Unit Three “Cell Proliferation and Genetics” “Foundations of Genetics” Gregor Mendel and the Pea Gregor Mendel, a Catholic monk, was the first to focus on using Quantitative observations (numbers) to better understand the mechanisms of heredity Prior to his initial research in the mid 1800’s there were others before him who carried out similar crosses Because Mendel used numbers and kept very good records, his results were more reliable and less likely to be refuted http://www.catholicnewsagency.com/news/googlerecalls-catholic-priest-who-was-father-of-moderngenetics/ Gregor Mendel Mendel’s Experimental System 1. Mendel selected seven pairs of lines that differed in easily distinguishable traits (smooth versus wrinkled, green versus yellow etc.) 2. He knew from the anecdotal findings of previous researchers, he would be able to quantify his results 3. Pea plants are small, easy to grow, produce many offspring, and mature quickly 4. Mendel carried out the fertilization process in a controlled environment (stop uncontrolled variables from negatively affecting results) Mendel’s Experimental Design 1. Mendel began by letting each variety selffertilize for several generations; this ensured “True-Breeding”…producing same expression of trait repeatedly; these lines were called the “P Generation” (P = Parental) 2. Mendel then conducted his experiment: crossed two pea varieties exhibiting different expression of a trait: green versus yellow pea; the offspring that resulted were called “F1 Generation” (First Filial) 3. Lastly, Mendel allowed the plants produced in the previous cross to self-fertilize; the offspring that resulted were called “F2 Generation” (Second Filial) Mendel’s Observations For each pair of contrasting expressions of a trait that Mendel crossed, he observed the same result: an expression of a trait disappeared in the First Filial generation, but then reappeared in the Second Filial generation In the case of flower color, when he crossed purple and white flowers, all the First Filial plants he observed were purple….Mendel called this “Dominance”, and he called the disappearance of the white color, “Recessive” Mendel’s Observations In the Second Filial generation, he observed there were some white flowers along with the predominant color of purple The white color (Recessive) physically disappeared from the First Filial generation, and then reappeared in the Second Filial generation Mendel determined the Recessive white flower color did not really disappear, but rather, was not expressed in the First Filial generation Mendel was beginning to realize the difference between “Phenotype” and “Genotype” Phenotype Genotype 3:1 1:2:1 Second Filial Generation Mendel’s Hypotheses….Theory Mendel proposed a series of hypotheses that later came to be known as the “Theory of Heredity” (now Laws) – Hypothesis 1: Parents do not transmit traits directly but rather transmit coded information that generates the expression of traits – Hypothesis 2: Each parent contains two copies of the trait, both can either be the same expression of the trait in question or differing expressions Mendel’s Hypotheses….Theory Hypothesis 3: Alternate expressions of a trait are called “Alleles”; “Phenotype” is the physical expression of the trait, “Genotype” is the allelic composition present which is not always the same as what is physically expressed Hypothesis 4: The two alleles that an individual possesses do not affect each other/change each other Hypothesis 5: The presence of an allele does not mean that allele will necessarily be expressed (remember Phenotype versus Genotype) Analyzing Mendel’s Results An individual receives one allele for each trait from each parent as a result of Meiosis Dominant alleles are designated with capital letters, and recessive alleles are designated with lowercase letters – Example: Purple flower – “A” White flower – “a” Analyzing Mendel’s Results AA = Homozygous Dominant – Purple Aa = Heterozygous – Purple aa = Homozygous Recessive - White Punnett Squares In a Punnett Square, the possible gametes of one individual are listed along the horizontal side of the square, while the gametes of the other individual are listed along the vertical side Two types of crosses are completed using Punnett Squares: – Monohybrid Cross – one trait focus – Dihybrid Cross – two trait focus Monohybrid Cross Dihybrid Cross Testcross To determine the genotype of individuals, a “Testcross” is conducted: Mendel’s Laws First Law = Segregation = two alleles of a trait separate from each other during the formation of gametes, so that half of the gametes will carry one copy and half will carry the other copy Second Law = Independent Assortment – genes located on different chromosomes are inherited independently of one another Mendel’s Laws Facts about DNA & Traits DNA makes RNA makes Protein Genes influence the phenotype (physical expression of trait) by specifying the types of proteins found in the body, which greatly influences how well the body functions Mutations (changes) alter the DNA, which in turn, alters the expression of traits Mutations cause Evolution to occur, which then leads to Natural Selection, then finally, Adaptations The Exceptions to Mendelian Inheritance Polygenic Inheritance – expression of trait is determined by several genes (segments of DNA), therefore, there can be many variations of said trait; many genes affect one trait Pleiotropic Effects – an individual allele (one section of DNA on one chromosome) has more than one effect on phenotype The Exceptions to Mendelian Inheritance Incomplete Dominance – alleles are neither dominant nor recessive and as a result, produce a heterozygote that is intermediate between both parents; example is a red flower and white flower produce a pink flower progeny Codominance – often in heterozygotes there is not a dominant allele; instead the effects of both alleles are expressed; example is blood type: A, B, O Chromosomes are the vehicles of Mendelian Inheritance Sex Linkage – a trait determined by a gene on the sex chromosome; recall the sex chromosomes are the last two, and for humans that means they are the 45th and 46th A commonly used example that demonstrates sex linkage is “Hemophilia” Hemophilia Pedigree Recessive Chromosomal Disorders Often, disorders/diseases manifest themselves only when a person is homozygous recessive A commonly used example is Sickle-cell Anemia Dominant Chromosomal Disorders Disorders/diseases that only manifest themselves when someone is homozygous dominant or heterozygous A commonly used example is Huntington’s Disease Genetic Counseling and Therapy The older a person is, the higher the chances his/her children could have a genetic disorder Also, a propensity toward a certain disease/disorder can “run in the family” For the above two reasons, it is recommended that couples who are preparing to have children get genetic counseling Once a woman is pregnant, ultrasounds and amniocenteses can detect genetic disorders DNA Screening with IVF patients….ethical questions!?!