How many X chromosome (s) and how many autosomes (s) are present in human ovum?

The chromosome is the thread-like structures found inside the nucleus of eukaryotic cells. These are classified into two category autosomes and sex chromosomes or allosomes. In humans, there are a total of 46 chromosomes (23 pairs) are present in which 44 (22 pairs) are autosomes and 2 (1 pair) sex chromosomes. Ovum is the haploid female gamete that is produced by the meiosis and hence have the half number of chromosomes, 23. In this, 22 chromosomes are autosomes and 1 sex chromosome (X).

Describe the symptoms of Down's syndrome.

Down's syndrome is caused due to chromosome 21 trisomy. It is the most common type of chromosomal aneuploidy that causes severe mental retardation in children. It is an autosomal chromosome disorder caused due to non-disjunction of chromosome. Symptoms of this syndrome are: Short stature Rounded face Constantly opened mouth and protruded tongue. Extra folds in eyelids, Broad forehead Flattened nasal bridge Short neck Flat hands Stubby fingers Undeveloped genital and gonads.

Why is a recessive allele unable to express itself in the heterozygous state?

Genes are the basic unit of heredity. Any of two or more alternative forms of a gene is called an allele. In a diploid cell, different alleles of the same gene are present in the same locus of a homologous chromosome. The allele can be present in the homozygous or heterozygous state. In a heterozygous state, one dominant and one recessive allele is present at the same locus. The dominant allele masks the effect of the recessive allele and inhibits it from expression.

Mention one cause of frameshift mutation.

Mutations are alterations made in the genetic material of a living organism. They occur mostly due to errors during DNA replication. It can be caused due to exposure to radiation or highly reactive chemicals. Some mutations are hereditary. They are passed on to the offspring through genetic material. A frameshift mutation is a type of mutation whose cause is the deletion or insertion in a DNA sequence that shifts the way the sequence is read. Crohn's disease, cystic fibrosis, etc., are some examples of genetic disorders caused due to frameshift mutation.

A haemophilic man marries a woman whose mother was normal but her father was haemophilic. What percentage of childern would be there to be haemophilic. Explain it with necessary crossing.

As Haemophilia is an X-linked recessive genetic disorder, so when a haemophilic man (X h Y) marries a woman whose mother was normal but her father was haemophilic, then she would be a carrier of haemophilia (X h X). The offspring produced by them will be a haemophilic female (X h X h ), carrier female (X h X), haemophilic male (X h Y) and a normal male (XY). So, the percentage of offspring that would be haemophilic is 50%.