Probability and Heredity 3:2

Just a little recap of Mendel’s experiments with pea plants

Evidently the F1 “tall” offspring must have been carrying the short trait, but it had been hidden. The short trait had been passed down to the offspring and it reappeared in the F2 generation.

Why did the recessive allele seem to disappear in the F1 generation and then reappear in the F2 generation Mendel realized that organisms have two alleles for every trait. These two alleles are inherited, one from each parent. If the offspring receives a dominant allele from one parent, that dominant trait will appear in the offspring. Recessive traits show up in the offspring only if the offspring receives recessive alleles from each parent.

Gametes are: sex cells or egg and sperm cells. Remember: the capital letter, T, represents a dominant allele. The lower case letter, t, represents a recessive allele. During meiosis, the DNA is _replicated once and then goes through two separations to form _4 gametes . In this way, a parent passes one allele for each gene to their offspring.

What Is Probability? Probability - a number that describes how likely it is that an event will occur. The laws of probability predict what is likely to occur, not what will occur. # of Tosses Probability of Heads Probability of Tails 1 20 50% 50% Invite students to describe situations in which they have used a coin toss to decide an issue. Ask: Why did you toss a coin in these situations? Sample answer: It was the fairest way to make a decision. Ask: Why is a coin toss fair? Answer: Each person has a 50-50 chance of winning. Have a student read aloud the definition for probability. Explain that each time you toss a coin, there are two possible ways it can land—heads up or tails up. You can say that the probability that a tossed coin will land heads up is 1 in 2. There’s also a 1 in 2 probability that the coin will land tails up. A 1 in 2 probability is expressed as the fraction ½ or as 50 percent. Click to reveal the second statement. Have a student read aloud the second statement. Explain that if you toss a coin 20 times, you may expect it to land heads up 10 times and tails up 10 times. But you may get 11 heads and 9 tails, or 8 heads and 12 tails. The more tosses you make, the closer your actual results will be to those predicted by probability. Have students complete the table by writing the probability of heads and tails when tossing a coin once and when tossing a coin 20 times. Click to reveal the correct answers. Ask: What is probability? Answer: a number that describes how likely it is that an event will occur Ask: Does probability predict what will definitely occur? Answer: No; it predicts what is likely to occur. Ask: What does probability predict will happen if you toss a coin ten times? Answer: The most likely result is that the coin will land heads up five times and tails up five times. 50% 50%

Probability Probability - a number that describes how likely it is that an event will occur. The result of one toss will not affect the results of the other. Each event occurs independently.

Probability and Genetics Punnett square: a chart showing all the ways alleles can combine in a genetic cross. In a genetic cross, the combination of alleles that parents can pass to an offspring is based on probability. Remind students that Mendel carefully counted the offspring from every cross. When he crossed two plants that were hybrid for stem height (Tt), about three-fourths of the F2 plants had tall stems. About one-fourth of the plants had short stems. Each time Mendel repeated the cross, he observed similar results. He realized that the principles of probability applied to his work. He found that the probability of a hybrid cross producing a tall plant was 3 in 4. The probability of producing a short plant was 1 in 4. Mendel was the first scientist to recognize that the principles of probability can predict the results of genetic crosses. Click to reveal the Punnett square. Read the definition of a Punnett square aloud. Explain that geneticists use Punnett squares to see allele combinations and to determine the probability of a particular outcome, or result. Tell students that in the Punnett square on the slide, the cross is between two hybrid pea plants with round seeds (Rr). The allele for round seeds (R) is dominant over the allele for wrinkled seeds (r). Each parent can pass one allele or the other to an offspring. The boxes in the Punnett square show the possible combinations of alleles that the offspring can inherit. The male parent’s alleles are written along the top of the square. Ask a student to write in the alleles for the female parent along the left side of the Punnett square. Answer: R; r Click to reveal the correct answer. Explain to students that combinations are pairings of the alleles from a particular row and column. Remind students that by convention, the letter representing the dominant allele is always written first, regardless of which parent was the source of the allele. Click to reveal another Punnett square. Explain that this Punnett square will show the possible offspring in a cross between a purebred pea plant with purple flowers and a purebred pea plant with white flowers. Ask: What are the alleles of the plant with purple flowers? Answer: PP Ask: What are the alleles of the plant with white flowers? Answer: pp Click to show the alleles on the Punnett square.

The alleles of the male parent (or 1st parent mentioned) will be placed on top of the square. The alleles of the female parent (or 2nd parent mentioned) will be placed along the left side of the square. The possible gene combinations of the offspring are placed inside the squares. Letters represent the alleles. A capital letter represents a dominant allele. A lower case letter represents a recessive allele.

