Genetics is the study of hereditary information, which is organized
into genes made of DNA (except in a few RNA viruses).
Transmission of hereditary information can be studied my making crosses.
Gregor Mendel discovered that genes are inherited as discrete particles that
do not blend. The genes usually exist in pairs. Genes in a pair separate
(or "segregate") during meiosis when gametes form.
- Gene: a hereditary determinant of a trait.
More precisely, a continuous or interrupted sequence of DNA that determines
one polypeptide.
- Allele: one of several possible variants of a gene
- Genotype: all of the hereditary traits of an organism, as revealed
by breeding experiments.
- Phenotype: all of the visible traits of an organism that can be revealed
by examining it closely (including microscopically, biochemically, behaviorally,
etc.) without any breeding.
- Homozygous: genotype with two identical alleles of the same gene
- Heterozygous: genotype with two unlike alleles of a particular gene
- Dominant: an allele expressed in the phenotype even when only one such
allele is present
- Recessive: an allele expressed in the phenotype only when two such
alleles are present. NOTE: The resulting phenotypes may be
called dominant and recessive as well.
- Blending theory (before Mendel): Early observers examined many traits
at once and thus noted resemblances to both parents. They thought that
inheritance was like the mixing of fluids; hence, expressions like
"pure blood" and "half blood."
- Particulate theory (discovered by Mendel in 1865; ignored until
rediscovered in 1900):
- Genes come in pairs (in nearly all animals and plants; haploid organisms
such as yeast and bacteria may differ).
- Genes come in differing types (dominant and recessive alleles).
- Genes do not blend; they remain discrete.
- Phenotypes are usually not intermediate. When two alleles occur
together, only the dominant one is expressed ("law of dominance").
- Important precautions taken by Mendel:
- He used plants of known parentage, derived from pure lines.
- He examined only one trait at a time.
- He examined several generations, but examined each separately.
- He counted large numbers of offspring of different types and
analyzed the results mathematically.
- Mendel's monohybrid (single-gene) crosses:
Mendel chose parents from pure lines, so they were always homozygous.
He crossed parents with opposite traits (dominant x recessive).
First generation offspring (F1) all showed dominant trait.
Second generation offspring (F2) showed 3:1 ratio of dominant
to recessive phenotypes. Here is Mendel's explanation:
- Use capital A for dominant allele, small a for recessive allele.
Parents can thus be represented as AA and aa.
- F1 are all heterozygous Aa; one gene comes from each parent.
- A and a will separate in the F1 gametes
(law of segregation).
- Separate gametes A and a recombine in all posible ways, giving
4 possibilities; 3/4 of these (AA, Aa, and aA) have at
least one A and will show the dominant phenotype, while 1/4
will be aa (recessive).
- Other single-gene crosses:
- First, determine the genotypes of the parents.
- Each parent produces 1 or 2 types of gametes; combine them to determine
the F1.
- Cross F1 x F1 if you want to get F2.
- Determine offspring of each cross by a Punnett square:
- List all possible female gametes across the top, and all possible male
gametes down the left-hand column. (The number of possibilities will
be either 1, 2, 4, or 8, etc., not always the same for both sexes.)
- Fill in each square with the combined genotypes of male and female gametes.
- For traits showing dominance, any genotype with a dominant allele
shows the dominant phenotype; a genotype with all recessive alleles
shows the recessive phenotype.
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