Question

State one difference between
(i) homozygous and heterozygous individuals
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(ii) dominant and recessive traits
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(iii) genotype and phenotype
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(iv) monohybrid and dihybrid crosses.
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Answer 1

Answer:

HOMOZYGOUS AND HETEROZYGOUS INDIVIDUALS:

Homozygous and heterozygous are terms that are used to describe allele pairs. Individuals carrying two identical alleles ( RR or rr) are known as homozygous. While individual organisms bearing different alleles (Rr) are known heterozygous. To better understand let us consider that we have two pairs of alleles for a hair colour gene – one allele codes for brown (R) and the other allele codes for black (r). Homologous pairs of chromosomes would either consist of same alleles (both black or brown) or different alleles (black and brown). On the basis of these possible outcomes, we can differentiate whether they are same alleles, i.e. homozygous or different alleles i.e. heterozygous.

DOMINANT AND RECESSIVE TRAITS:

Dominant and recessive traits exist when a trait has two different forms at the gene level. The trait that first appears or is visibly expressed in the organism is called the dominant trait. The trait that is present at the gene level but is masked and does not show itself in the organism is called the recessive trait.

GENOTYPE AND PHENOTYPE:

The phenotype is the physical appearance of an organism, while the Genotype is the genetic composition of an organism. Phenotype are observable and are the expression of the genes of an individual. So even the organism with the same species may differ, with a minute difference in their genotype. This is the main difference between the two.

MONOHYBRID AND DIHYBRID CROSSES:

Mendel's earlier experiments in garden pea (Pisum sativum) were mono hybrid crosses. A cross involving contrasting expression of one trait is transferred to as monohybrid cross. For example, in order to learn inheritance of plant height, a tall pea plant was crossed with a dwarf one; all other traits were ignored.

Inheritance of two pairs of alleles through a number of generations was studied by Mendel through dihybrid crosses. In one experiment Mendel crossed a round green’ with a wrinkled yellow. The F1 individuals, called dihybrid, were all round yellow. Two members of the F1 were crossed to raise the F2 . The result was 315 round yellow + 108 round green + 101 wrinkled yellow + 32 wrinkled green. This observed figures closely approximated a 9:3:3:1 ratio.

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