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Biology 110 - Basic Concepts and Biodiversity

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Predicting Phenotypes and Genotypes

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Recall from Tutorial11, that homologous chromosomes pair during Prophase 1 of meiosis, and later, each member of the pair will be packaged into separate gametes. Therefore, as we discussed in Tutorial 28, alleles on homologous chromosomes will segregate during Meiosis I (The Law of Segregation), and if the genes for different characters reside on different chromosomes, they will also independently assort during Metaphase I.  (The Law of Independent Assortment).

As the connection between genes and chromosomes started to become more clear in the years after Mendel's death, scientists noted what seemed to be a paradox in Mendel's Law of Independent Assortment and the behavior of chromosomes during gamete formation. Complex organisms must have huge numbers of genes to govern their biochemical processes, and yet, the number of chromosomes in these organisms is often small. For example, humans have 23 pairs of chromosomes, whereas pea plants have 7 pairs. Therefore, each chromosome must have many genes. 

But if there are many genes on each chromosome, how can two genes on the same chromosome  be separated from one another? The chromosome doesn't break apart during meiosis; therefore, there must be another explanation. The answer is that some genes are physically linked to one because they are located on the same chromosome, but they appear to independently assort because the chromosomes recombine (exchange genetic material) with one another. The frequency with which they do this enables scientists to "map" gene locations on chromosomes. We will examine recombination in greater detail in upcoming exercises.

Transcript for Punnet Square - part I

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Using Probability

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As depicted here, the probability of getting an offspring with the genotype Pp is equal to the probability of getting Pp (P from the ova, p from the sperm) plus the probability of getting pP (p from the ova, P from the sperm). Therefore, the probability would be 1/4 + 1/4 = 2/4 = 1/2. This example demonstrates how both the Rule of Multiplication and the Rule of Addition can be used to predict the outcome of genetic crosses. To ensure that you understand the Rule of Addition, here is a non-genetic example.

What is the probability of getting heads, at least once, in two flips of a coin? There are three possible ways to do this: heads on both flips, heads on the first flip, or heads on the second flip.

(a) Use the Rule of Multiplication to calculate the probabilities of each event that satisfies the conditions of the question.

  • Probability of getting heads on first flip, heads on second flip = (1/2)(1/2) = 1/4
  • Probability of getting heads on first flip, tails on second flip = (1/2)(1/2) = 1/4
  • Probability of getting tails on first flip, heads on second flip = (1/2)(1/2) = 1/4

(b) Use the Rule of Addition to calculate the overall probability.

  • Probability of getting heads, at least once, in two flips of a coin = 1/4 (heads on the first flip, tails second) + 1/4 (tails first, heads second) + 1/4  (heads both times)= 3/4

 

Transcript for Probability - part I

 

Transcript for Probability - part II

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Test your Understanding

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 Questions?  Either send Send your instructor a message through ANGEL or attend an office hour (the times and places are posted on ANGEL).Canvas!

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