Patterns of Inheritance Chapter 14 Mendel and the

Patterns of Inheritance Chapter 14: Mendel and the Gene Idea

Patterns of Inheritance FParents and offspring often share observable traits. FGrandparents and grandchildren may share traits not seen in parents. FWhy do traits disappear in one generation and reappear in another?

Possible Hypotheses F The “blending” hypothesis states that • Example: Blue and yellow paint blend to make green F The “particulate” hypothesis states that • These heritable units are that can be passed on to the next generation F Gregor Mendel documented a through his experiments with

Gregor Mendel �� FAustrian monk FAnalyzed • Asked why • Tested his theories • Worked with

Pea plants Pisum sativum F Advantages of pea plants for genetic study: • Many varieties with distinct heritable features, or (such as flower color) § Character variants (such as purple or white flowers) are called • • Each pea plant has spermproducing organs (stamens) and egg-producing organs (carpels) •

Traits Mendel used

True breeding plants FBegan with true-breeding varieties (pure-bred) • • F Crossed with other truebreeding variety • Offspring called

Generations F Mendel mated two contrasting, true-breeding varieties • Process called F The true-breeding parents are the F The hybrid offspring of the P generation are called the • Referred to as hybrids F When F 1 individuals selfpollinate, the is produced

Crosses F Had lots of varieties • 7 traits with two forms of each trait F Crossing a tall plant with a short plant is a • A monohybrid cross is F Trait • Two variations F Generations • P - parental (true-breeding): • F 1 - first filial or son (children): • F 2 - next generation (grandkids): F What happened? How did the short plants reappear?

Mendel’s Conclusions 1. Alternative versions of genes account for F For example, the gene for flower color in pea plants exists in two versions: purple flowers and white flowers F These alternative versions of a gene are now called F Each gene resides at a specific F Therefore, we distinguish between an organism’s • •

Alleles

Genotype Terminology FIf alleles are identical = • If both alleles are recessive = § Genotype is • If both alleles are dominant = § Genotype is FIf both alleles are different = • • One dominant allele and one recessive allele ( ) FAn organism’s traits do not always reveal

Mendel’s Conclusions 2. For each character, an organism inherits F Factors (genes) that determine traits can be F Alleles may be • traits expressed in the F 1 generation • traits not expressed in the F 1 generation F Mendel observed the same pattern of inheritance in 7 pea plant characters, each represented by two traits F What Mendel called a “heritable factor” is what we now call a

Mendel’s Conclusions 3. If two alleles at a locus differ, then one (dominant allele) determines the organism’s appearance, F When Mendel crossed contrasting, true-breeding white and purple flowered pea plants, F When Mendel crossed the F 1 hybrids, F Mendel discovered a ratio of about

Mendel’s Law of Segregation 4. The law of segregation states that the two alleles for a heritable character F Thus, an egg or a sperm gets only F This segregation of alleles corresponds to the distribution of

Principle of Segregation 1. Two copies of each trait (gene) • Fully expressed gene • Other gene 2. Gametes only have 3. Fertilization restores

Monohybrid & Dihybrid Crosses FMendel derived the law of segregation by following FMendel identified his second law of inheritance by following FCrosses involving two traits are called FA dihybrid cross can determine whether two characters are transmitted to offspring FUsing a dihybrid cross, Mendel developed the

Law of Independent Assortment F The law of independent assortment states that each pair of alleles F This law applies only to genes on F Genes located near each other on the same chromosome

Probability Rules F Mendel’s laws reflect F When tossing a coin, the outcome one toss has on the outcome of the next toss F In the same way, the alleles of one gene F The multiplication rule states that the F Example: probability of 2 coins landing heads up is of

Probability Rules F Each gamete has a chance of carrying the and a chance of carrying the • Similar to heads and tails F Another rule is needed to figure out the probability that an F 2 plant from a monohybrid cross will be heterozygous rather than homozygous F The rule of addition states that the probability that any one of two or more exclusive events will occur is calculated F Example: probability of one heads & one tails is

Probability Rules F These rules can be used to predict the outcome of crosses involving F A dihybrid or other multicharacter crosses are equivalent to two or more F In calculating the chances for various genotypes, each character is

Punnett squares F Probability can be depicted through the use of a • A diagram for predicting the results of a genetic cross between F Predicts all possible gametes with F Same letter used for trait • • for

Setting up a Punnett Square F Step 1. Designate letters which will represent the genes/traits. • T = tall t = short F Step 2. Write down the genotypes (genes) of each parent. These are often given to you or are possible to determine. • TT (tall) X tt (short) - both homozygous or purebred F Step 3. List the genes that each parent can contribute. Parent 1 Parent 2

Setting up a Punnett Square F Step 4. Draw a Punnett square and write the possible gene(s) of one parent across the top and of the other parent along the side. 

 F Step 5. Fill in each box of the Punnett square by transferring the letter above and in front of each box into each appropriate box. As a general rule, the capital letter goes first and a lowercase letter follows. FStep 6. List the possible genotypes and phenotypes of the offspring for this cross. FGenotypic Ratio: FPhenotypic Ratio:

Practice! F 1. Cross a homozygous tall plant with a short plant. What are the genotypic and phenotypic ratios? F 2. Cross a heterozygous tall plant with a homozygous tall plant. What are the genotypic and phenotypic ratios? F 3. Cross a heterozygous tall plant with a short plant. What are the genotypic and phenotypic ratios?

Testcross F Used to determine • Individual expressing • Could be F Cross with F Make prediction with Punnett square F If homozygous dominant: • TT x tt • F If heterozygous: • Tt x tt •

More Practice!!! 1. In rabbits, the allele for black fur (B) is dominant over the allele for brown fur (b). If a heterozygous male mates with a heterozygous female, what are the chances that the offspring will have black fur? 2. In humans, dimples are dominant to no dimples. If a homozygous dominant man reproduces with a heterozygous female, what are the chances of having a child with no dimples? 3. In humans, freckles are dominant over no freckles. A man with freckles reproduces with a woman with freckles, but the children have no freckles. What chance did each child have for freckles? 4. If a man is homozygous for widow’s peak (dominant) reproduces with a woman homozygous for straight hairline (recessive), what are the chances of their children having a widow’s peak? A straight hairline? 5. In humans, pointed eyebrows (B) are dominant over smooth eyebrows (b). Mary’s father has pointed eyebrows, but she and her mother have smooth. What is the genotype of the father?

Review Questions 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Differentiate between the blending and particulate hypotheses of inheritance. Explain the importance of Gregor Mendel’s work with garden peas. Also, explain why he used garden peas. Define the following vocabulary associated with basic genetics: character, trait, hybrid, gene, allele, locus, genotype, phenotype, dominant, recessive, homozygous, & heterozygous. Differentiate between the P, F 1, and F 2 generations. Differentiate between monohybrid and dihybrid crosses. Explain Mendel’s four basic conclusions regarding inheritance patterns. Explain the three parts to the law of segregation. Explain the law of independent assortment. Properly construct a Punnett square for use in solving a genetics problem involving probability. Explain the idea of a testcross.