What is Inheritance? Every living thing has a set of characteristics inherited from its parent or parents. Genetics is the scientific study of heredity.
11-1 The Work of Gregor Mendel He was an Austrian monk He worked with different true-breeding (pure bred) pea plants Pea plants were a good choice because: They were self-pollinating Seed in one season Many different true-breeding types
Mendel worked with 7 different traits: Seed Shape & Color Seed Coat Color Pod Shape & Color Flower Position Plant Height
Genes and Dominance What is a trait? A characteristic like eye color Example: Tall vs.. Short – Height Round vs. Wrinkled – Pea Shape
Mendel called the original plants the P 1 (parent) generation Offspring = F 1 generation If you cross two parents with different traits, the offspring are called hybrids.
This is what Mendel saw. The offspring had the characteristics of only one of its parents Mendel’s F1 Cross
Mendel concluded: 1.Inheritance is determined by factors that are passed from one generation to the next 2.Chemical factors that determine traits are called genes 3.Different forms of the same gene are called alleles Example: Gene for height Alleles: tall vs. short 4. The Principle of Dominance : Some alleles are dominant and some alleles are recessive.
Dominant Traits are always expressed Recessive Traits are can only be expressed when the dominant allele is not present
Mendel wondered if the recessive alleles had dissapeared or were they still present in the F 1 plants He decided to allow all seven kinds of F 1 hybrids to produce F 2 offspring.
Segregation Mendel looked at the results of his F 1 and F 2 crosses: P 1 tall plants x short plants F 1 tall plants ( F 1 become the parents for the next generation ) P 2 tall plants x tall plants F 2 tall plants, short plants
Because the trait, short, reappeared, Mendel reasoned that the alleles for tallness and shortness had separated from each other when gametes (sex cells) form. T = Tall t = shortF 1 : the F 1 plant produces 2 kinds of sex cells T t Tt
11-2 Probability and Punnett Squares The chances that a particular event will happen is called probability. The principles of probability can be used to predict the outcomes of genetic crosses.
Punnett Squares Punnett Squares can be used to determine the genetic combinations that might result from a genetic cross. The letters in a Punnett Square represent alleles: Capital Letters = Dominant Alleles (G) Lowercase Letters = Recessive Alleles (g)
Predicting Averages Probabilities can predict the average outcome of genetic crosses. The larger the number of offspring resulting from a cross, the closer the results will be to the expected values. Offspring Ratios: P - 75% Tall, 25% short G – 1:2:1
11-3 Independent Assortment What happens if there is more than one gene? Does inheriting a certain gene for seed color affect the inheritance of another trait like plant height?
2 Factor Crosses Mendel performed experiment to follow two different genes as they passed from one generation to the next. These experiments are known as two factor (Dihybrid) crosses.
Mendel crossed plants that were true-breeding for two different traits. Round, yellow peas Genotype RRYY Wrinkled green peas Genotype rryy Which traits are dominant and which traits are recessive?
Why are there so many boxes? Each parent can produce 4 different kinds of sex cells (gametes) Each gamete has an equal chance of combining with each of the other parents 4 types of gametes.
F 2 : Dihybrid Cross Each of the offspring in the example are hybrids for BOTH traits - Dihybrids Mendel crossed these offspring to produce another generation of plants (F 2 ) If the genotype of each parent is RrYy, What kinds of gametes will each parent produce?
The results of the F2 cross showed that the alleles for the two different traits segregated independently into the gametes. The offspring from this cross showed a 9:3:3:1 ratio of the different phenotypes.
Summary of Mendel’s Principles Inherited traits are determined by genes. Genes are passed from parents to offspring Some forms of the gene may be dominant and others may be recessive The genes segregate during meiosis so only one copy of a gene goes into the gamete Alleles for different genes usually segregate independently of one another.
Exceptions to Mendel’s Principles Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes.
Incomplete Dominance. One allele is not completely dominant over another. The heterozygous phenotype is somewhere in between the two homozygous phenotypes.
R = red flowers r = white flowers Rr = pink flowers. ____
Codominance Both alleles contribute to the phenotype of the organism Example: Horses, allele for red hair is codominant with allele for white hair. Animals with both alleles have both red and white hairs. The color is called roan.
Polygenic Traits Traits produced by the interaction of many genes. Examples: human skin color, height, cystic fibrosis
Applying Mendel’s Principles Albinism in humans is caused by a recessive trait. If two people with normal skin color have a child with albinism, what are the odds that a second child will also have albinism?
A = normal, a = albino Chances for a normal child? ___________ Changes for an albino child? ___________ ____
11-4 Meiosis Introduction Multicellular organisms use mitosis to replace cells that are lost due to injury or damage or to grow. These cells (somatic cells) are identical to the parent cells because all of the DNA is first copied and then two copies of the DNA separate when the daughter cells form. The daughter cells are identical to the parent cells
Meiosis is Different Multicellular organisms reproduce sexually. In order to keep the number of chromosomes the same from generation to generation, the sex cells have to reduce the number of chromosomes to one half of the number that you find in a somatic cell (body cell) Meiosis is the process that reduces the number of chromosomes to 1/2.
Meiosis I The chromosomes have replicated during S phase of the cell cycle. During Prophase I, the chromosomes become visible and the chromosomes pair off--that is chromosomes that carry the same information called homologs, and form structures called tetrads.
Something important happens during this process--the homologous chromosomes can twist around each other and some times they break off. When they re-attach, they may attach to the other chromosome. This event is called crossing over and is an important process in genetics.
After prophase I, the tetrads of chromosomes line up at the equator of the cell for metaphase I In anaphase I, the pairs of chromosomes separate from each other. Each daughter cell receives one copy of each type of chromosome.
Meiosis II In this second cell division, the two daughter cells from meiosis I undergo one more cell division The steps in this cells division are very much the same as mitosis except the DNA is NOT copied again