Patterns of Inheritance Chapter 14: Mendel and the Gene Idea

Patterns of Inheritance Parents and offspring often share observable traits. Grandparents and grandchildren may share traits not seen in parents. Why do traits disappear in one generation and reappear in another?

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

Gregor Mendel Austrian monk Analyzed Asked why Tested his theories Worked with

Pea plants Pisum sativum 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 sperm-producing organs (stamens) and egg-producing organs (carpels)

Traits Mendel used

True breeding plants Began with true-breeding varieties (pure-bred) Crossed with other true-breeding variety Offspring called

Generations Mendel mated two contrasting, true-breeding varieties Process called The true-breeding parents are the The hybrid offspring of the P generation are called the Referred to as hybrids When F1 individuals self-pollinate, the is produced

Crosses Had lots of varieties 7 traits with two forms of each trait Crossing a tall plant with a short plant is a A monohybrid cross is Trait - Two variations - Generations P - parental (true-breeding): F1 - first filial or son (children): F2 - next generation (grandkids): What happened? How did the short plants reappear?

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

Alleles

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

Mendel’s Conclusions For each character, an organism inherits Factors (genes) that determine traits can be Alleles may be traits expressed in the F1 generation traits not expressed in the F1 generation Mendel observed the same pattern of inheritance in 7 pea plant characters, each represented by two traits 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, When Mendel crossed contrasting, true-breeding white and purple flowered pea plants, When Mendel crossed the F1 hybrids, 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 Thus, an egg or a sperm gets only This segregation of alleles corresponds to the distribution of

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

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

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

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

Probability Rules Each gamete has a chance of carrying the and a chance of carrying the Similar to heads and tails Another rule is needed to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous The rule of addition states that the probability that any one of two or more exclusive events will occur is calculated Example: probability of one heads & one tails is

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

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

Setting up a Punnett Square Step 1. Designate letters which will represent the genes/traits. T = tall t = short 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 Step 3. List the genes that each parent can contribute. Parent 1 Parent 2

Setting up a Punnett Square 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.

 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. Step 6. List the possible genotypes and phenotypes of the offspring for this cross. Genotypic Ratio: Phenotypic Ratio:

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

Testcross Used to determine Individual expressing Could be Cross with Make prediction with Punnett square If homozygous dominant: TT x tt 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 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, F1, and F2 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.