Chapter 11 introduction to genetics 11.1 The work of Gregor mendel
What is inheritance? Two uses of the word inheritance Things that are passed down through generations Factors we get from our parents which determine your characteristics
What is genetics? Every living thing has a set of characteristics inherited from its parent or parents For many years it remained unclear why people were like they are Possible to identify family resemblance - but why? Delivery of characteristics from parents to offspring hereditary Genetics scientific study of hereditary The key to understanding why each organism is unique!
Who was Gregor Mendel? An Austrian monk who lived from 1822 1884 Also a high school teacher In the monastery he was in charge of the garden Became fascinated with garden peas Due to their simplicity peas were an excellent model system on which to perform experiments
Mendel s experiments on peas Mendel knew that the male part created pollen and contained the plants male reproductive cells Also knew that the female part produced eggs Sexual reproduction is required to join the two sex cells through fertilization, and produce a new cell Grows into a seed Pea flowers are self pollinating Pea plants have a single parent Monastery garden had a series of true breeding pea plants Offspring were identical each generation
Mendel s pea experiments continued. Trait specific characteristic of an individual E.g. Seed colour, plant height etc Some of Mendel s pea plants only produced tall plants, others only produced short etc Mendel forced breeding stocks to reproduce with each other Crossing cross pollination Mendel studied 7 contrasting traits (e.g. green vs yellow seed) Studied offspring are called hybrids
Mendel s results
Mendel s results Original plants = P, or parental generation Offspring F1, or filial generation Mendel s offspring only had the characteristic of one of the parents A mixture was never observed Mendel had two conclusions An individual's characteristics are determined by factors that are passed from parents to their offspring Some traits are dominant over other traits
Genes and Alleles Gene Factor that is passed from parent to offspring Allele The different possible expressions of a gene Can you think of any examples of genes and their subsequent alleles?
Dominant and recessive alleles Mendel s second conclusion lead to the principle of dominance Some alleles are dominant, others are recessive An organism with a dominant allele will exhibit that trait Recessive alleles will only be exhibited when the dominant allele is not present Which of Mendel s alleles were dominant? Which were recessive?
Variation in the classroom Turn to page 311 in the textbook
Segregation Mendel was confused. Had the recessive trait disappeared, or was it still there somewhere? He crossed all the F1 plants together, producing an F2 generation What do you think happened?
Results of F1 cross
Explaining the F1 cross During gamete formation, alleles segregate from each othe so that each gamete carries only a single copy of the gene Alleles are paired up again during fertilisation
Gametes formation During Gamete formation, alleles for each gene segregate from each other so that each gamete only carries one allele for each gene Dominant alleles are represented by a capital letter Recessive alleles are dominated by lower case letter
Summary Gregor Mendel father of genetics Recognized that traits are passed down from parents to offspring Recognized that some traits can be dominant, others can be recessive Gene= factor that is passed down from parent to offspring Allele Different form of a gene
Section 11.2 - applying Mendel s principles
Nothing in nature is certain The best we can do it discuss what is probable We can calculate the probability that offspring will express a certain characteristics Probability can be used to explain Mendel s genetic crosses
How does probability work? When you flip a coin, what is the chance it will land on heads? What is the chance it land on heads three times in a row? Key point past outcomes do not affect future ones
Using segregation to predict outcomes The probability of each gamete carrying the t allele is 50% So the probability of a fertilized egg containing 2 t alleles is 0.5 X 0.5 0.25 Not all organisms with the same characteristics have the same alleles
Genotype vs Phenotype Every organism has a genetic make up (Genotype) AND every organism has a physical trait (phenotype) Two organisms can share a phenotype but had a different genotype Organisms that have two identical alleles are referred to as homozygous Organisms that have two different alleles for the same gene are heterozygous
Punnett Squares Punnett squares use mathematical probability to predict genotype and phenotype combinations
Punnett squares for monohybrid crosses Page 316. Key points Figure out possible gametes Draw the table with enough squares for each pair of gametes from each parent Fill the table by combining the gametes genotypes
Unfortunately it s not always that simple Mendel wondered if segregation of one pair of alleles can affect another pair of alleles He investigated whether shape of the seed will affect seed colour This is known as a dihybrid cross Two factors are involved
Dihybrid crosses All of the F1 generation will have the same genotype and phenotype IF the genes are unlinked, the pattern on the right is produced If genes are linked (found together on the same chromosome it gets a lot more complicated.) What phenotypes would you expect to see if the two genes were linked?
