12 Biology

INHERITANCE AND VARIATION

SEX DETERMINATION

XO Type Sex Determination: In a large number of insects, all the eggs bear an additional X-chromosome apart from other chromosomes (autosomes). Some sperms bear the X-chromosome while some do not. An egg which is fertilized by sperm (having X-chromosome) becomes female. An egg which is fertilized by sperm (without X-chromosome) becomes a male.




XY Type Sex Determination: This type of sex determination is seen in many insects and in mammals (including man). In this case, both male and female have same number of chromosomes. The X-chromosome is larger, while the Y-chromosome is somewhat smaller. The somatic cell of a male contains an X and a Y chromosome in the last pair, while both chromosomes in the last pair of somatic cell of female are X-chromosomes. Thus, each egg has a single X-chromosome; as the sex-chromosome. 50% of sperms have X-chromosome, while the remaining 50% sperms have Y-chromosome as the sex-chromosome. If a sperm with X-chromosome fertilizes the egg, the zygote would develop into female. If a sperm with Y-chromosome fertilizes the egg, the zygote would develop into male. Examples: Human, Drosophila.

ZW Type Sex Determination: This type of sex determination is seen in birds. The female bird produces two types of gametes, i.e. bearing Z-chromosome or W-chromosome. The male bird produces only one type of gametes, i.e. Z-chromosome. When an egg with Z-chromosome is fertilized, the zygote would develop into a male. When an egg with W-chromosome is fertilized, the zygote would develop into a female.

Mutation: Alteration of DNA sequences and consequent results in changes in genotype and phenotype of an organism is called mutation. When mutation arises due to change in a single base pair of DNA, it is called point mutation.



GENETIC DISORDERS

Pedigree Analysis: Study of the family history about inheritance of a particular traits over several generations of a family is called pedigree analysis.

Mendelian Disorders: Mendelian disorders are mainly determined by alteration or mutation in the single gene. Transmission of these disorders generally obeys the principles of inheritance. Most common Mendelian disorders are: Haemophilia, Cystic fibrosis, Sickle-cell anaemia, Colour blindness, Phenylketonuria, Thalesemia, etc.

Haemophilia: This is a sex-linked recessive disease. It is transmitted from unaffected carrier female to some of the male progeny. In this disease, a single protein (that is as part of a cascade of proteins involved in clotting of blood) is affected. In an affected individual, a simple cut will result in non-stop bleeding. The heterozygous female is the carrier. She may transmit the disease to sons. The possibility of a female becoming a haemophilic is extremely rare, because mother of such a female has to be at least carrier and the father should be haemophilic. But a haemophilic rarely survives up to the age at which he can become a father.

Sickle Cell Anaemia: This is an autosome-linked recessive trait that can be transmitted from parents to the offspring when both the partners are carrier for the gene (or heterozygous). The disease is controlled by a single pair of allele; HbA and HbS. There are three possible combinations, i.e. HbAHbA, HbAHbS and HbSHbS. Out of them, only homozygous individuals with HbSHbS genotype are affected. Individuals with HbAHbS are carriers of this disease.

This defect is caused by the substitution of Glutamic acid (Glu) by Valine (Val) at the 6th position of the beta globin chain of the haemoglobin molecule. The mutant haemoglobin molecule undergoes polymerization under low oxygen tension causing the change in the shame of the RBC from biconcave disc to elongated sickle-like structure.

Phenylketonuria: This is an autosome-linked recessive trait. The affected individual lacks an enzyme that converts the amino acid phenylalanine into tyrosine. As a result, phenylalanine is accumulated and converted into phenylpyruvic acid and other derivatives. Accumulation of these substances in the brain results in mental retardation. These substances are also excreted through urine because of poor absorption by kidney.



Chromosomal Disorders: These disorders are caused due to absence or excess or abnormal arrangement of one or more chromosomes. Failure of segregation of chromatids during cell division cycle results in the gain or loss of a chromosome. This phenomenon is called aneuloploidy.

Down’s Syndrome: This genetic disorder is caused due to the presence of an additional copy of the chromosome number 21 (trisomy of 21). The affected Individual is short statured with small round head, furrowed tongue and partially open mouth. Palm is broad with characteristic palm crease. Physical, psychomotor and mental development is retarded.

Klinefelter’s Syndrome: This genetic disorder is caused due to the presence of an additional copy of X-chromosome. The affected individual has overall masculine development, but the feminine development is also expressed. Such individuals are sterile.

Tuner’s Syndrome: This genetic disorder is caused due to the absence of one of the X chromosomes. Such females are sterile. Ovaries are rudimentary. Such females lack other secondary sexual characters.