CRISS-CROSS INHERITANCE:

• It is the transmission of a gene from mother to son or father to daughter.

• Those patterns of inheritance are called crisscross inheritance or skip generation inheritance, in which a character is inherited to the second generation through the carrier of first generation.

• The sex linked characters exhibit criss-cross inheritance

Genes on one or other of the sex chromosomes produce inheritance pattern different from that

shown by autosomes:

Autosomal genes

• All individuals carry two allels of each genes.

• Dominance operates in both males and females.

• Reciprocal crosses produce the same results.

• Alleles passed equally to male and female offsprings.

Sex-linked genes

• Males carry only one alleles of each gene (hemizygous).

• Dominance operates in females only

• Reciprocal crosses produce different results.

• ‘criss cross’ inheritance pattern: father to daughter to grandson, etc.

RECEPROCAL CROSS:• It involves a pair

of crosses in which the phenotypes of the partners are reversed.

• Different results from the two crosses indicates that sex linkage of the gene is controlling the trait.

CRISS-CROSS INHERITANCE:

Sex-linked characters, which are controlled by genes on sex chromosomes are divided into two categories:

•Sex-influenced traits

•Sex-limited traits

Sex-influenced traits:

•These are phenotypes affected by whether they appear in a male or female body.

•Even in a homozygous female the condition may not be expressed fully.

•Example: baldness in humans.

Sex-limited traits:

•These are characters only expressed in one sex.

•They may be caused by genes on either autosomal or sex chromosomes.

•Examples: female sterility in Drosophila

Sex linkage & criss-cross inheritance• Sex-linkage was first discovered by Thomas H. Morgan

(father of modern genetics), (1910) in Drosophila melanogaster.

• When the white-eyed male is crossed to a normal red-eyed female, in the first generation all the males and females were red-eyed.

• First generation red-eyed female was crossed to a red-eyed male, in the second generation all the females were red-eyed and 50% males were white-eyed.

• The white-eyed character from the initial male parent is inherited to the male of the 2nd generation through carrier female.

Sex linkage & criss-cross inheritance• In a reciprocal cross that involved a second cross of same

traits but carried by sexes reversed, the results were different.

• When a white-eyed female was crossed to a red-eyed male, in the first generation all the females were red-eyed and males were white-eyed.

• The difference was due to the inheritance of a recessive gene located on the X chromosome.

• Males always get the sex-linked recessive characters from females due to a single recessive allele on X-chromosome.

CRISS-CROSS INHERITANCE:

• Genes on X but not on Y – leads to a “criss-cross” pattern of inheritance where sons receive their X chromosome from the maternal parent and daughters receive an X from both parents

• There are several, well-known sex-linked traits in humans: examples include hemophilia, color blindness, and the fragile X syndrome

• Holandric inheritance: Genes on the Y but not the X

Sex linked recessive inheritance:

For a recessive trait controlled by a gene on the X chromosome, the features of inheritance are:

•More males than females show the trait.

•A male with the trait cannot pass the allele to sons.

•Carrier females do not show the trait but pass it to sons.

•All daughters of affected males will at least be carriers of the trait.

Sex linked dominant inheritance:

•This is rarer since all the daughters of affected males will be affected (the heterozygous condition is not a carrier)

X- linked dominant inheritance:

X-linked dominant inheritance works differently depending upon whether the mother (left image) or father (right image) is the carrier of a gene that causes a disease or disorder.

X- linked recessive inheritance:In X- linked recessive inheritance mainly the male offsprings are affected due to presence of single X chromosome.

EXAMPLE OF CRISS-CROSS INHERITANCE- Haemophilia

Transmission of a character from 1st generation male to 3rd generation male including a 2nd generation female shows the criss-cross inheritance.

EXAMPLE OF CRISS-CROSS INHERITANCE- Colour blindness

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