E F H A B G D C ‡ MENDEL AND THE GARDEN PEAS
he first established true-­‐breeding varieties by allowing plants to self-­‐
fertilize for several generations; in this way he ensured that each variety contained only one type of trait; he named these pure lines the P generation ;WĨƌŽŵůĂƚŝŶ͞ƉĂƌĞŶƚŝƐ͟Ϳ he then crossed two varieties exhibiting alternative traits, he named the resulting offspring the F1 generation (F from latin ͞ĨŝůŝƵƐ͟Ϳ What Mendel observed:
Mendel performed crosses for each of the seven characters and repeatedly made his observations of F1: TASK: Examine the figure and write down inferences to explain why Mendel got this results.
Results:
For each pair of contrasting varieties that he crossed, one of the traits disappeared in the F1 generation! Mendel called: ͻƚŚĞƚƌĂŝƚĞdžƉƌĞƐƐĞĚ in the F1 generation the dominant trait ͻĂŶĚŶĂŵĞĚƚŚĞƚƌĂŝƚŶŽƚĞdžƉƌĞƐƐĞĚ in the F1 generation the recessive trait The F1 offspring of these crosses are called hybrids. The specific trait of one of the parents seemed to have vanished! But was it true? To find an answer, Mendel continued his experiments. He left the F1 generation self-­‐pollinate and called this offspring F2 generation. Incredibly enough, for each of the seven traits, the form that have vanished in F1 reappeared in the F2 generation! Mendel counted the number of each type of plant in the F2 generation and he found:
TASK: Examine the figure and try to explain the results.
‡ Three quarters of the F2 individuals expressed the dominant trait ‡ while one quarter expressed the recessive trait the dominant : recessive ratio among the F2 plants was always close to 3:1 (three to one) that the inheritance of each trait is ĚĞƚĞƌŵŝŶĞĚďLJ͞ŝŶĚŝǀŝƐŝďůĞƵŶŝƚƐ͟Žƌ ͞ĨĂĐƚŽƌƐ͟that are passed on to descendants unchanged. Mendel ĐĂůůĞĚƚŚŝƐƵŶŝƚĂ͞ŵĞƌŬŵĂů͟;сĐŚĂƌĂĐƚĞƌ
in german). that an individual must contain at least two alternatives for a trait. When the two alternatives differ, the dominant one masks the presence of the other. This is called now Principle (or law) of dominance (however, the dominant factor does not alter the recessive one in any way, and both can be passed on to the next) 3
During the production of gametes the two copies of each hereditary factor segregate so that offspring acquire one factor from each parent. This is called also law of segregation. THE END
dK^^/E'K/E'Ed/^͙ ‡ The rules of probability can be applied to Mendelian crosses to determine the phenotypes and genotypes of offspring. ‡ Mendel's laws of segregation and independent assortment reflect the same laws of probability that are applied to tossing coins. Probability is the chance that something happens. ‡ When you toss a coin there is a same probability of 50% that you obtain a head or a tail. The next time you toss the coin the ƉƌŽďĂďŝůŝƚLJĚŽĞƐŶ͛ƚĐŚĂŶŐĞĂƐĞǀĞƌLJĨůŝƉŝƐĂŶ
independent event. ‡ In the same way, when eggs (or sperm) are produced, each allele has the same chance (50% of probability) of ending up in an egg or sperm cell. ‡ Thus there is an equal probability of ½ (50%) for each egg or sperm to join forming a zygote; Example: ‡ in a Mendelian cross between pea plants that are heterozygous for flower color (Pp), what is the probability that the offspring will be homozygous recessive (pp)? ‡ The probability that an egg from the F1 (Pp) will receive a p allele = 1/2. ‡ The probability that a sperm from the F1 will receive a p allele = 1/2. ‡ The overall probability that two recessive alleles will unite, one from the egg and one from the sperm at fertilization is: 1/2 X 1/2 = 1/4. (25%) This is called the Product Rule: the probability that independent events will occur simultaneously is the product of their individual probabilities