Introduction to genetics

INTRODUCTION TO GENETICS

The Work of Gregor Mendel
Inheritance is represented by our genes. Genetics is the scientific study of heredity.

Gregor Mendel’s Peas
The work of Gregor Mendel, an Austrian monk with pea plants laid the foundations of the science of
genetics. Mendel carried out his work with ordinary garden peas. Each flower contains pollen and egg
cells which join during asexual reproduction process known as fertilization. Fertilization produces a new
cell, which develops into a tiny embryo encased within a seed. The seeds that are produced by self
pollination inherit all of their characteristics from the single plant that bore them.
Mendel worked with several types of pea plants which were true-breeding, meaning that if they were
allowed to self-pollinate,they would produce offspring identical to themselves.
Mendel produced seeds by joining male and female cells from different types of plants (cross-polli-
nation) This made it possible for Mendel to cross-breed plants with different characteristics and then
study the results.

Genes and Dominance
Mendel studied seven different pea plant traits. A trait is a specific characteristic, suchas seed color
or plant height that varies from one individual to another. Each of the seven traits had two contrasting
characters. Mendel crossed plants with each of the seven contrasting characters and studied their
offspring. The offspring of crosses between parents with different traits are called hybrids.
When Mendel crossed plants with contrasting characters for the same trait, theresulting offspring
had only one of the characters. From these experiments, Mendel concluded that some alleles (different
forms of a gene) are dominant and others are recessive.
From his experiments, Mendel drew two conclusions or principles:
First conclusion—biological inheritance is determined by factors (genes) that are passed from one generation to the next
Second conclusion(principle of dominance)—states that some alleles are dominant and others are recessive.
An organism with a dominant allele for a particular form of a trait will always exhibit that form of
the trait. An organism with a recessive allele for a particular form of a trait will exhibit that form only
when the dominant allele for the trait is not present.

Segregation
When Mendel allowed the F1plants to reproduce by self-pollination, the traits controlled by recessive alleles reappeared in about one fourth of the F2 plants in each cross.
In trying to explain the F1 cross, Mendel assumed that a dominant allele had masked the corresponding recessive allele in the F1 generation. However, the trait controlled by the recessive allele showed up in some of the F2 plants. During gameteformation (formation of sex cells), alleles segregate (separate) from each other so that each gamete carries only a single copy of each gene. The alleles are paired up again when gametes fuse during fertilization.

Probability and Punnett Squares
When Mendel performed a cross with pea plants, he carefully categorized and counted the many offspring. Mendel realized that the principles ofprobability could be used to explain the results of genetic crosses.
Genetics and Probability
The likelihood that a particular event will occur is called probability. The way in which alleles segregate is completely random, like a coin flip.

Punnett Squares
The gene combinations that might result from a genetic cross can be determined by drawing a diagram known as a Punnett square. The typesof gametes produced by each parent are shown along the top and left sides of the square. The possible gene combinations for the offspring appear in the boxes that make up the square.
Organisms that have two identical alleles for a particular trait are said to be homozygous Organisms that have two different alleles for the same trait are heterozygous. Homozygous organisms are true-breeding...