Probability and Genetics Copy the male parent’s alleles into the boxes beneath them. A completed Punnett square shows all the possible allele combinations the offspring can have. p p r p R P P p P Read the statement at the top of the slide aloud to students. Ask: What are the offspring’s alleles in these two crosses? Click to reveal the completed Punnett squares for both crosses. Ask: In the hybrid cross, what is the probability that an offspring will have wrinkled seeds? Answer: one fourth, or 25 percent Ask: What is the probability that an offspring will have round seeds? Explain. Answer: three fourths, or 75 percent: one fourth, or 25 percent, round seeds RR; plus one half, or 50 percent, round seeds Rr Explain to students that Mendel did not know about alleles. But a Punnett square shows why he got the results he saw in the F2 generations. In the hybrid cross, plants with alleles RR would have round seeds. So would plants with alleles Rr. Only plants with alleles rr would have wrinkled seeds. Ask: In the purebred cross, what alleles do the offspring have? Answer: All have Pp. Ask: What color flowers will these plants have? Answer: All will have purple flowers. R R P p P p P Hybrid cross Purebred cross

The physical appearance of an organism, or its physical traits Phenotype: The physical appearance of an organism, or its physical traits In Mendel’s pea plants, the tall allele was dominant over the short allele Genotype: The genetic combination of an organism or its alleles Genotype Phenotype TT Tall Tt Tall tt short

Homozygous And Heterozygous An organism having two identical alleles for a trait In Mendel’s pea plants, the tall allele was dominant over the short allele Heterozygous: An organism having two different alleles for a trait Genotype Phenotype TT Tall Tt Tall tt short

Phenotypes and Genotypes SS Homozygous Ss Heterozygous Tell students that an organism can be described by its phenotype or genotype. Explain that an organism’s phenotype is its physical appearance, or visible traits. An organism’s genotype is its genetic makeup, or alleles. In other words, genotype is an organism’s alleles. Phenotype is how a trait looks or is expressed. Tell students to look at the Phenotypes and Genotypes table. The allele for smooth pea pods (S) is dominant over the allele for pinched pea pods (s). All the plants with at least one S allele have the same phenotype. That is, they all produce smooth pods. However, these plants can have two different genotypes—SS or Ss. If you were to look at the plants with smooth pods, you would not be able to tell the difference between those that have the genotype SS and those that have the genotype Ss. The plants with pinched pods, in contrast, would all have the same phenotype—pinched pods—as well as the same genotype—ss. Ask students to fill in the missing information under the column Genotype. Click to reveal the correct information. Explain that an organism that has two identical alleles for a trait is said to be homozygous for that trait. A smooth-pod plant with the alleles SS and a pinched-pod plant with the alleles ss are both homozygous. An organism that has two different alleles for a trait is heterozygous for that trait. A smooth-pod plant with the alleles Ss is heterozygous. Recall that Mendel used the term hybrid to describe heterozygous pea plants. Ask students to fill in the missing information under the column Homozygous or Heterozygous. Ask: How man genotypes are there for the smooth-pod phenotype? Answer: two; SS and Ss Ask: How do the two smooth pods differ? Answer: They don’t differ at all. Ask: Why can you be certain of the genotype of the plant that produced the pinched pod? Answer: Pinched pod is recessive, so the only way this form of the trait could be seen is if the plant is homozygous, ss. ss Homozygous

What is the genotype of the first parent mentioned? T T Mendel began his experiments using purebred pea plants. Cross a purebred tall pea plant with a purebred short pea plant. What is the genotype of the first parent mentioned? T T TT t T t T t What is the genotype of the second parent mentioned? t T t T t tt Place the alleles of the first parent on the top of the square. Place the alleles for the second parent on the left side of the square. Fill in the Punnett square to show the possible combinations. What is the ratio of tall:short? 4:0

What is the genotype of each parent. In the previous problem, none of the offspring will show the short trait. As we learned earlier, Mendel wondered what had happened to the short trait. He allowed the F1 generation to self-pollinate. Show this cross using the Punnett square below. What is the genotype of each parent. Tt x Tt T t T TT Tt TT Tall Tt Tall tt Short t Tt tt

What is the genotype of the woman? N n Nn Normal skin(N) is dominant over albino skin (n). A woman who has normal skin, but whose father was albino, marries a heterozygous, normal skinned man. What type of offspring might they expect? Describe the probability of each genotype and phenotype in the data table. What is the genotype of the woman? N n Nn What is the genotype of the man? NN N Nn Nn % Genotype Phenotype probability n Nn nn 25% NN Normal skin 50% Nn Normal skin 25% nn Albino skin What are the possible genotypes? What are the possible phenotypes? What percentage of offspring are homozygous dominant? What percentage of offspring are heterozygous? What percentage of offspring are homozygous recessive? NN, Nn, nn Norman skin, Albino skin 25% 50% 25%

Having dimples is dominant over the absence of dimples Having dimples is dominant over the absence of dimples. Cross a heterozygous dimpled man with a woman who does not have dimples. Show all work in the Punnett square. Describe the probability of each genotype and phenotype in the data table. What is the genotype of the man? Dd What is the genotype of the woman? dd D d % Genotype Phenotype probability Dd d dd 50% Dd dimples d Dd dd 50% dd No dimples

In dogs, the allele for short hair (B) is dominant over the allele for long hair (b). Two short haired dogs have a litter of puppies. Some of the puppies have short hair and some of the puppies have long hair. Describe the probability of each genotype and phenotype in the data table. What are the genotypes of the parents? Bb and Bb B b % Genotype Phenotype probability BB B Bb 25% BB Short hair 50% Bb Short hair b Bb bb 25% bb Long hair If the litter of puppies contained 12 pups, how many would you expect to have short hair? 9 pups or 75% How many would you expect to have long hair? 3 pups or 25%