Transcript Ch 11- Introduction to Genetics
BIG IDEA: How does biological info. pass from one generation to another?
Genetics is the science of heredity, or how traits are passed through each generation. • Gregor Mendel is considered the father of genetics: Austrian monk • Worked with pea plants
Why might someone choose to work with pea plants to study genetics?
Fertilization occurs when the male and the female sex cells join together to form a new cell.
Traits are genetic characteristics (ex: hair color, eye color, height, etc.) -To prevent plants from self-pollinating he cut the male parts from the flower and dusted pollen from them onto the desired flowers. (Cross pollination)
Mendel’s Principles of Inheritance:
1. The factors that pass from one generation to the next are called
- The different forms of each gene are alleles
2. Some alleles are dominant and some are recessive - Dominant traits mask recessive traits and are seen if 1 of the 2 alleles present are dominant
3. Allele pairs separate or segregate during gamete formation, and randomly unite at fertilization.
- A trait may not show up in an individual but can still be passed on to the next generation.
Generations in crossing
P generation= Parental generation, each being true-breeding (SS or ss) F 1 = 1 st generation cross. Cross between 2 true breeding organisms F 2 =2 nd generation cross. Cross between two F offspring. The generation where recessive alleles may re-appear 1
During gamete formation, the alleles for each gene segregate from each other, so that each gamete carries only 1 allele for each gene
See if link works??
Probability is the likelihood that a particular event will occur. - The principles of probability can be used to predict the outcomes of genetic crosses. Homozygous organisms have 2 identical alleles (Ex: TT/tt) Heterozygous organisms have 2 different alleles (Ex:Tt)
Probabilities predict averages/ratios : F 1 produce all tall plants, or 4:0, or 100% F 2 produce mostly tall plants, or 3:1, or 75%:25%
genetic makeup Example: Tt=genotype, tall=phenotype
Punnett squares are used to predict the genotype and phenotype combination in genetic crosses.
How to for 1 factor and 2 factor crosses: pg. 316
To test if certain traits were linked to each other Mendel performed a “Dihybrid” cross, involving 2 traits.
-F 1 - rryy x RRYY, all offspring were heterozygous for each trait -F 2 generation produced approximately 9:3:3:1 He found out that seed shape and color are not related
Independent assortment states that genes for different traits can segregate independently.
A summary of Mendel’s Principles:
-Genes are passed from parents to offspring -2 or more alleles exist for each trait, they may be dominant or recessive -Genes segregate from each other when forming offspring -Alleles for diff. genes usually sort independently
There are some exceptions to Mendel’s principles
Some alleles are neither dominant or recessive
-Incomplete dominance shows the heterozygous phenotype to be an intermediate between the 2 dominant parental phenotypes
Codominance produces offspring that show both dominant phenotypes at the same time Ex: Roan cow
Many genes exist in several different forms and are therefore said to have multiple alleles
Polygenic traits are the traits that are produced by the interaction of several genes. -Ex: height, skin color
Environmental conditions can affect gene expression and influence genetically determined traits.
Examples: different temperatures, diets, toxins
How many sets of genes are found in most adult organisms?
All offspring receive 1 chromosome from each parent, forming homologous pairs. Diploid (2n)=2 sets Haploid (1n)=1 set
The diploid cells of most adult organisms contain 2 complete sets of inherited chromosomes and 2 complete sets of genes.
Meiosis is the process by which the # of chromosomes per cell is cut in ½ through the separation of homologous chromosomes in a diploid cell Remember: PMAT
Each replicated chromosome pairs w/ its corresponding homologous chromosome.
Crossing-over produces new combinations of allleles
Metaphase I: Paired homologous chromosomes line up across center of the cell Anaphase I: Spindle fibers pull each homologous chromosome pair toward opposite ends of the cell Telophase I: Nuclear membrane forms around each cluster of chromosomes.
Cytokinesis follows, forming 2 new cells
Prophase II: Chromosomes become invisible
Metaphase II, Anaphase II, Telophase II:
Similar to meiosis I, but the result is 4 haploid daughter cells Fertilized egg= zygote
Produces sex cells Result= 4 1N cells Reduces chromosome number by half
Produces body cells Result=2 2N cells No change in chromosome number
Alleles of different genes tend to be inherited together from one generation to the next when those genes are located on the same chromosome.
Genes that are far apart assort independently, genes that are linked are on the same chromosome.
Genes that are close together on a chromosome are NOT likely to cross-over. The frequency of crossing over between genes can determine the distances those traits are from each other on chromosomes. = Gene mapping