Heredity, variation and Mendel's laws

Heredity and variation

A kitten looks like a small cat, and a seedling grows the same kind of leaves as its parent plant. The passing of similar features from parents to offspring, generation after generation, is heredity. Heredity is controlled by genes. Yet puppies of one mother can have different coat colours, and roses in one garden can differ in colour. Such differences between members of the same kind are variations. They come from different combinations of genes, and sometimes from the environment. The science that studies heredity and variation is genetics.

Activity: free and attached ear lobes in your family

1. 1
   Look at the lower end of your own ear lobe in a mirror. It may hang loose below its joint with the head (free ear lobe), or be joined to the side of the head (attached ear lobe).
2. 2
   Check the ear lobes of your parents, brothers and sisters, and of a few friends. Write each person's type in a table.
3. 3
   Note which parent has the same type as you. That is the parent from whom you may have inherited it.
4. 4
   Repeat for two more inherited features: rolling the tongue into a tube, and bending the tip of the thumb backwards (the hitch-hiker's thumb).
5. 5★
   Conclusion: these features pass from parents to children (heredity), and the differences you recorded between people are variations.

★ key step

Mendel and his two laws

• Gregor Johann Mendel (1822 to 1884) was an Austrian monk. He grew pea plants for years, counted each kind of offspring and worked out the first laws of inheritance. He is called the father (founder) of genetics.
• His work was recognised only after his death, when Correns, Tschermak and de Vries reached the same results on their own.
• First law: every feature, such as plant height or flower colour, is controlled by a pair of factors. When gametes (sex cells) form, the two factors separate (segregate), one into each gamete. Your lesson calls this first law universal; scientists later found exceptions to Mendel's other laws.
• Second law: of the two factors, the dominant one shows even when the other is present. The recessive one shows only when the dominant one is absent. Tallness in peas is dominant; dwarfness is recessive.
• Mendel also gave the law of parental equivalence and the law of independent assortment; your lesson only names them.
• In 1902, Sutton, studying grasshopper chromosomes, showed that Mendel's factors lie on chromosomes. Later the word gene replaced the word factor.

One cross that shows both laws: tall × dwarf pea plants

1. 1
   Parents: pure tall (TT) × pure dwarf (tt). T is the factor for tallness, t for dwarfness.
2. 2
   Gametes: the two factors of each pair separate, so the tall parent gives only T and the dwarf parent only t.

   This is the first law at work.
3. 3★
   F1 generation (first generation of offspring): all plants are Tt, and all are tall. The dwarf factor is hidden, not lost.

   This is the second law: T is dominant.
4. 4
   F1 plants self-pollinated: Tt × Tt. Each plant makes gametes T and t in equal numbers.
5. 5★
   F2 generation: TT, Tt, Tt, tt, that is 3 tall : 1 dwarf in what we see, and 1 TT : 2 Tt : 1 tt in factor make-up.
6. 6
   Conclusion: dwarf plants come back in F2, so the F1 plants carried both factors, but only the dominant one showed.

★ key step

F2 checkerboard (Tt × Tt)

How to draw the cross

• Write Parents: TT (tall) × tt (dwarf).
• Below, write the gametes in small circles: T and t.
• Draw an arrow down to F1: Tt (all tall).
• Write F1 × F1 (selfing): Tt × Tt.
• Draw a 2 × 2 checkerboard with the gametes T and t along the top and down the side, and fill in TT, Tt, Tt, tt.
• Under it, write both ratios: 3 tall : 1 dwarf and 1 TT : 2 Tt : 1 tt.