BIOL 112 Lecture Notes - Lecture 3: Meiosis, Zygote, Zygosity

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Gregor Mendel (1822-1884)
Children resemble their parents
Siblings are not identical
For a given character (color, stature, facial features etc.) offspring share traits (light or dark color; short or tall stature) with
their parents.
Siblings will not have identical traits for a given character.
In the language of genetics
"Blended inheritance"
Infinite number of traits fro a given character that fall along a continuous spectrum (i.e. height, skin color).
Continuous Variation:
There are only two or a few traits for a given character. (fur color in mice)
Mendel tested this by crossing peas, which resulted in may progeny, so that he could do statistics. And starting with a
trait that bred true; as long as he bred a stain with itself, all the progeny had the same traits for the characters
Discrete Variation:
Types of Variation
A round pea plant that always gave round peas when fertilizing itself.
A wrinkled pea plant that likewhise always gave wrinkled peas.
Mendel used true-breeding strains
Gene = unit of heredity (pea shape)
Alleles = different gene flavors (round, wrinkled traits)
Homozygous = having two of the same allele (RR, rr, SS, ss)
Let S=round allele (dominant) and s=wrinkled allele (recessive)
Heterozygous = having two different alleles (Rr, Ss)
Segregation of the alleles into each gamete
Law of Segregation
The probability of two independent events is the product of the probability of each event.
OR, we can draw the Punnett square...
Probability Calculation (probability theory 101):
Genotype = the set of alleles an organism has
Phenotype = the set of traits an organism displays
Genotype determines phenotype, but because of dominance, phenotype may not uniquely determine genotype.
Test Cross
The Number of Heads Expected
When Flipping 4 Coins
1/16 1/4 1/4 1/16
heads 2
heads 3
heads 4
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