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Basic Genetics
The Father of Genetics
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Gregor Mendel – Austrian monk
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Carried out the first studies of heredity
Successfully predicted how traits would be
passed
Studied the garden pea plant
Basic terminology
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Heredity – passing of characteristics from
parents to offspring
Gametes –male and female sex cells
Fertilization – process where male and
female sex cells unite
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Produces a zygote (fertilized egg)
Cross – pollination
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Mendel used to fertilize
pea plants
He would take pollen
from one plant and dust
on another
Rule of unit factors
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Mendel concluded that two factors control
each trait
Each factor is called an allele
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An allele is the different forms of a gene
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Examples: blue, green, brown are different alleles for the
eye color gene
Rule of dominance
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One allele is usually seen more often than
the other
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Dominant alleles are observed more often
Recessive alleles disappear
To write alleles we abbreviate:
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Capitol letter for the dominant
Lower case letter for the recessive
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Always use the first letter of the dominant allele for both
the dominant and the recessive alleles
Genotype vs. Phenotype
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Phenotype: the physical appearance
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Can be determined by looking at the organism
Tall, short, blue eyes, brown hair
Genotype: the allele combination
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Cannot be determined by looking at the organism
Can be homozygous (TT, tt, AA, aa)
Can be heterozygous (Tt, Aa)
Law of segregation
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Every individual has 2 alleles of each gene
and when gametes are produced, each
gamete receives one of these alleles
During fertilization, gametes randomly pair to
produce 4 combinations
Punnett squares
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Shorthand way of finding possible genotypes
of the offspring
Monohybrid cross – 4 boxes in square
Dihybrid cross – 16 boxes
Punnett squares
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Parental gametes go along the top and down
the side
Inside the boxes, the possible combinations
of the offspring
Dihybrid crosses
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Involves two traits
Follows the law of independent assortment
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Genes for different traits are inherited
independently of each other
Dihybrid Punnett squares
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Must use all possible combinations of alleles
when setting up the Punnett square
AATt x Aatt
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Must use the foil method to place gametes along
the top and side
Combine as usual for the combinations in the
offspring
Genotypic and phenotypic ratios
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Once Punnett square is complete, you must
calculate the genotypic ratios and the
phenotypic ratios