Gene – A segment of DNA which codes for a given trait
Allele – The form of a gene coded for
Trait – an appearance or result coded for by a gene or genes
Genotype – The actual alleles present
Phenotype – The appearance resulting from the alleles present
Homozygous – both alleles are the same - can produce only one type of gamete
Heterozygous – having different alleles
Dominant – The gene that is expressed in a heterozygote
Recessive – The gene masked in a heterozygote
Codominant – When a mixed expression results from a heterozygote
Incomplete Dominance – When a blended expression results from a
heterozygote
Inheritance
Gregor Mendel
Austrian monk born in 1822
Worked with garden peas
Pea flowers have male parts which produce pollen and female parts which
produce eggs
Mendel had some true-breeding lines of peas
True breeding plants produce offspring looking like themselves when allowed
to self-pollinate
Mendel had true-breeding tall, short, green seeding, and yellow seeding plants
Mendel forced cross pollination amongst his peas to do his research
Some terminology:
P generation = parental generation = what you start with
F1 generation = first filial generation = crosses from parent stock
F2 generation = 2nd filial generation = crosses from F1
When Mendel crossed plants of different characteristics, all of the F1 plants
looked like only one of the parents. For example when he crossed green seeded
plants with yellow seeded plants, all of the offspring had yellow seeds.
From this Mendel concluded 2 things:
1. Biological inheritance is passed from 1 generation to the next.
Today we call the chemical factors controlling inheritance genes. Genes
code for traits. The different possible forms of a gene are called alleles.
For example, yellow-seeded and green-seeded are 2 alleles of the gene
for seed color
2. The principal of dominance: Some alleles are dominant and some are
recessive. Dominant alleles mask the expression of recessive alleles
when both are present. For example: If a plant has the alleles for both
yellow and green seeds, all of its seeds will be yellow
Mendel carried his research further
He crossed F1 plants with each other and produced plants in the F2
generation that had both characteristics. From this Mendel concluded that
alleles are segregated when gametes are formed
Parent generation
YY (yellow seeded)
yy (green seeded)
F1:
Y
Y
y
Yy
Yy
y
Yy
Yy
All of the F1 plants were heterozygous (had 2 different alleles)
In fact, all were Yy. The dominant Y (yellow seed) allele is expressed and
all plants had yellow seeds.
F2:
Y
y
Y
YY
Yy
y
Yy
yy
The F2 plants were either YY (yellow seeded), Yy (yellow seeded), and yy
(green seeded)
About 3/4ths of the F2 plants were yellow seeded
From this, Mendel inferred the basics of all reproduction
Genes are controlled by factors coming from parents
There are 2 alleles inherited, 1 from mom and 1 from dad
Some alleles are dominant and some are recessive