Heredity And Variation 8 - From Past 28 Years Questions Free MCQ Practice

Consideration of one character with two contrasting conditions at one time contributed to Mendel's success in proposing the laws of inheritance of traits. His choice of experimental plant as pea and use of statistical methods also contributed to his success.

In haploid organisms every gene, irrespective of dominant or recessive and every mutation finds expression because there is only one allele for each gene in the haploid individual. Recessive allele is able to express as there is no alternative dominant allele for producing its masking effect on recessive allele.

Humans have 22 pairs of autosomes and 1 pair of sex chromosomes which are XX in females and XY in males. So, everytime the chances of son or daughter is 50-50% depending upon which sex chromosome is contributed by father in sperm.

Induced mutations are produced in response to external factors of chemicals called mutagens. In agriculture mutagens are used to induce desired mutations in crop plants.

Chromosomes occur in homologous pairs. Somatic cells have diploid number of chromosomes. Humans have 23 pairs i.e. 46 chromosomes. 22 pairs of autosomes and 1 pair of sex chromosomes ie. XX in females and XY in males.

Two linked genes a and b show 20% recombination. The individuals of a dihybrid cross between + + / + + × ab/ ab shall show gametes + + 40 : ab40: + a 10 : +b : 10.

Triticale is a man made cereale produced by crossing two generate wheat (Triticum) and rye (Secale). Hexaploid Triticale was obtained by crossing Triticum durum and Secale cereale.

Besides nucleus some genes are also present in the cytoplasm of the female parent, these genes are called plasmogenes. In the given example the progeny is albino because of inheritance of plastids from female parent.

On mating tt with Tt, 50% individual offsprings are recessive and 50% are heterozygous dominant.

A gene may have more than two alternative forms occupying the same locus on a chromosome such alleles are known as multiple alleles and the phenomenon is turned as multiple allelism. ABO blood group has 3 alleles. I^O, I^A or A, I^B or B.

In recessive epistasis, the recessive alleles suppress the expression of a trait. Recessive epistasis is a type of intergenic interaction in which one non allelic gene can produce its effect independently in the dominant state but second gene can produce its effect when they are present together. Here homozygous recessive allele for a gene mask the expression of dominant allele of another gene.

AaBB × aaBB on crossing gives 50% individuals having genotype AaBB and 50% individuals having genotype aaBB.

Nucleosomes are oblate spherical structure having an octamer of four histone proteins (constituting 2 molecules of each type).

The probability of the daughter becoming colour blind arises only when the father is also colour blind.
Husband – XY Wife – XXC (wife is carrier because her father is colour blind)

So, probability of their daughter becoming colour blind is 0%.

Phenylketonuria is a human genetic disorder in which the body does not contain the enzyme phenylalanine hydroxylase, necessary to metabolize phenylalanine to tyrosine, and converts phenylalanine instead to phenylpyruvic acid. As PKU is an autosomal recessive genetic disorder each parent must have at least one defective allele of the gene for PAH, and the child must inherit a defective allele from each parent. As such, it is possible for a parent with a PKU phenotype to have a child without PKU if the other parent possesses at least one functional allele of the gene for PAH. A child of two parents with the PKU phenotype will always receive two defective alleles so will always have PKU. The gene for PAH is located on chromosome 12, at location  12q22-q24.2.

Haemophilia is a sex linked recessive trait  carried by X chromosome (also known as bleeder’s disease).

Bateson and Punnett (1906) discovered coupling and repulsion in sweet pea (Lathyrus odoratus).

Down’s syndrome is due to trisomy of 21st chromosome and is an autosomal abnormality.

Incomplete dominance is the phenomenon of none of the alleles being dominant, with the effect that the hybrid produced by crossing two pure individuals is a mixture between the parents.

The allele which can not impressits effect in the hybrid or in the presence of dominant allele is termed as recessive. It is denoted by small letter.

The brown eyed man will have the genotype Bb and his wife bb. Hence Bb × bb = Bb : bb. Hence ratio is 1 : 1.

Dihybrid condition involves simultaneous inheritance of two pairs of mendelian factors or genes.

According to the law of Independent Assortment, alleles of different characters located in different pairs of homologous chromosomes are independent of one another in their separation or segregation during gamete formation.

Gregor Johann Mendel formulated the laws of heredity and is regarded as the father of genetics.

Two allelomorphs are the different forms of a gene which are responsible for different expression of same characters, e.g. for colour of flower is R and r.