Mendelian Genetics


Mendel's work: Gregor Mendel accomplished most of his work in the 1860's in the small country of Austria. Working with garden peas, he worked with clear cut characteristics so they could be easily followed. Each of his chosen traits occurred in alternate forms: round and wrinkled seeds, yellow and green color, tall and short, etc. Mendel mated pure (homozygous) individuals, one dominant the other recessive (TT x tt ). He called this the P generation. The offspring he received from this mating was labeled the F1 generation. If the mated 2 F1 individuals an F2 generation was formed. From these matings Mendel discovered several rules concerning how these traits were transferred from generation to generation.

Genetic Vocabulary:

  • Phenotype. The outward appearance of a characteristic. Brown hair, blue eyes, etc.
  • Genotype. The genetic make up of an organisms. Tt, Rr, etc.
  • Allele. Alternate forms of a gene. T and t. each are alleles of each other.
  • Homozygous. The alleles in a pair are identical. TT, HH, etc.
  • Heterozygous. The alleles in a pair are different. Tt, Hh, etc.
  • Test Cross. Mate a homozygous recessive individual with an unknown genotype for that characteristic.
  • Punnett Square. A chart used to solve genetic problems.

Mendel's Laws:

1. Law of Unit Characters: Mendel deduced that there were units in the cell that were responsible for these traits, and that these units came in pairs. Since sexual reproduction was the mode of gene transfer, he figured that each offspring received one unit from each of the two parents.

2. Law of Segregation: During gamete (sex cells) formation gene pairs separate. This is evident in meiosis. When a sperm or egg cell is being produced it under goes a reduction in the number of chromosomes by 1/2. This will allow the normal number of chromosomes to occur in the offspring at fertilization.

3. Law of Independent Assortment: Mendel deduced the first 2 laws by using a monohybrid cross. This is a genetic cross containing 1 trait. Mendel was lucky in choosing the characteristics he worked with, since the were not linked and found on separate chromosomes. In a dihybrid cross, 2 trait cross, there are many possible chromosome combinations during gamete formation. Each chromosome seems to have a mind of its own when choosing which sperm or egg cell to enter. Each chromosome does not have a mind but the rules of chance take over in determining where they are to go. This is called independent assortment. Let us look at a simple example: RrYy represents one parent with the characteristics R and r for skin texture and Y and y for hair texture. The parent is heterozygous for both characteristics. During gamete formation one of the R's and one of the Y's need to be in each sperm cell. If this does not happen the offspring will have too many Y's or too few R's and visa versa. The possible gametes must have one R and one Y to be effective. RY, rY, Ry, and ry are the only possible combinations allowed. This individual has the possibility of producing any one of these gametes from the original RrYy cell. If the other parent has the same genotype ( arrangement of genes) the gametes would be the same. If a genetic cross were made there would be 16 possible combinations of offspring from that mating. 9 would be dominant for both characteristics, 3 would be dominant for R and recessive for y, 3 would be dominant for Y and recessive for r, and 1 would be recessive for both characteristics. This can only occur if 2 or more traits are being used and they are on separate chromosomes.

4. Law of Dominance: Within any characteristic one allele appears more often than the other. This may give the appearance that that allele is stronger and the other is weak. The fact is that it has nothing to do with strength. The dominant allele is naturally selected to appear more often than the other allele. In some cases the dominant allele is lethal and in a homozygous condition kills the offspring during some point in its life cycle. The recessive allele appears least often and in many cases is less selective of the two alleles. In some cases a trait may have more than one allele representing the characteristic. This is called multiple alleles. An example of this condition is Human ABO blood typing. A and B type blood are co-dominant while blood type O is recessive to both A and B. Hence your genotypes for A type blood are: AA and AO; B type blood: BB and BO; AB blood is AB, and O type is OO.


Exceptions to the Rules:

1. Incomplete dominance. The appearance of the F1 hybrids appear half way in between the two parents. If a red flowering plant were mated to a white flowering plant one would expect red or white to be the color of their offspring, depending on which allele is dominant. In this case all the offspring were pink in color. The reason for this is each allele for red allows 1/2 of the red pigment to be produced. RR = red or 100% of the pigment; Rr = 50% of the pigment or pink; and rr = 0% of the pigment or white. Pink is a hybrid and can never breed true. Two pinks would yield 25% red, 50% pink and 25% white offspring.

2. Pleiotropy. A gene can sometimes affect another characteristic. This ability of having multiple effects is called pleiotropy. Genes that control fur pigmentation in cats may have an influence on the cats eyes and brain.

3. Epistasis. One gene may interfere with the expression of another gene that is independently inherited. In flower color a P is required for it to exhibit purple color. PP and Pp = purple colored flowers. This can only happen if a dominant allele is present for another characteristic. PPCc =purple Ppcc = white. The C characteristic has an effect on the color of the flower.

4. Polygenic Traits. Quantitative traits or having 3 sets of alleles for a characteristic. Skin color is polygenic. There are 6 genes responsible for this characteristic. BBBBBB= Very dark pigmentation where as bbbbbb = the opposite very light pigmentation. All the other genotypes are intermediates of these combinations.

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