Pseudoalleles Sex-influenced and sex-limited traits Pedigree analysis Pseudoalleles • Pseudoalleles refer to closely linked and functionally related genes. • Two or more genes with similar functions located close to each other on the same chromosome • Pseudoalleles are genetically linked and are always nearly inherited together • A cluster of pseudoalleles is known as pseudoallele series or a complex locus or a complex region. • Eg. Red, White and Apricot eye colour in Drosophila, • Main characteristics of pseudoalleles are given below: 1. Pseudoalleles govern different expressions of the same character. In other words, they are functionally related. 2. Pseudoalleles are considered to occupy a complex locus which is divided into sub loci. Thus, they occupy different positions, but on the same complex locus. 3. They exhibit low frequency of genetic recombination by crossing over. In other words, crossing over occurs between pseudoalleles, but at a very low frequency. Sex-influenced and sex-limited traits • The chromosomes present in the diploid cells of the majority of the sexually reproducing animals are of two types: autosomes bearing genes for somatic characters and sex chromosomes bearing genes for sex. • Sex chromosomes also carry some genes for non-sexual characters such as colour blindness and haemophilia. • Such genes which are always associated with sex chromosomes are called sex-linked genes. • In humans and Drosophila, the sex chromosomes (X and Y) are unequal in size and shape, X being larger and rod shaped whereas Y is small and slightly curved. • There are three types of sex-linked genes depending upon their association with particular chromosome. They are as follows: (i) The genes which are located on X-chromosomes are called X-linked genes or sex linked genes. (ii) The genes which are located on Y chromosomes are called Y-linked genes or holandric genes. (iii) Certain genes are found to occur in both X and Y chromosomes. Such genes are called incomplete sex-linked genes. Characteristics of Sex Linked Inheritance: (a) It is a criss-cross inheritance as the father passes its sexlinked character to his daughter who in turn passes it to the grandson. (b) Daughter does not express the recessive trait but act as carrier in the heterozygous condition. (c) Female homozygous for recessive trait expresses the trait. (d) Any recessive gene borne by the X chromosome of male is immediately expressed as Y chromosome has no allele to counteract. Pedigree analysis • Pedigree is a table, chart or diagram representing the ancestral line of an in-dividual man or animal having a long span of life and low reproductive ability. • Pedigree analysis is the process of determination of the exact mode of inheritance of a gene when sufficient family information is available. • By this, determination of genotypes and phenotypes of past, present as well as future generations; the nature of inheritance, whether sex-linked or autosomal and dominant or recessive; probable ratio of inheritance of a gene and segregation are possible. • To express the phenotype of individuals of a family for a few generations various symbols are used • The symbols used are: i. Circle (O) a female ii. Square (□) a male iii. Diamond (◊) a sib (sibling) of un-known (either) sex iv. Darkened or shaded symbols for the rarer genetic phenotype v. Connection between circle and square (□—O), marriage line vi. Horizontal line (O—□) below the parents connecting off-springs (siblings) is sibship line vii. Generations, denoted by roman letter I, II, III, IV etc., on a side viii. Off-springs in each generation are numbered from left side in order of birth ix. Identical (monozygotic) twin (arise from splitting of a common zygote) represented by a common connection with sib-ship line (II -5 & 6) x. Dizygotic twins with separate connection with sib-ship line (IV-6 & 7) xi. Numbers within the symbols (II-7 to 10) are sibs not individually listed
