Heredity is the transfer of characteristics from parent to offspring through their genes. A gene is the unit of heredity made of deoxyribonucleic acid (DNA). There are many genes on each molecule of DNA.
* Relationship between Genes, Chromosomes & DNA *
A molecule of DNA is a single chromatin strand which, when condensed, is called a chromosome. Eukaryotic DNA is organized in chromosomes that exist within the nucleus of each cell. Genes have specific places on chromosomes.
* Diploid Organisms *
Organisms that reproduce sexually are diploid. This means that they received one set of chromosomes from the male parent (the genetic material that was carried in the sperm) and a corresponding set of chromosomes from the female parent (the genetic material that was carried in the egg or ovum).
Sperm and eggs are gametes. They are haploid, each containing half the number of chromosomes present in the diploid parent. When these haploid gametes fuse at fertilization, they create a diploid zygote with all the genetic material necessary to grow into an embryo, fetus and ultimately adult.
* Genotypes & Phenotypes *
The genotype is an organism’s genetic makeup. For example, all of your genes are what comprise your genotype.
The expression of your genes is called your phenotype, your traits that result when the instructions in your genes are carried out.
* Alleles *
Alleles are variations of a gene. A diploid organism gets one set of alleles (genes) from one parent and the other set of alleles from the other parent.
Some of the traits that we have are based on simple inheritance where one version of a gene (dominant allele) masks the expression of the other version of that gene (recessive allele).
In writing, we represented dominant and recessive alleles with letters that distinguish the different types of alleles. A capital letter is used to represent the dominant allele and a lower-case letter is used to represent the recessive allele (example: dominant allele = P; recessive allele = p)
* Homozygous and Heterozygous *
When an organism has a pair of identical alleles for a character, they are said to be homozygous for that characteristic (PP, pp) When an organism has two different alleles for a gene (Pp) they are said to be heterozygous for that characteristic.
* Dominant or Recessive Phenotypes *
In cases if simple inheritance, where a characteristic is controlled by one pair of alleles and one allele is dominant over the other, this is called complete dominance. For that characteristic, an organism will have one of two phenotypes.
When an organism has the recessive phenotype, this means that both recessive alleles must be present (bb). With recessive traits, we clearly know what the genotype is if the phenotype is recessive.
An organism showing the dominant phenotype can be homozygous or heterozygous for dominant allele (PP, Pp). It is more difficult to know genotype when dominant allele is involved.
* ‘Bent Little Finger’ Example *
Lay both of your hands flat on a table and relax your muscles. Take a look at your pinky fingers and note whether they noticeably bend towards your ring finger or whether they are straight. Most of us have reasonably straight little fingers. If your little finger is straight, you have the recessive phenotype for the trait of ‘bent little finger.’ Your genotype for this characteristic would be represented with ‘pp.’
It is a dominant allele causes the last joint of the little finger to dramatically bend inward toward the 4th finger. If you have bent little fingers you probably do not know your genotype, as it could be either PP or Pp, both of which would show the bent finger phenotype.
* Sources *
Thorpe ed. (2007) Biology 120 Lab Manual. Grand Valley State University.
Doezema, B. (2006) Biology 101 Lab Manual. Grand Rapids Community College.