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Genetics
The scientific study of heredity
Gregor Mendel is considered to be the father of genetics due to his work with
in the 1800’s.
Pea Plants
Genes are the Units of Heredity
The Laws of Genetics
• Traits or characteristics are passed from parents to offspring via genes.
• The traits of an organism are determined by the combination of genes.
• Organisms inherit two genes, one from each parent.
The Laws of Genetics
• Some genes are dominant and some are recessive.
• Dominant genes mask recessive genes when both are present in an organism.
The Laws of Genetics
• Some genes are codominant (neither dominant or recessive). These genes show incomplete dominance.
Geneticists use letters to represent different genes.
Dominant = capital letter
Recessive = lower case letter
Every organism has 2 forms of each gene, which we call alleles
(uh LEELZ).
(One from each parent)
Punnett Squares
A Punnett Square is used to show all the possible outcomes of a genetic cross and to determine the probability of a particular outcome.
Punnett Squares
Genotype = an organism’s genetic makeup
Phenotype = an organism’s physical appearance
Example of a Punnett Square:
B is the gene for a brown-haired guinea pig (this is a dominant gene)
b is the gene for a white-haired guinea pig (this is a recessive gene)
Problem: If the genotype for a brown haired guinea pig is BB, (remember you have 2 sets of genes, one from each parent) and you cross (mate) this guinea pig with a white haired guinea pig whose genotype is bb, determine the possible genotypes and phenotypes for their offspring.
b
b
B
Bb
Bb
B
Bb
Bb
Parents are homozygous (or homogenous).
Bb
Bb
100% of offspring will be brown-haired.
Bb
Bb
b
b
B
BB
Bb
B Bb
bb
Parents are heterozygous (or heterogenous).
BB
Bb
Bb
bb
Offspring will be 75% brown-haired,
and 25% white-haired.
b
b
b
Bb
Bb
B
bb
bb
What percentage of the offspring will be brown or white?
B
b
B
BB
Bb
B
BB
Bb
What percentage of the offspring will be brown or white?
(fin)
