Genetics and Plant Breeding

Mendel formulated the law of independent assortment based on his dihybrid cross experiments, where he studied two different traits simultaneously. These experiments showed that alleles for different traits assort independently during gamete formation.

Genes are specific segments of DNA that code for proteins and determine heritable traits. Each gene contains instructions for a particular characteristic, such as eye color or blood type.

The classic dihybrid phenotypic ratio is 9:3:3:1, where 9 show both dominant traits, 3 show one dominant trait, 3 show the other dominant trait, and 1 shows both recessive traits. This ratio appears when two heterozygous parent plants (TtRr x TtRr) cross.

Homozygous dominant means having two dominant alleles for each gene. TTRR shows dominant alleles (TT) for one trait and dominant alleles (RR) for another trait. TtRr is heterozygous, Ttrr is heterozygous-homozygous recessive, and ttRr is homozygous recessive-heterozygous.

When crossing a homozygous dominant plant (TTRR) with a homozygous recessive plant (ttrr), all F1 offspring will be heterozygous (TtRr) and show only dominant traits: tall plants with round seeds. This demonstrates Mendel's Law of Dominance.

Fathers have one X and one Y chromosome (XY). When they pass an X chromosome to their child, the child will be female (XX) because the mother always contributes an X chromosome. If the father passes his Y chromosome, the child will be male (XY).

Members of species interbreed to produce fertile offspring. They are almost identical in their anatomy, physiology and behaviour.

The DNA fingerprint of a male child has 50%-50% similarity to both parents.

Polymorphism refers to a slight difference in the base-pair sequence of the common genes between individuals.

Since both parents are homozygous recessive (aa), all of their children will be as well. As a result, none of them will be free of this recessive trait in their phenotypes.

There is a 50% chance that the children will be heterozygous (Aa) and a 25% chance that they will be homozygous dominant (AA). Children with either of these genotypes will have this trait expressed in their phenotypes.

The frequency of heterozygotes will grow while the frequency of homozygotes will correspondingly shrink. This is because individuals who have the same genotype for a trait would not mate. As a result, heterozygotes, or crosses, are most commonly born.

This is a definition of his principle of independent assortment. In other words, the genes that determine a trait assort independently of the genes for other traits. As a result, new combinations of genes, present in neither parents, are possible.

An allele is one of two or more alternate forms of a gene. If an individual is homozygous (YY or GG) for a trait, it has inherited the same allele from both parents. If it is heterozygous (YG), it has inherited different alleles for the trait.

The reason is that mating is between individuals with the same genotypes -- AA X AA, aa X aa, or Aa X Aa. The first 2 mating combinations only produce homozygous offsprings, while 50% of the offsprings of the latter are homozygous as well.