Genetics is the science of heredity. Heredity is the transmission (giving) of genetic, and therefore physical traits, from parent to offspring.
Heredity Parents pass on their genes through their reproductive cells. The Zygote (offspring) has one set of chromosomes from each parent that are paired up during fertilization form a whole set of 23 pairs.
What are Genes? • A gene is a segment of DNA on a chromosome. • A gene codes for a specific trait. • Gene is inherited from the biological parents. Therefore, the traits of any organism is from the biological parents. • Genes come in pairs.
What is a dominantgene? • The gene that expresses itself; the most powerful and dominant gene used by the cell. • It has the power to overshadow the recessive gene when there is complete dominance. • Some examples are: brown hair, right handed, etc.
What is a recessive gene? • The gene that is overshadowed by a dominant gene • Recessive genes can only express themselves in the absence of the dominant gene. • A recessive trait can only show in a person with both genes in a pair are recessive genes.
Gregor Mendel • Austrian Monk and science teacher 1822-1884 • Considered the “Father of Heredity” • He conducted plant breeding experiments in a monastery garden. • In 1865 he made his work public about heredity.
Gregor Mendel • It took him 2 years to select the pea plant as his subject. • He collected data for 8 years. • His sample sizes were large; he tabulated results from 28,000 pea plants. • He repeated his experiments. • He analyzed his data with statistics (probability theory). • His work was not well known until early 1900s—34+ years later!
Mendel (nor anyone else) in the1800’s did not know of DNA in the nucleus as the material of inheritance. • He found the traits were passed down in not always in a continuous way but skipped generations and were not blended but distinct.
Mendel observed seven pea traits that are easily recognized and apparently only occur in one of two forms: The trait on the left is dominant and the trait on the right is recessive.
Mendel’s Pea Plant Experiment 3. He recorded data on the offspring of this cross (First Filial, F1) or first generation. 2. He crossed a true breeding plant with a plant of the opposite trait (purple x white). He called this the Parental (P) generation. 1. Mendel chose true breeding lines of each plant/trait he studied (true breeding lines always produced offspring of the same type). 5. He recorded data on the offspring of the second generation, calling it the Second Filial generation (F2) 4. He self pollinated the F1 offspring
Mendel’s Experiment This 3:1 ratio occurs in later generations as well. Mendel realized that this was the key to understanding the basic mechanisms of inheritance. However, the following generation (f2) consistently has a 3:1 ratio of yellow to green. In cross-pollinating plants that either produce yellow or green peas exclusively, Mendel found that the first offspring generation (f1) always has yellow peas.
He came to three important conclusions from these experimental results: • A trait may not show up in an individual • but can still be passed on to the next generation. • An individual inherits one such unit from • each parent for each trait. 1. The inheritance of each trait is determined by "units" or "factors" that are passed on to descendents unchanged (these units are now called genes).
Mendel established three principles (or Laws of Genetics) from his research: 3.Principle of Independent Assortment - factors of a trait separate independently of one another during gamete (sex cell) division called meiosis Another way to look at this is, whether a flower is purple has nothing to do with the length of the plants stems - each trait is independently inherited. 2.Principle of Segregation - the two factors (alleles) for a trait separate during gamete formation; the pair of alleles of each parent separate and only one allele passes from each parent on to an offspring. Which allele in a parent's pair of alleles is inherited is a matter of chance. We now know that this segregation of alleles occurs during the process of sex cell (gamete) division (meiosis). • The Principle of Dominance and Recessiveness - one trait is masked or covered up by another trait. • We now know that there are alleles (genes) that are more dominant than others. Chromosomes, genes, or DNA was not known of during Mendel’s lifetime in the mid 1800s. Neither did he know the process of how sex cells (gametes) are made.
What are genes made of? • Chromosomes are made of DNA • Genes are located on Chromosomes • Genes are a section of DNA that codes information for making proteins.
Genetic Terms to know • Gene-A sequence of base-pairs (ATCG) of various lengths that code for proteins. • Loci- Location of a gene on the chromosome. • Phenotype- Outward appearance of a trait, for example: hair color, size, horn shape. • Genotype- Genetic classification of a gene, AA, Aa, aa. Always come in pairs. • Homozygous- contains 2 identical genes for the same trait, AA, BB, cc – • Heterozygous- contains 2 different genes for the same trait, Aa, bB, Cc-- “Purebred” “Hybrid”
Genotype • Genotypes of your body cells contain two copies of the gene. Aa or AA or aa However, Gamete (sex cells) genotypes contains only one copy (allele) of the gene. A or a
Steps for Solving a Genetics Problem: • Trait – dominant = A (AA or Aa) • Trait – recessive = a (aa) • =genotype • PunnettSquare • The Punnett Square diagram is used to see the probable outcome of a cross. Aa Aa _______ ________ x A a Parent A a A A A Parent a a a a A
Offspring from a Collie Cross! Who has the dominant allele? The dog on the lower left side What percentage of the offspring will probably have fur that is black and white? 50%
Each parent has just one version of the gene in each of its sex cells. Not paired. It is random what each sex cell gets as the gene.
Fill in the punnet square of this cross (fertilization). If a heteroyzous round seed is crossed with itself (Rr x Rr) a Punnett square can help you figure out the ratios of the offspring. Practice!
Showing a genetic cross! Heterozygote cross: Rr x Rr What percentage of the offspring will probably have white flowers? Do you see why dominant traits are more common? 1 out of 4 or 25%
Example 2: Answer the Question • What is the probable offspring phenotype ratio for a cross involving two parents that are heterozygotes for both traits? • 9/16 Pink, tall • 3/16 Pink, short • 3/16 Pink, tall • 1/16 White, short
Incomplete Dominance: • Incomplete Dominance: is a condition in which the dominant allele cannot completely mask the expression of another allele. • For example, red-flowered snapdragons crossed with white ones yield pink in the first generation. • The heterozygote phenotype is somewhere between that of two homozygotes.
Incomplete Dominance A blend of dominant and recessive traits Pink
Codominance • Codominance: is a condition in which both alleles are expressed in heterozygotes. • It is NOT a middle combination of two alleles like the red and white snapdragons make pink. • Instead it is where both alleles and their traits are both equally expressed. • Best example is blood type. IA and IB are each dominant to i (type O), but are codominant to each other. Therefore, some persons can express both genes and have AB blood.
Mendel’s Experiment However, the dominant yellow allele does not alter the recessive green one in any way. Both alleles can be passed on to the next generation unchanged. With all of the seven pea plant traits that Mendel examined, one form appeared dominant over the other. Which is to say, it masked the presence of the other allele. For example, when the genotype for pea color is YG (heterozygous), the phenotype is yellow. Note that each of the f1 generation plants (shown above) inherited a Y allele from one parent and a G allele from the other. When the f1 plants breed, each has an equal chance of passing on either Y or G alleles to each offspring.