incompletely dominant, recessive, or a combination of these (it’s all relative).

Dominant and recessive alleles are straight-forward. Dominant alleles are typically denoted with a capital letter (example:

T). A dominant allele completely masks the expression of the other allele. Both heterozygotes and homozygous dominant

people will express this phenotype. Recessive alleles are typically denoted with a lowercase letter (t). A recessive allele is

masked by any allele that is dominant to it. Homozygotes would be the only ones to express the phenotype of these alleles.

Co-dominance and incomplete dominance are two distinctly different things, but both are a result of a combination of

different alleles (heterozygotes). The big difference between these two categories is how the phenotype is affected. In

both of these cases, the denotation can be arbitrarily defined, with one allele represented by a capital letter and the other

lowercase, or two different letters may be used all together.

Co-dominant alleles express both phenotypes simultaneously. An example of this would be a flower with red and white

petals where one parent had white petals and the other had red. The colors are both present, not mixed together to make

a new color. Another example of allele co-dominance is the human ABO blood typing system. Type O is recessive to both

types A and B, however types A and B are co-dominant. This is how a person can have AB blood; both A and B alleles are

being expressed. However if their genotype is AO, the allele for A is the only one that will be expressed, so the phenotype

would be Type A.

Incomplete dominance also describes simultaneous expression of both alleles, however it is more like a blending of the

two phenotypes of the homozygotes instead of both homozygous phenotypes being present. An example of this would be

a flower with pink petals (Rr), with one parent having red petals (RR) and the other with white (rr). The phenotypes mixed

together to produce a third intermediate phenotype.

Ptc Genetics Lab Student Worksheet Page 9