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Ch 11- Introduction to Genetics

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1 Ch 11- Introduction to Genetics
Genetics- scientific study of heredity Gregor Mendel- father of genetics, laid the foundation of the science of genetics Used garden peas for his experiments He cross-breed true-breeding plants with different characteristics and study the results

2 Genes and Dominance Mendel studied seven different pea plant traits
Trait- specific characteristic, such as seed color or plant height P (parental) generation-original pair of plants F₁ (first filial) generation- offspring of original pair of plants

3 What did Mendel notice with the offspring?
All F₁ offspring had the characteristic of only one of the parents What did Mendel conclude about inheritance? Traits are inherited through the passing of factors from parents to offspring Genes- chemical factors that determine traits The traits studied were controlled by one gene that occurred in two contrasting forms Alleles- different forms of a gene What is the principle of dominance? Some alleles are dominant and others are recessive Allele for tall plants-dominant, allele for short-recessive

4 Segregation He allowed F₁ plants to self-pollinate – produced F₂ generation Results of F₂- traits controlled by recessive allele represented one fourth of F₂ plants Segregation- separation of alleles Gametes- sex cells What happens during segregation? The two alleles segregate from each other so that each gamete carries only a single copy of each gene Each F₁ plant produces two types of gametes, those with allele for tallness and those with allele for shortness Capital T represents a dominant allele, lowercase t represents recessive allele. Result= F₂ generation with new combination of allele

5 Sec 2- Probability and Punnett Squares
Probability- the likelihood that a particular event will occur What is chance of that a coin lands heads up or heads down? How is coin flipping relevant to genetics? The way in which alleles segregate is completely random, like a coin flip The principles of probability can be used to predict the outcomes of genetic crosses

6 Punnett Squares Punnett squares- diagram that shows gene combinations as a result of genetic crosses Punnett squares can be used to predict and compare the genetic variations that will result from a cross Homozygous- organisms that have two identical alleles for a particular trait- TT or tt Heterozygous- organisms that have two different alleles for the same trait- Tt Phenotype- physical characteristics All tall plants have same phenotype Genotype- genetic makeup Genotype of one third of tall plants is TT while two thirds is Tt


8 Sec 3- Exploring Mendelian Genetics
Does the gene that determines whether a seed is round or wrinkled in shaped have anything to do with the gene for seed color? Mendel’s crossed plants that produced only round yellow peas (genotype RRYY) with plants that produced wrinkled green peas (genotype rryy) All offspring produced round yellow peas Yellow and round are dominant over green and wrinkled Mendel allowed the F₁ generation to self-pollinate to produce an F₂ generation

9 What did Mendel find out?
F₂ plants produced 556 seeds seeds were round and yellow, 32 were wrinkled and green seeds had combinations of phenotypes and alleles that weren’t found in parents What does this mean? The alleles for seed shape segregated independently of those for seed color Genes for seed shape and seed color in pea plants do not influence each other’s inheritance Results-9:3:3:1 ratio -punnett squares predict this What is the principle of independent assortment? Genes for different traits can segregate independently during the formation of gametes


11 Dominant and Recessive Alleles
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 Red plants (RR) crossed with white plants (WW)- pink (RW) Codominance- both alleles contribute to the phenotype Black feathered chicken (BB) crossed with white (WW)- all speckled offspring (BBWW) Multiple alleles- many genes have more than two alleles Human genes for blood type Polygenic traits- traits controlled by two or more genes Variation in skin color Applying Mendel’s Principles- Thomas Hunt Morgan uses fruit flies to test Mendel’s principles Produce large # of offspring

12 Sec 4- Meiosis Where are genes located?
On chromosomes in cell nucleus Fruit fly, Drosophila, has 8 chromosomes 4 from male parent, 4 from female parent. These sets of chromosomes are homologous- each of the 4 chromosomes that came from the male parent has a corresponding chromosome from the female parents Diploid- cell that contains both sets of homologous chromosomes Represented by 2N For Drosophila, the diploid number is 8, 2N=8 Diploid cells contain 2 complete sets of chromosomes and 2 complete sets of genes Haploid- gametes of sexually reproducing organisms, contain only single set of chromosomes, only a single set of genes For Drosophila, haploid number is 4, N=4

13 Phases of Meiosis Meiosis- process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell 4 haploid cells are genetically different from one another and from the original cell Involves two distinct divisions- meiosis I and meiosis II Produces gametes

14 Meiosis I Interphase, prophase, metaphase, anaphase, telophase
Interphase I- each chromosome is replicated Prophase I- each chromosome pairs with its corresponding homologous chromosome to form a tetrad 4 chromatids in tetrad Crossing over- chromosomes exchange portions of chromatids Produces new combinations of alleles Telophase I and Cytokinesis- Nuclear membranes form, cells separate into two cells

15 Meiosis II After meiosis I, the two cells have sets of chromosomes and alleles that are different from each other and from diploid cells Prophase II- meiosis I results in two haploid daughter cells, each with half the number of chromosomes as original cell Metaphase II- chromosomes line up in a similar way to the metaphase stage of mitosis Anaphase II- sister chromatids separate and move toward opposite ends of the cell Telophase II and Cytokinesis- meiosis II results in four haploid daughter cells Four daughter cells contain just 2 chromosomes each Meiosis

16 Gamete Formation In males, meiosis results in four equal sized gametes called sperm In females, only one large egg cell results from meiosis Other three cells called polar bodies are not involved in reproduction How is mitosis different than meiosis? Mitosis results in the production of two genetically identical diploid cells, whereas meiosis produces four genetically different haploid cells Comparison of Meiosis and Mitosis

17 Sec 5- Linkage and Gene Maps
Thomas Hunt Morgan- researched fruit flies in 1910 and discovered that chromosomes assort independently, not individual genes Each chromosome is actually a group of linked genes How did Mendel manage to miss gene linkage? Six of the seven genes he studied are on different chromosomes, the two genes found on same chromosome are so far apart they assort independently

18 Gene Maps If two genes are found on the same chromosome, does this mean that they are linked together forever? Crossing over during meiosis separates genes on same chromosomes What does this lead to? Genetic diversity Alfred Sturtevant- created gene map showing the relative locations of each known gene on one of the Drosophila chromosomes He showed genes close to each other on chromosome are usually inherited together His method has been used to construct genetic maps- including maps of human genome

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