Mendel's First Law

1. What is mendelism?
   • It is the attempt to use breeding analysis to follow one or more hereditary traits across one or more generations.
   • It assumes that hereditary traits are determined by hereditary units. Today we call these units the genes.
   • It assumes that genes mutate or change on occasion and the changed form of the gene alters its expression.

2. How does mendelism work?
   • We will study monogenic traits (traits involving a single gene and its mutant varieties.
   • The normal gene and the mutant form of it that arises by mutation are called alleles.
   • We are the products of fertilization by a sperm (from our father) and an egg (from our mother). We thus have a biparental inheritance. Hence we have two genes (or two alleles) for each possible trait our parents give us. This double state we call the diploid condition.
   • Genes can be represented by symbols. Mendel used yellow peas and green peas in one of his crosses. The offspring of a cross between these two strains produced only yellow peas. Thus yellow is the dominant trait and green is the recessive trait . We can represent the allele for the recessive trait, green , with the letter g. The dominant yellow trait is then caused by the allele G.

3. Where does the recessive go when the offspring are yellow?
   • It remains silent or hidden. The offspring that show yellow peas are hybrid or heterozygous. We represent their status as Gg. If the original strains of yellow and green peas that Mendel used were from true breeding strains (always yellow or always green, respectively), we could represent these purebred stocks as GG for yellow and gg for green. In this case the GG is homozygous for the yellow allele and gg is homozygous for the green allele. The hybrid offspring Gg, expresses the dominant allele for yellow but not the recessive allele for green. The reason for this is usually that the recessive trait is a loss of the trait that the dominant allele expresses. Yellow peas have a pigment that is yellow. Green peas lack that pigment and thus the chlorophyll, which is green, shows through. The hybrid expresses yellow because it masks or hides the green chlorophyll. For most hybrid monogenic traits the one dose of the dominant allele is identical in expression to the two doses of the dominant allele when it is in homozygous form.

4. What happens to the hidden or recessive allele in the hybrid?
   • The recessive allele is intact but not ex-pressed. We know this because it can be extracted and made to express when it becomes a gamete (sex cell) and encounters another recessive like itself. Mendel did this by crossing two of the hybrid yellow pea offspring to one another. They produced 75% yellow pea plants and 25% green pea plants.

5. How would such a cross be represented to show the three generations involved?
   • The original purebred strain would be represented as:
     P1 GG x gg

   • Note that we use P1 to represent the parent generation or first generation. We also use the homozygous form or dual state of heredity of the parents, one parent being homozygous for yellow peas[the one we designate as GG] and the other parent being homozygous for green peas [the one we designate as gg]. We call these the genotypes of the mating individuals.
   • The offspring of that cross we represent as: Gg
   • We call the offspring of that cross the F1 generation [the F is Latin for filial or offspring] and thus we could also represent them as:
     F1 Gg

   • When we cross the two hybrids together we establish a second generation cross:
     F1 Gg x F1 Gg

   • From this F1 x F1 cross we get the F2 generation or second generation of offspring [the equivalent of grandchildren for the original parents who were from the purebred lines]: F2 25% GG 50% Gg 25% gg

   • This F2 generation is not easy for us to see. This is how it comes about: The F1 Gg produces two types of gametes (in plants this would be pollen or ovules) or 0.5 G and 0.5 g. This true for each of the two hybrids who are mating with one another. If we take one of the plants and consider its two types of pollen, 0.5 G will encounter either of two types of ovules from the other plant, 0.5 G and 0.5 g. This gives us .25 GG and .25 Gg. Similarly for the other half of the pollen donator, we would expect those 0.5 g to encounter 0.5 G or 0.5 g from the other parent. This gives us 0.25 Gg and 0.25 gg. If we add it all up, one-fourth are homozygous for the yellow allele or 0.25GG; one half are heterozygous Gg [0.25 Gg plus 0,25Gg]; and one quarter are homozygous for green pea plants or 0.25 gg.

6. Isn't there an easier way to figure this out?
   • Yes, you can use a grid, called a Punnett square, which gives you a visual way of looking at the gametes from one parent on the vertical side of a square and the gametes from the other parent will be on the horizontal side. The encounters of the pollen and ovules are then read off inside the grid:

     	0.5 G	0.5 g
     0.5 G	0.25 GG	0.25 Gg
     0.5 g	0.25 Gg	0.25 gg

7. Why is this called Mendel's law of segregation?
   • At the time that the parent [in this case the F1 Gg] produces gametes it undergoes reduces its diploid state to the haploid state among its reproductive cells. Thus Gg becomes 0.5 G and 0.5 g.
   • note that the F2 generation from the F1 x F1 cross of two hybrids gives a genotypic ratio of 0.25 GG to 0.50 Gg to 0.25 gg.
   • If we convert those genotypes to what they express (what we can call their phenotypes), we use a pair of parentheses and insert the allelic letter of the trait expressed, in this case G for yellow and g for green):
     
     0.25 (G) from the homozygous GG; 0.50 (G) from the sum of the heterozygous Gg F2 progeny [0.25 Gg plus 0.25 Gg equals 0.50 Gg]; and 0.25 (g) from the homozygous gg. Since the yellow peas color is the same for the homozygous GG and the heterozygous Gg, we have a total of .75 (G), which is derived from the 0.25 GG which expresses (G) plus 0.50 Gg which expresses (G).

   • This three to one ratio, 0.75 (G) to 0.25 (g), is called the phenotypic ratio.
   • Note that inside a pair of parentheses only one of the two alleles is used instead of two, namely, the one that is expressed. Thus GG = (G); Gg = (G); and gg = (g).

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