Objective SWBAT complete Punnett squares for monohybrid crosses.

Segregation of gametes

Genes are on chromosomes

Alleles

Probability & Punnett Squares VOCABULARY: Probability Punnett Square Homozygous Heterozygous Phenotype Genotype KEY CONCEPTS: How do geneticists use the principles of probability?

A little background info… Every time Mendel performed a cross with his pea plants, he carefully counted the offspring Out of the over 20,000 plants he counted he noticed there was a pattern Whenever he crossed 2 plants that were hybrid for stem height (Tt), about ¾ of the resulting plants were Tall and ¼ were short (3:1 ratio) He realized that the principles of probability could be used to explain his results!

The work of Gregor Mendel worked with pea plants… …he called them his children! Why pea plants??? There was a long-standing tradition of breeding pea plants at the monastery where Mendel lived and worked

So…they were readily available and they come in lots of varieties! …there were plants with different flower colors, seed color, flower position etc..

Mendel’ Pea Plants Mendel based his laws on his studies of garden pea plants. Mendel was able to observe differences in multiple traits over many generations because pea plants reproduce rapidly, and have many visible traits such as: Pod color Seed Color Plant Height Green Yellow Green Yellow Seed Shape Short Pod Shape Tall Wrinkled Round Smooth Pinched

Probability Definition: the likelihood that a particular event will occur Ex: flip a coin: ½ or 50% chance it will come up heads If you flip a coin 3 times in a row, what is the probability it will come up heads 3 times in a row? Each event is an independent therefore it is: ½ x ½ x ½ = 1/8 Past outcomes do not influence future outcomes The principles of probability can be used to predict the outcomes of genetic crosses

Punnett Squares The gene combinations that MIGHT result from a genetic cross can be determined by drawing a Punnett Square The parents go on the outside of the square along the side and top The possible outcomes of the cross are on the inside of the square The letters in the Punnett Square represent alleles

Alleles in a Punnett Square “T” represents the Dominant allele: TALL “t” represents the recessive allele: short Organisms with 2 identical alleles (TT or tt) for a trait are said to be HOMOZYGOUS Mendel called these: true-breeding Organisms with 2 different alleles for a particular trait are said to be HETEROZYGOUS Mendel called these hybrids

Punnett Squares A capital letter represents the dominant allele for tall. A lowercase letter represents the recessive allele for short. In this example, T = tall t = short The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants.

MORE VOCABULARY FUN! Phenotype: the physical characteristics of the organism (what the organism looks like) Ex: Tall; short Genotype: the actual genetic makeup of the organism (the 2 alleles the organism inherited) Ex: TT; Tt: tt

Punnett Squares The plants have different genotypes (TT and Tt), but they have the same phenotype (tall). Although these plants have different genotypes (TT and Tt), they have the same phenotype (tall). TT Homozygous Tt Heterozygous

Probabilities Predict Averages Probabilities predict the outcomes of a LARGE number of events Probabilities cannot predict precise outcomes of an event Ex: flip a coin twice you may get 100% heads You need to flip the coin many, many times to get close to 50%

Probability and Segregation One fourth (1/4) of the F2 plants have two alleles for tallness (TT). 2/4 or 1/2 have one allele for tall (T), and one for short (t). One fourth (1/4) of the F2 have two alleles for short (tt). The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants.

Because the allele for tallness (T) is dominant over the allele for shortness (t), 3/4 of the F2 plants should be tall. The ratio of tall plants (TT or Tt) to short (tt) plants is 3:1. The predicted ratio showed up in Mendel’s experiments indicating that segregation did occur.

What is the expected ratio for this cross? Tt x tt

Punnett Square Rules 1. Choose a letter to represent the dominant allele and capitalize it (choose a letter that is easy to distinguish between upper-case and lower-case). 2. Use the same letter but use lower case to represent the recessive allele. 3. Put the male on the left of the square and the female on the top. 4. Write "genotype" and "phenotype" below the square. 5. Use "x" to indicate a cross. 6. Write the dominant allele first if you are writing out a hybrid.

Question 1 Probability can be used to predict: A. average outcome of many events. B. precise outcome of any event. C. how many offspring a cross will produce. D. which organisms will mate with each other.

Question 2 Compared to 4 flips of a coin, 400 flips of the coin is A. less likely to produce about 50% heads and 50% tails. B. guaranteed to produce exactly 50% heads and 50% tails. C. more likely to produce about 50% heads and 50% tails. D. equally likely to produce about 50% heads and 50% tails.

Question 3 Organisms that have two different alleles for a particular trait are said to be A. hybrid. B. heterozygous. C. homozygous. D. recessive.

Question 4 The Punnett square allows you to predict A. only the phenotypes of the offspring from a cross. B. only the genotypes of the offspring from a cross. C. both the genotypes and the phenotypes from a cross. D. neither the genotypes nor the phenotypes from a cross.

Question 5 Two F1 plants that are homozygous for shortness are crossed. What percentage of the offspring will be tall? A. 100% B. 50% C. 0% D. 25%