MONOHYBRID CROSS ONLY ONE TRAIT Monohybrid Cross It

MONOHYBRID CROSS ONLY ONE TRAIT

Monohybrid Cross • It is the principle of Dominance and Segregation. • A single trait is studied at a time. • For example, –Height of the plant (Tall or Dwarf).

• It is a cross in which only one of the two contrasting characters appeared in the hybrids. • When these hybrids were selffertilized, they produced two types of progeny, each resembling one of the plants in the original crosses.

MENDEL’S KEY PRINCIPLES We summarize Mendel’s analysis of monohybrid crosses by stating two key principles that he discovered: 1. THE PRINCIPLE OF DOMINANCE 2. THE PRINCIPLE OF SEGREGATION

THE PRINCIPLE OF DOMINANCE • In a heterozygote, one allele may conceal the presence of another. • This principle is a statement about genetic function. • Some alleles evidently control the phenotype even when they are present in a single copy.

THE PRINCIPLE OF SEGREGATION • In a heterozygote, two different alleles segregate from each other during the formation of gametes. • This principle is a statement about genetic transmission. • An allele is transmitted faithfully to the next generation, even if it was present with a different allele in a heterozygote.

BIOLOGICAL BASIS FOR SEGREGATION

PRINCIPLE OF SEGREGATION AND MEIOSIS

PRINCIPLE OF SEGREGATION AND MEIOSIS

PUNNETT SQUARE • Punnett square is a diagram to understand Mendel's conclusions. • It starts with genes and alleles. • A Punnett square illustrates how the parents’ alleles might combine in offspring.

Punnett Square

Different Genotypes Can Produce the Same Phenotype

TEST CROSS & MENDELIAN GENETICS • Is there a way to distinguish the genotype of an organism expressing the dominant phenotype? • The answer is YES! • Mendel devised a method called test cross, where an organism of dominant phenotype but unknown genotype is crossed with a homozygous recessive individual.

CASE 1 • If a purple-flowered plant (PP) is testcrossed to a white-flowered plant (pp), all offspring will produce purple flowers (Pp). • All purple-flowered offspring show that the unknown parental genotype is homozygous dominant.

CASE 2 • If a purple-flowered plant (Pp) is testcrossed to a white-flowered plant (pp), half of the offspring will produce purple flowers (Pp) and the other half will produce white flowers (pp). • A 1: 1 ratio of purple: white offspring shows that the unknown parental genotype is heterozygous dominant.

BACKCROSS • Backcrossing is a crossing of a hybrid with one of its parents or an individual genetically similar to its parent, in order to achieve offspring with a genetic identity which is closer to that of the parent. • It is used in horticulture, animal breeding and in production of gene knockout organisms.