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| LEARNING OUTCOMES |
The following topics are covered in this chapter:
• The Mendelian principles of transmission:
♦ unit inheritance: genes and alleles;
♦ dominance: allelic relationships;
♦ segregation – single gene inheritance patterns, Punnet squares;
♦ independent assortment – inheriting two or more genes.
• Exceptions to the rules:
♦ mitochondrial inheritance;
♦ penetrance;
♦ genomic imprinting;
♦ sex-related effects;
♦ mutations;
♦ genetic linkage;
♦ polygenic and multifactorial inheritance;
♦ epistasis;
♦ pleiotropy.
INTRODUCTION
The fact that biological traits can be inherited has long been established. The first significant discoveries regarding the mechanisms of inheritance resulted from the work of Gregor Mendel in the late nineteenth century.
Mendel studied the patterns of inheritance within pea plants while he was working as a monk. His work went largely unnoticed until after the start of the twentieth century. Scientists who were studying the function of chromosomes rediscovered Mendel’s publications and realised that Mendel had discovered the way in which biological traits were inherited. Mendel became known as the Father of Genetics, and the branch of genetics involved with simple inheritance is known as Mendelian genetics.
Although Mendel’s work involved plants, his findings are relevant to human genetics. From his work, he derived certain laws that have become the principles of transmission genetics. Mendel proposed four principles of inheritance: unit inheritance, dominance, segregation and independent assortment. It is these four principles that form the basis of inheritance today.
1. The Principle of Unit Inheritance
Biological traits are determined by genes. Genes are the basic units of heredity. Strands of DNA that encode for one protein form a gene. As chromosomes occur in pairs after fertilisation, genes can be found on both the paired chromosomes. The individual ‘genes’ on each chromosome are termed alleles.1 An allele is a version of a gene that has a paired version of the same gene in the same location on the opposite chromosome (see Figure 2.1).
Figure 2.1 A gene is usually made of two alleles, one on each of the paired chromosomes.
2. The Principle of Dominance
Alleles can present as different versions of the same gene. If two alleles carried a different sequence of DNA, the effect of one allele might be masked by its partner allele. A dominant allele will be expressed regardless of any instructions carried by the other allele.
In humans, the allele that codes for freckles is dominant over the allele for no freckles. Therefore, an individual who carries two different alleles for this gene – an allele for freckles and an allele for no freckles – will have freckles on their skin. This is because the freckles gene is dominant and will be expressed in that individual. An individual who has two different types of alleles for a single trait (like freckles) is said to be heterozygous for that trait.
An allele that is not expressed, due to the presence of a dominant partner allele, is termed recessive. Recessive alleles are only expressed when both alleles are in a recessive form. Individuals who have either two recessive alleles or two dominant alleles (i.e. two identical alleles) are said to be homozygous for that trait.
Whether an individual is said to be homozygous or heterozygous for a particular trait indicates whether they carry the same or different alleles within that gene. This can be described as the individual’s genotype. A person’s genotype is the genetic make-up for a particular trait. The term phenotype is used to describe the expression of the gene (or paired alleles) for the same trait (Table 2.1).
Table 2.1 Genotypes and phenotypes for freckles
| Genotype | Classification | Phenotype | |
| Allele 1 | Allele 2 | ||
| freckles | freckles | homozygous | has freckles |
| freckles | no freckles | heterozygous | has freckles |
| no freckles | freckles | heterozygous | has freckles |
| no freckles | no freckles | homozygous | no freckles |
Allelic relationships
Dominant alleles are phenotypically expressed in both heterozygotes and homozygotes. Recessive alleles are only expressed if the alleles are both in a recessive form (homozygous recessive).
Upper and lower case letters are used to represent dominant and recessive alleles. Upper case letters are used to represent a dominant allele and lower case for a recessive allele. If the letter ‘F’ was chosen to represent the gene for freckles, then ‘F’ would represent the dominant allele and ‘f’ would represent the recessive allele. An individual who is heterozygous for the freckles gene would be represented as an ‘Ff’ genotype. A homozygous dominant genotype would be ‘FF’, while a homozygous recessive would be ‘ff’.
Any letter can be chosen to represent different allelic traits. However, it is good practice to choose a letter that has a different form in upper case compared with lower case. For example, A and a, B and b would be good to use but avoid C and c. This helps when drawing out inheritance patterns as different forms can be visually recognised as dominant or recessive, and it avoids errors due to poor handwriting.
Examples of Mendelian traits
Cleft chin
A cleft chin is due to a dominant allele (Figure 2.2). A person without a cleft chin has two recessive alleles for no cleft in their chin.
Figure 2.2
