POLA-POLA HEREDITAS part 1 (Istilah2 hereditas, Hukum I Mendel, Hukum II Mendel)| Biologi XII SMA

Heredity and Mendelian Laws

The video introduces heredity—the passing of traits from parents to offspring—and explains key genetics terms before demonstrating Mendel’s First and Second Laws using pea plants.

Key Terms

• Parental (P): The parent organisms in a cross. The lesson refers to P1 and P2 for parents involved in successive crosses.
• Filial (F): The offspring generation. F1 is the first generation of offspring, and F2 is the next.
• Gene: A factor that influences a trait. A gene may have dominant or recessive forms.
• Alleles: Alternative forms of a gene, typically represented by the same letter in uppercase and lowercase—for example, A and a.
• Dominant allele: Expressed when present; represented by a capital letter.
• Recessive allele: Typically expressed only when no dominant allele is present; represented by a lowercase letter.
• Genotype: An individual’s genetic makeup. Examples include:
  • Homozygous dominant: AA
  • Homozygous recessive: aa
  • Heterozygous: Aa
• Phenotype: An observable trait, such as flower color or seed shape.
• Hybrid: Offspring of parents with differing traits. A monohybrid cross considers one trait; a dihybrid cross considers two.
• Genetics: The branch of biology that studies inheritance.

The video notes that Gregor Mendel used pea plants because they have clear, observable varieties, including differences in flower color, seed color, and seed shape. Mendel is described as the “father of genetics.”

Mendel’s First Law: Segregation

The Law of Segregation states that the two alleles for a trait separate when gametes form, so each gamete receives one allele. The video demonstrates this with a monohybrid cross.

Example: Purple and White Flowers

• Purple is treated as dominant, and white as recessive.
• Cross homozygous purple with homozygous white: PP × pp.
• The parents produce gametes P and p, respectively.
• All F1 offspring are Pp and have purple flowers.
• Cross the F1 offspring: Pp × Pp.
• The resulting F2 ratios are:
  • Genotype: 1 PP : 2 Pp : 1 pp
  • Phenotype: 3 purple : 1 white

Mendel’s Second Law: Independent Assortment

The Law of Independent Assortment states that alleles for different traits are assorted into gametes independently in the example discussed. The video illustrates this with a dihybrid cross involving seed shape and seed color.

Example: Round Yellow and Wrinkled Green Seeds

• Round and yellow are treated as dominant; wrinkled and green as recessive.
• Cross homozygous round-yellow with homozygous wrinkled-green: RRYY × rryy.
• All F1 offspring are RrYy, with round yellow seeds.
• Each F1 parent can produce four kinds of gametes: RY, Ry, rY, ry.
• Crossing the F1 plants (RrYy × RrYy) gives the F2 phenotype ratio:
  • 9 round yellow
  • 3 round green
  • 3 wrinkled yellow
  • 1 wrinkled green

This is the classic 9:3:3:1 ratio.

The video’s spoken notation for the dihybrid example is garbled in places; the symbols above express the described crosses using standard notation.

Speakers and Source

• Presenter/biology teacher: One presenter explains the lesson; the subtitles do not clearly identify the person by name.
• Source: “POLA-POLA HERITAS part 1 (Istilah2 hereditas, Hukum I Mendel, Hukum II Mendel)” by Sri Mulyani Biology.