Mendelian Genetics Notes

Dominant character: In a cross between two pure parents with contrasting characters, the character that appears in the F₁ generation is the dominant character. It is expressed even when only one copy of its allele is present (TT and Tt). Example: tallness (T) in pea plants.

Recessive character: The character that stays hidden (suppressed) in the F₁ generation is the recessive character. It is expressed only when two copies of its allele are present (tt). Example: dwarfness (t) in pea plants.

Gene → DNA molecule → Chromosome → Nucleus → Cell

In blood group AB the person has the alleles Iᴬ and Iᴮ. Iᴬ makes antigen A and Iᴮ makes antigen B. Neither allele hides the other and neither blends with it, so both are fully expressed at the same time and both antigens are found in the blood. Both alleles are expressed equally in the F₁ hybrid, which is the meaning of co-dominance. Cross: IᴬIᴬ (group A) × IᴮIᴮ (group B) gives IᴬIᴮ (group AB).

Law of dominance: When two pure organisms having a pair of contrasting characters are crossed, only one character of the pair appears in the F₁ generation while the other remains hidden. The character that appears is dominant and the hidden one is recessive.

Example: A pure tall pea plant (TT) crossed with a pure dwarf plant (tt) gives all tall plants (Tt) in F₁. So tallness is dominant and dwarfness is recessive.

Mendel chose the garden pea (Pisum sativum) because:

The seven pairs of contrasting characters he studied:

The allele for round leaves is dominant.

Explanation: A heterozygous plant has one allele for round and one for oval leaves. Only the round shape appears, so the round allele hides the effect of the oval allele. This is Mendel’s law of dominance: the allele that is expressed in the heterozygote is dominant and the hidden one is recessive. The oval allele will show only in plants with two copies of it.

Law of independent assortment: When two or more pairs of contrasting alleles are inherited together, the inheritance of one pair occurs independently of the other. The alleles of different genes assort into gametes randomly and independently.

Dihybrid cross (height and flower colour):

Phenotypic ratio = 9 : 3 : 3 : 1. Genotypic ratio = 1 : 2 : 2 : 4 : 1 : 2 : 1 : 2 : 1.

Conclusion: The new combinations tall white and dwarf red, which were not in the parents, show that the gene for height is not linked with the gene for flower colour. They assort independently.

The Punnett square below has 16 boxes. The ttrr genotype is found in only one box (marked in red), when a tr egg meets a tr pollen grain.

Chance = 1/4 × 1/4 = 1/16 (6.25%).

The full F₂ ratio is 9 tall red : 3 tall white : 3 dwarf red : 1 dwarf white.

The man got b from his mother, so he got B from his father. The father has at least one B. He may be BB or Bb (both brown-eyed).

Man Bb (gametes B and b) × woman bb (gametes b only). Offspring: 50% Bb (brown eyes) and 50% bb (blue eyes), a 1 : 1 ratio.

Phenotypic ratio = 3 tall : 1 dwarf. Genotypic ratio = 1 : 2 : 1.

Explanation of the result: Total plants = 302 + 99 = 401. For a 3 : 1 ratio the expected numbers are 3/4 × 401 ≈ 301 tall and 1/4 × 401 ≈ 100 dwarf. The observed 302 tall : 99 dwarf is very close to 3 : 1, so the result agrees with Mendel’s law of segregation.

In this standard cross (R = round, Y = yellow), the dominant traits are round seed (R) and yellow seed (Y). The recessive traits are wrinkled seed (r) and green seed (y). A capital letter stands for the dominant allele and a small letter for the recessive allele.

The table has 16 boxes and four types of gametes on each side, so the parents of this cross were F₁ plants heterozygous for both genes: RrYy × RrYy. These F₁ plants came from RRYY × rryy.

Counting the 16 boxes gives 9 round yellow (R_Y_), 3 round green (R_yy), 3 wrinkled yellow (rrY_) and 1 wrinkled green (rryy). The F₂ phenotypic ratio is 9 : 3 : 3 : 1.