Types of inheritance

Introduction

Previous chapters looked at such genetic terms as genotype and phenotype. This chapter looks at how the genes are distributed when organisms reproduce. Different types of inheritance and genetic laws will be discussed.

Types of genetic traits

When reproductive cells, called gametes, combine to form a new organism they both contribute their hereditary factors. Usually these factors do not blend with each other and act independently. In his studies Mendel discovered that hereditary factors, called genetic traits, are either dominant or recessive. The trait that will phenotypically appear in the offspring if it is contributed by only one of the parents is called dominant. A recessive trait is the opposite of a dominant trait. In order to be phenotypically expressed it must be contributed by both parents. If a recessive trait is contributed by one parent only, it is suppressed by the dominant trait. Recessive genes can be carried by an organism without being phenotypically expressed.

Different genes control different characteristics. Several versions of a gene that is responsible for the same characteristic are called alleles. For example, humans can have blue, brown, green or grey eyes. Different forms of the gene responsible for eye colour are alleles.

An organism carrying two different alleles of the same gene is called heterozygous or hybrid. Organisms carrying the same alleles of the gene are called homozygous or true-breeding. All alleles occupy a specific location on chromosomes, called the locus.

Laws of genetics

Mendel showed that genetic traits are inherited according to particular laws, which were later named after him and which are sometimes referred to as the laws of genetics or the principles of inheritance. See image 1.

These laws are.

1. Law of Segregation

Mendel's law of segregation states that allele pairs separate or segregate during gamete formation and then randomly unite at fertilisation. The specific gene that is included in a gamete is determined by chance.

2. Law of Independent Assortment

Alleles for different traits are distributed to gametes and offspring independently of one another. During his research, Mendel noticed that different features of the plants did not influence one another. That also applies to other living organisms. For example, blue eyes do not come in one set with large feet in humans. In other words, different traits are inherited independently.

Punnett squares

Scientists study genetics in order to better predict ways of inheriting particular traits.Today genetics is a very important part of agriculture and medicine. Thanks to genetic research, it is possible to develop plant and animal breeds with particular qualities. Also, by applying the laws of genetics it is possible to predict inheritance patterns in family lines.

One of the easiest ways to calculate the mathematical probability of inheriting a specific trait was invented by an early 20th century English scientist, Reginald Punnett. He developed a chart that helps to predict genotype of the offspring. This chart is now called Punnett squares.By using Punnett squares it is possible to predict all potential genotype combinations of the offspring, given the genotypes of their parents. It also shows the likelihood of each offspring's genotypes occurring. See image 2.

Types of inheritance

Sometimes an organism's feature or characteristic inheritance depends only on dominant and recessive alleles. In other cases, alleles form a different mix resulting in a type of inheritance called codominance. Codominance occurs when both alleles are expressed in an organism's phenotype. Sometimes the phenotype of heterozygous organisms is a mixture of the parents' phenotypes. This type of inheritance is called incomplete dominance.

Consider what happens if plants with red flowers and plants with white flowers are cross-bred. The allele for red-coloured flowers is dominant and the allele for white-coloured flowers is recessive. In the case of codominance, the offspring will have red flowers with white spots or white flowers with red spots. In the case of incomplete dominance, offspring will have pink flowers because that is the colour resulting from mixing red and white.