Mendelian GeneticsMohd Aminudin Bin Mustapha

Centre for Pre-University StudiesUniversiti Malaysia Sarawak

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Learning Objectives

• Define the basic principles of heredity

• Explain law of segregations and law of independent assortments

Heredity

Heredity, the transmission of genetic information from parent to offspring, generally follows predictable pattern in organisms.

One possible explanation of heredity is a “blending” hypothesis.

An alternative to the blending model is the “particulate” hypothesis of inheritance: the gene idea

Heredity

Allele is alternative form of a genetic locus: a single allele for each locus is inherited separately from each parent.

Locus is the specific location of a particular gene on a chromosome.

Gene is the fundamental physical and functional unit of heredity.

Heredity

Phenotype – Observable trait or traits of an individual

Genotype – The genetic constitution of an organism.

Heterozygous - Two alleles for a trait are different (Aa)

Homozygous - Two alleles for a trait are the same (AA or aa)

Heredity

Dominant allele- a gene is said to be dominant if it expresses its phenotype even in the presence of a recessive gene.

Recessive allele –In heterozygous, an allele whose expression is fully or partially masked by expression of its partner; fully expressed only in the homozygous recessive condition.

Law of Segregation

Different variations of genes accounts for variations in inherited characters.

An organism inherits two alleles for every characters from each parents

If the two alleles differ, the dominant allele will be fully expressed in its phenotype, while the recessive does not.

The two alleles of each character segregate during gamete production

Law of Segregation

To test the hypothesis, Mendel predicted the outcome of a breeding experiment.

He crossed heterozygous round peas (Rr) with homozygous wrinkled peas (rr) (test cross), then predicted the result would be 1 round : 1 wrinkled.

Mendel crossed pea plants that differed in six other qualitative traits and the results supported his hypothesis.

Law of Independent Assortment

During meiosis, members of a pair of genes on homologous chromosomes get distributed into gametes independently of other gene pairs.

The genes for different traits are inherited independently of each other.

Many gene pairs tend to sort into gametes independently of one another.

Mendel realized that the results could only be explained if the genes for seed shape and color were inherited independently.

The second law states that the alleles for one trait may be distributed to the gametes independently of the alleles for another traits.

When paired chromosomes line up during metaphase I, the orientation of the homologous chromosome of one pair does not influence the orientation of other pairs.

Law of Independent Assortment