Mendel
performed crosses between plants with purely tall and purely short (dwarf)
traits. The resulting offspring were all tall. When the offspring of the first
generation (F1) were crossed, he observed a ratio of 3:1.
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| Monohybrid Inheritance |
The
reason behind the appearance of the short trait is due to the fact that the
parents were both homozygous for tall and short traits. The F1 generation
appeared tall but were heterozygous. When two F1 offspring were bred, Mendel
observed that three were tall and one was short. This outcome occurred because
one tall plant was homozygous, two tall plants were heterozygous, and one short
plant was homozygous for the short trait. The short trait is recessive,
manifesting only in individuals with a homozygous condition—when both parents
contribute the gene. Hence, among every four plants resulting from meiosis,
three were tall and one was short. Mendel derived the Law of Segregation from
these experiments, now known as Mendel's First Law of Segregation.
"This
principle states that every organism possesses a pair of factors for each characteristic,
and these factors segregate during gamete formation, resulting in each gamete
carrying only one factor from the pair." In simpler terms, "when two
different traits are crossed, the traits will segregate in a 3:1 ratio in the
second generation." In the second generation (F2), Mendel observed the
following:
Tall
Plants: (i) One-third of the tall plants produced only tall offspring.
(ii)
Two-thirds of the tall plants produced both tall and short offspring.
Short
Plants: The F2 short plants produced only short offspring.
Gene
Pool: The complete collection of genes within a population at a specific time
is known as the gene pool. It serves as the source from which the next
generation derives their genetic makeup, encompassing all alternate forms (alleles)
present in the population.
Some Definitions
Gene: A gene, in Mendelian Genetics, determines the expression of a
phenotypic trait.
Genome: The entire genetic information contained within an organism or
cell.
Locus: The position of a gene on a chromosome is referred to as its locus
(plural: loci).
Alleles: Variants of genes are termed alleles or allelomorphs.
Gene
Pool: The sum total of all genes across
all individuals in a population.
Dominant: An allele that expresses its trait when present with another
allele of a different kind in a heterozygous individual is termed dominant.
Recessive: An allele whose trait is suppressed in a heterozygous state.
Phenotype: The observable appearance of a trait or gene is called the
phenotype.
Genotype:
The genetic constitution of a trait, whether homozygous or
heterozygous.
Homozygous: Having identical alleles, e.g., TT or tt.
Heterozygous: Possessing dissimilar alleles, e.g., Tt.
First
Filial Generation (F1): The immediate
offspring of the parent generation is designated as the first filial
generation.
