Alleles

Box 1.1 ). Mendel’s principles of inheritance can be summarized as follows.

(1) The heredity and variation of characters are controlled by factors, now called genes, which occur in pairs. Mendel called these factors Formbildungelementen (form-building elements).

(2) Contrasting traits are specified by different forms of each gene (different Alleles).

(3) When two dissimilar Alleles are present in the same individual (i.e. in a heterozygote), one trait displays dominance over the other: the phenotype associated with one allele (the dominant allele) is expressed at the expense of that of the other (the recessive allele).

Table 1.1: Definitions of some common terms used in transmission genetics

Term	Definition
Allele	Broadly, a variant form of a gene specifying a particular trait. At the molecular level, a sequence variant of a gene (q.v. wild-type allele, mutant allele, polymorphism)
Character	A biological property of an organism which can be detected or measured
Character mode	A general type of character, e.g. eye color
Character trait, trait, variant	A specific type of character, e.g. blue eye color
Gene	Broadly, a hereditary factor controlling or contributing to the control of a particular character. At the molecular level, a segment of DNA (or RNA in some viruses) which is expressed, i.e. used to synthesize one or more products with particular functions in the cell (q.v. gene, cistron, gene expression)
(Gene) locus	The position of a gene (or other marker or landmark) on a chromosome or physical or genetic map. A useful term because it allows discussion of genes irrespective of genotype or zygosity
Genetic	Pertaining to genes. Of characters, heredity and variation arising from the nucleotide sequence of the gene (cf. epigenetic, environmental

Alleles , which code for different versions of a particular inherited character (such as red versus white flower color). The different Alleles of the same gene are closely related, but have minor chemical variations that may produce significantly different outcomes.

The overall nature of an organism is due to the sum of the effects of all of its genes as expressed in a particular environment. The total genetic make-up of an organism is referred to as its genome . In lower organisms such as bacteria, the genome may consist of approximately 2,000 to 6,000 genes, whereas in higher organisms such as plants and animals, there may be up to 50,000 genes.

Etymological Note

Genes Determine Each Step in Biochemical Pathways

2.7 Summary

Chapter 17 ).

Does selection or drift determine the future of an allele?

•The effective population size •The selection coefficient •The type of selection •The frequency of the allele under selection

T , found in these 334 nucleotides are:

Note that the allele count is determined by the genotype numbers multiplied by the number of copies of the allele of interest borne by individuals with a specific genotype. The gene pool is now described by the allele frequencies that are derived by dividing the allele counts by the total number of sampled genes (or nucleotides, in this case):

These allele frequencies are the essential description of the Ashkenazi gene pool for this nucleotide. Similarly, the allele frequencies in the sub-Saharan African population are: (5  ×  2  +  16  ×  1  +  85  ×  0)/212  =  0.123 for the C allele and (5  ×  0  +  16  ×  1  +  85  ×  2)/212  =  0.877 for the T allele.

An alternative definition of the gene pool is the population of potential gametes that can be generated from the individuals of the deme. This definition is illustrated in Fig. 1.2 for the Ashkenazi Jewish population. Starting with the population of diploid individuals as characterized by genotype frequencies, the rules of inheritance are applied to each genotypic class to predict the probabilities of all the types of gametes each genotype can produce. Assuming that there is no mutation and normal meiosis, the CC homozygote will produce C -bearing gametes with a probability of 1, and T- bearing gametes with a probability of 0. Similarly, the TT homozygote will produce C -bearing gametes with a probability of 0 and T- bearing gametes with a probability of 1. Given no mutation and normal meiosis, the only rule of inheritance that is relevant to the CT heterozygotes is Mendel's first law of segregation, which states that the probability of a C- bearing gamete is ½ and the probability of a T -bearing gamete is ½. These meiotic probabilities assigned to each diploid genotype are transition probabilities; that is, they describe the probabilities by which a given diploid genotype produces a given haploid gamete type through the process of meiosis. The transition arrows shown in Fig. 1.2 illustrate this. The genotype frequencies and the meiotic transition probabilities are sufficient to calculate the allele frequencies in the population of gametes, as shown in Fig. 1.2

Appendix 3 Population genetics of single-gene disorders Maintenance of gene frequencies

Maternal gametes	Paternal gametes
A (p )	a (q )
A (p )	AA (p 2 )	Aa (pq )
a (q )	Aa (pq )	aa (q 2 )

The most important application of this law is the calculation of carrier frequencies for autosomal recessive traits.