BME 80H Lecture Notes - Lecture 6: Dihybrid Cross, Mendelian Inheritance, Gamete
11 Jun 2018
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6th lecture Mendelian Genetics II (chapter 3, p. 50-54/48-52)
I. The Principle of Segregation: Mendel's First Law
A. The alleles in a gene pair segregate (separate) during gamete
(reproductive cell) formation with only one member of the pair going into
each gamete (egg or sperm/pollen). When fertilization occurs, the gene
pair is restored (figure 3.5/3.4).
B. Predicting outcomes of crosses using Punnett Squares (figure 3.6/3.5)
-Ss x Ss so the idea is that each has two S’s which separate from each
other during gamete formation
-got a 3:1 phenotypic ratio
The expected genotypic ratio would be 1:2:1 (1SS:2sS:1ss)
(1 true-breeding Smooth: 2 Smooth carrying the wrinkled allele: 1
true breeding wrinkled). The expected phenotypic ratio is 3:1
(Smooth:wrinkled). This is very close to what Mendel saw with his
experiments following pea shape (2.96:1) as well as other
monohybrid crosses (figure 3.4/3.3)
II. Mendel's Law of Independent Assortment
The segregation of alleles for one character is not influenced by the
segregation of alleles for another character.
A. Mendel's Dihybrid cross: Following the inheritance of two different
characteristics.
1. Characteristics examined
seed shape
S = smooth and s = wrinkled
seed color
Y = yellow y = green
-dihybrid was two heterozygous ones combined
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Document Summary
When fertilization occurs, the gene pair is restored (figure 3. 5/3. 4): predicting outcomes of crosses using punnett squares (figure 3. 6/3. 5) Ss x ss so the idea is that each has two s"s which separate from each other during gamete formation. The expected genotypic ratio would be 1:2:1 (1ss:2ss:1ss) (1 true-breeding smooth: 2 smooth carrying the wrinkled allele: 1 true breeding wrinkled). This is very close to what mendel saw with his experiments following pea shape (2. 96:1) as well as other monohybrid crosses (figure 3. 4/3. 3) The segregation of alleles for one character is not influenced by the segregation of alleles for another character: mendel"s dihybrid cross: following the inheritance of two different characteristics, characteristics examined seed shape. S = smooth and s = wrinkled seed color. P: the cross (figure 3. 8/3. 6) pure-breeding smooth, yellow pea plant (ssyy) is crossed to a pure breeding wrinkled, green plant (ssyy) all progeny seeds are smooth yellow (ssyy) dihybrids.
