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Sex linked inheritance – criss cross inheritance – reciprocal difference; holandric genes; sex influenced and sex-limited inheritance - Genetic disorders
Sex linked inheritance:
The characters for which genes are located on sex or ‘X’ or analogous ‘Z’ chromosomes are known as sex linked traits. Such genes are called sex linked genes and linkage of such genes is referred to as sex linkage.
Inheritance of such genes or characters is known as sex linked inheritance. The sex linkage was first discovered by T.H. Morgan in Drosophila and the first sex linked gene found in Drosophila was recessive gene ‘w’ responsible for white eye colour.
When white eyed females are crossed with wild type (red eye) males, all the male offspring have white eyes like the mother and all the female offspring have red eyes like their father.
In human beings, colour blindness and haemophilia (bleeder’s disease) are well known hereditary characters showing a peculiar relationship to sex.
Colour blindness: A marriage between colour blind woman and a normal man gives rise to all normal daughters and colour-blind sons.
The allele for colour blindness (c) is found on both ‘X’ chromosomes of mother and therefore she is colour blind.
The only one ‘X’ chromosome of father in this marriage carries a wild type (+) allele and hence he has normal vision.
The ‘Y’ chromosome lacks both the alleles (+ and c). The reduction division produces one kind of egg in contrast to two kinds of sperms.
Fertilization results in the usual sex ratio of 1 male: 1 female. All the daughters have normal vision since they receive dominant allele ‘+’ from their father.
All the sons are colour blind, because their single ‘X’ chromosome derived from mother carries the allele ‘c’ for colour blindness. This result is known as crisscross inheritance because daughters are normal like father and sons have colour blindness like mother.
However, the daughters are heterozygous carriers. This crisscross method of inheritance is characteristic of sex-linked genes.
This peculiar type of inheritance is due to the fact that Y chromosome carries no alleles homologous to those on the X chromosome.
Thus, males carry only one allele for sex linked traits. This one allelic condition is termed as hemizygous in contrast to homozygous and heterozygous possibilities in female.
The expression of recessive gene in hemizygous condition is termed as pseudo-dominance.
The inheritance of colour blindness can be studied in the following three other possible types of marriages:
Results of possible four marriages make it clear why there are more colour-blind males than females in the population.
In three marriages colour blind sons were produced where as in only one of the marriages, colour blind daughters were observed, where the mother is heterozygous (carrier) and the father is colour blind.
Nearly all colour-blind women must come from the last type of marriage, since the only other possible source of colour-blind females is mating between two colour blind persons – naturally a rare occurrence.
Haemophilia is a recessive sex-linked disease and the inheritance pattern of haemophilia is similar to that of colour blindness in human beings.
Genes present in the non-homologous region of the Y chromosome pass directly from male to male. In man, the genes present on Y chromosome (holandric genes) such as the gene causing hypertrichosis (causing excessive development of hairs on the pinna of ear) are transmitted directly from father to son.
Sex influenced inheritance:
Sex influenced genes are the autosomal genes present in both males and females, whose phenotypic expression is different in different sexes in such a way that they act as dominant in one sex and recessive in the other i.e. in a pair of alleles one seems to be dominant in males while the other in females.
E.g.: Pattern baldness in human beings and horns in sheep.
Pattern baldness in human beings is a condition in which a low fringe of hair is present on the head in human beings.
It is a genetically inherited condition, where the allele for baldness B is dominant in males and recessive in females.
In heterozygous condition, males are bald and females are non-bald.
If a woman heterozygous for this gene marries a heterozygous bald man, in the offspring, the ratio of bald to non-bald in males is 3: 1, while in females it is 1: 3.
Sex limited characters or Secondary Sexual characters:
Sex limited genes are autosomal genes, whose phenotypic expression is limited to one sex only.
Their phenotypic expression is influenced by the sex hormones.
The sex-limited genes are mainly responsible for secondary sex characters in cattle, human beings and fowl.
E.g.: milk production in cattle, beard development in human beings, plumage in male fowls etc.
Milk production in cattle: Just as the cow, the bull carries genes for milk production, but the bull obviously cannot express this trait.
Bull may however transmit these genes for high milk production to the female progeny and the male progeny are unable to express this trait.
Some bulls are so well endowed with such genes that they are known to breed calves, which always yield greater milk than their mothers.
However, in plants no secondary sexual characters are known except the absence of one or the other sporangia.
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