Basic Genetic Mechanisms - Heredity

Overview

Why do children look similar to their parents, but not identical? Why do some diseases get passed on to all of the offspring while others don’t? These are the questions that will be answered in the next few lectures. Gregor Mendel set most of the groundwork that is the foundation of modern genetics today. 

 

There are several terms to be familiar with before continuing.

• Genes are genetic material on a chromosome that code for a trait. For example, you have a gene for eye color.
  
  
• Alleles are variations of genes. For example, you have the allele for brown eye color. Note that some alleles are dominant over others. That is, if a person inherits both the dominant and the recessive alleles, the dominant allele will be the one expressed.
  
  
• A genotype is the actual set of alleles an organims carries. For example, you have the genotype Bb since you have the allele for brown eye color (B) and the allele for blue eye color (b). An organism is said to be homozygous for a certain trait if both it carries two of the same alleles. It is homozygous dominant if it carries two dominant alleles and homozygous recessive if it carries two recessive alleles. The organism in our example is heterozygous -- it carries two different alleles.
  
  
• A phenotype is the expression of a gene. For example, since you have the genotype Bb with one dominant and one recessive allele, the dominant allele (B) will mask the recessive allele (b) and you will have the phenotype for brown eyes. 

Mendel's Discoveries

Gregor Mendel was an Austrian monk who lived in the middle of the 19th century. Mendel performed breeding experiments on bean plants by measuring the heredity patterns of a number of different characters of the plants, such as the color of fruits and flowers, the shape of seeds and fruits, and the size of plants. Mendel’s studies led him to propose two fundamental laws that govern the heredity of many characters in organisms – the law of segregation and the law of independent assortment. Although Mendel’s work was largely ignored for over 30 years after its completion, it ultimately has provided a strong foundation for the understanding of inheritance and gene expression.

Monohybrid Crosses

 

Monohybrid crosses deal with a single trait. When studying monohybrid crosses it is useful to make a Punnett square. In a Punnett square, the genotype of one parent is written across the top while the genotype of the other parent is written vertically to the left. The cross is made and the possible genotypes of the offspring show up in the four boxes below.

Example:
In humans, weirdness (W) is dominant over normal (w). A man who is heterozygous for weirdness marries a woman who is heterozygous for weirdness. What are the possible genotypes of their offspring?

Sometimes you are information about the offspring and are asked to obtain information about the parents. 

Example:
In pea plants, yellow seeds (Y) are dominant and green seeds (y) are recessive. A pea plant with yellow seeds is crossed with a pea plant with green seeds. The resulting offspring have about equal numbers of yellow and green seeded plants. What are the genotypes of the parents?

 
Dihybrid Crosses
 

In a dihybrid cross, two traits are studied at one time. 

Example:
A female guinea pig heterozygous for both fur color and coat texture is crossed with a male that has light fur color and is heterozygous for coat texture. Dark fur color is dominant (D) and light fur (d) is recessive. Rough coat texture (R) is dominant while smooth coat (r) is recessive. Find the possible genotypes of the offspring.

 

Types of Inheritance

While Mendel’s experiments revealed systematic laws involved in patterns of heredity, they certainly didn’t explain everything. As usually happens in scientific research, finding the answer to one question raises at least ten more. Clearly not every inherited trait follows Mendel’s laws, so what else is involved? In this lesson we will investigate some other types of inheritance that take Mendel’s laws another level deeper or put a twist on them.

While not all aspects of heredity follow strict Mendelian laws, Mendelian inheritance patterns have turned out to be very important and useful in fields such as plant and animal breeding and understanding and diagnosing inherited diseases in humans.

Non-Mendelian Inheritance

In eukaryotic organisms, the majority of genes are located on autosomes, or non-sex chromosomes. While many of these genes have Mendelian inheritance patterns, there are also many that do not. Inheritance patterns that differ from Mendelian patterns can occur for a number of reasons, including location of the gene(s) on the chromosome, environmental effects, and varying characteristics of the protein made by different alleles of the gene. Although these types of relationships lead to heredity patterns that would seem to disprove Mendel’s laws, this is not exactly the case. Rather, instances of non-Mendelian inheritance provide us with further insight into the link between genotype and phenotype, and patterns of heredity.

Sex-Linked Inheritance

As humans, we are very accustomed to considering others as male or female, girl or boy, or man and woman. It may be surprising to learn, however, that the majority of sexually reproducing organisms on earth, including many plants, algae, and even some animals, are functionally both male and female at the same time.

In species that do produce male and female individuals, sex may be determined in a number of different ways. In some cases, such as humans, sex is determined by special chromosome, called a sex chromosome. Often, genes for non-sexual characters are also found on sex chromosomes. In such cases, inheritance patterns of these characters may vary from typical Mendelian inheritance.