Independent assortment Genes for different traits can segregate independently during the formation of gametes Account for widespread genetic variation observed in organisms which share the same parents Common ratio of offspring phenotypes in a two-factor cross: 9 :3 : 3: 1
Practice problem 1 Let's say that in seals, the gene for the length of the whiskers has two alleles. The dominant allele (W) codes long whiskers & the recessive allele (w) codes for short whiskers. a) What percentage of offspring would be expected to have short whiskers from the cross of two long-whiskered seals, one that is homozygous dominant and one that is heterozygous? b) If one parent seal is pure long-whiskered and the other is short-whiskered, what percent of offspring would have short whiskers?
Practice problem 2 A green-leafed luboplant (I made this plant up) is crossed with a luboplant with yellow-striped leaves. The cross produces 185 green-leafed luboplants. Summarize the genotypes & phenotype of the offspring that would be produced by crossing two of the green-leafed luboplants obtained from the initial parent plants.
Practice problem 3 Yellow fruit and dwarf vines are recessive traits in tomatoes. Red fruit and tall vines are dominant. Complete a punnett square and answer the questions for a completely dominant red and tall plant crossed with a heterozygous red and dwarf plant. (You chose the letters you want to use) 1. What percent of the offspring will be totally heterozygous? 2. What is the phenotype ratio? 3. What percent of the offspring will have yellow fruit and dwarf vines? Using the same traits as above, cross a dwarf and homozygous red plant with a yellow and heterozygous tall plant. (You chose the letters you want to use) 1. What percent of the offspring will be totally heterozygous? 2. What is the phenotype ratio? 3. What percent of the offspring will have red fruit and dwarf vines?
Why is Mendel s work so important? Mendel s principles of hereditary, observed through patterns of inheritance, form the basis of modern genetics Inheritance determined by genes passed from parent to offspring When two or more traits exists some alleles can be dominant whereas others can be recessive Most organisms have two copies of a gene one from each parent Alleles usually segregate independently of each other Mendel s principles don t just apply to plants, but are valid for every organism that undergoes sexual reproduction
Other patterns of inheritance Section 11.3
Mendel s work is not infallible Life is not always so simple Many genes have more than two alleles Many traits are controlled by more than one gene Incomplete dominance some alleles are neither dominant or recessive Phenotype lies between two end members Example flower color in some plants Codominance Phenotypes from both alleles are expressed Possible to get black and white feathered chickens Seen in humans with proteins that control cholesterol
Multiple alleles Many genes exist in several forms and have multiple alleles A gene with more than 2 has multiple alleles Individuals still only have 2 copies of the genes Human blood type great example (page 320)
Polygenic traits Many traits are produced by the interaction of several genes Polygenic = many genes Wide range of phenotypes Example human skin colour
Genes and the environment Environmental conditions can effect gene expressions and influence genetically determined traits Phenotype is not just determined by genes Can you think of any examples in nature of how the environment can affect phenotype?
Meiosis Section 11.4
How many sets of genes are found in most adult organisms? Homologous chromosomes from the male parent have a corresponding chromosome from the female parent Diploid (2n) contains two complete sets of inherited chromosomes and two complete sets of genes Haploid (n) only a single set of chromosomes, and a single set of genes Gametes are haploid How many chromosomes do humans have?
Phases of meiosis
Meiosis Key points Crossing over during Prophase 1 leads to genetic diversity amongst cells produced Each replicated chromosome pairs with it s corresponding homologous chromosome to form a tetrad No Chromosome replication occurs before entering Meiosis II Four haploid daughter cells are produced at the end of Meiosis II
Comparing mitosis to meiosis Mitosis a form of asexual reproduction Daughter cells receive complete genetic copy Meiosis early stage of sexual reproduction Alleles are segregated Chromosome number halved
Gene linkage Each chromosome is a group of linked genes Independent assortment holds true BUT chromosomes assort independently, not individual genes Mendel missed it, because by luck or design, several of the studied genes were on different chromosomes, or were far apart
Gene mapping The further apart genes are on a chromosome, the more likely crossing over will occur The closer together the less likely they are to cross over Possible to work out where exactly on the chromosome a gene is