Question

1. Describe the differences in inheritance patterns for autosomes and allosomes.

2. What are the properties of independent assortment and segregation?

3. Would a human with two X chromosomes and a Y chromosome be male or female? When would you expect an alternative sex than what is expected?

Answer 1

1. The normal chromosomes, other than the sex chromosomes of an individual are known as autosomes. The chromosome which determines the sex of an individual in dioecious or unisexual organisms are called sex-chromosome, Allosomes or Idiosomes. Sex chromosome may be similar in one sex and dissimilar in the other. A sex chromosome which determine male sex is called endrosome eg., Y-chromosome in human and which determines female sex is called crymosome eg., X-chromosome in human.

There are 2 pattern of autosomal inheritance. Autosomal dominant inheritance and Autosomal recessive inheritance. Homologous chromosomes synapse during meiosis and then separate or segregate independently into different cells which establishes the quantitative basis for segregation and independent assortment of hereditary factors.

Sex chromosome inheritance is linked with the chromosome sex determination. Chromosomal sex determination is based on heterogamesis(formation of 2 type of gametes in one sex).

2. Law of independent assortment:

The law of independent assortment states that when a cross is made between two individuals different from each other in two or more characters, then the inheritance of one character is independent to the inheritance of another character. Because of their independent assortment, besides the parental types recombinants are also obtained. In dihybrid cross these combinations are obtained in the ratio of 9:3:3:1. This law is not universally applicable.

Law of segregation or Law of purity of gametes

This law states that both parental alleles(recessive and dominant) of F1 separate and are expressed phenotypically in F2 generation. This law is universally applicable. The F2 generation was produced by allowing F1 hybrid to self pollinate, to find out segregation or separation. It was observed that both dominant and recessive plants appeared in 3:1 ratio. Thus F2 progeny shows both parental forms.

This law suggested that in F1 generation both dominant and recessive traits of the pair segregates at the time of formation of gametes. The gametes are pure for a particular trait and may have either dominant allele or recessive allel of the trait.

3. In humans the female possess two homomorphic sex chromosome i.e. XX. The males contain two heteromorphic sex chromosome i.e.XY. The Y chromosome is often shorter and heterochromatic. When a person have XXY i.e. 2X and 1Y chromose it shows a chromosomal disorder. This kind of disorder is allosomal or sec chromosomal disorder.

Its called Klinefelter's syndrome. It occurs due to the trisomy of X- chromosome in male, resulting into a karyotype of 47, (44+XXY). Individuals have long legs, sparse body hair, small prostrate gland, small testes, reduced mental intelligence and enlarged breasts (Gynaecomastia). Such individuals are sterile .