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DNA & Heredity-II
- 1. DNA & Heredity II Also known as... Why you look like your mum
- 2. Last time.... • What is “Genetics”? • Structure, function of DNA • Genetic code Today.... • Basics of inheritance • Mendelian genetics • Sex determination • Pedigrees
- 4. Terminology • Locus (pl loci) = a location within the genome • Allele (pl alleles) = one particular version of a gene, locus • Diploid = cells contain two full homologous sets of chromosomes (2N, one from each parent) • Haploid = cells contain one set of chromosomes only (1N, e.g. gametes) • Gene = the smallest unit of inheritance, a region of DNA encoding an RNA or protein
- 5. Chromosomes & ploidy 3 nonhomologous chromosomes centromere 3 pairs of homologous chromosomes haploid (1N) diploid (2N)
- 6. Information content of DNA vs traits: Genotype: the information in DNA Phenotype: physical expression of DNA sickle phenotype normal phenotype red blood cells
- 7. SS AS AA heterozygote has a mix of red blood cell types => A and S are codominant for this trait Homozygote HomozygoteHeterozygote
- 8. More terminology... • Homozygous – having the same allele at a locus • Heterozygous – having two different alleles at a locus In heterozygotes: • Dominant - allele that determines the phenotype • Recessive – allele that has no noticeable effect on the phenotype • Codominant – simultaneous expression of two different alleles
- 9. Why cells divide Mitosis: simple cell division (2N 2N) • growth of multicellular organism from single cell: zygote • replacement of cells in organisms • asexual reproduction of unicellular forms of life Meiosis: reductional division (2N 1N) • formation of sex cells (gametes: sperm/egg)
- 10. mitosis meiosis
- 11. chromosomes consist of identical “sister chromatids”: replicated DNA chromosome at onset of cell division
- 12. Meiosis I Meiosis II ...produces 4 haploid gametes Meiosis 1 diploid cell...
- 14. Mendelian genetics : the fundamentals Principle of segregation: Alleles are placed in gametes at random (50:50) and combine at random in progeny Principle of independent assortment: Alleles at unlinked loci segregate independently
- 15. Principle 1 –Segregation Alleles segregate 50:50 and combine at random Yy y Yy Y y Y yyYY Yy Yy yyyy
- 16. Principle 2: Independent assortment Alleles at unlinked loci assort independently YySs YS Ys yS ys YS Ys yS ys YySs YYSS YYSs YySSYYSs YySsYySS YYss YySs YySs Yyss Yyss YySs yySS yySs yySs yyss all gamete combinations possible all zygote combinations possible (& predictable)
- 17. Sex Determination What is sexual reproduction? The fusion of gametes Many organisms have 2 mating types or sexes (usually, but not always, male and female) sea urchin egg & sperms
- 18. Sex Determination Chromosomal sex determination XX = female, XY = male X Y X XX XY X XX XY Dad Mom
- 19. XB Y Xb Xb XB Xb XB Xb Xb Y Xb Y X-linked & Y-linked loci do not assort independently from sex chromosomes
- 20. Single gene human disorders Recessive: Albinism Cystic fibrosis Phenylketonuria Sickle cell disease Dominant: Dwarfism Alzheimer’s (one type) Huntington’s disease
- 21. Single gene human disorders Recessive: Cystic fibrosis 1 in 1,800 European Americans Phenylketonuria 1 in 10,000 in US & Europe Sickle cell disease 1 in 500 African americans Dominant: Dwarfism 1 in 25,000 Alzheimer’s (one type) unknown Huntington’s disease 1 in 25,000
- 24. Often Environment Matters! Sometimes, genes and environment interact fairly simply Temperature-dependent expression of coat color genes in both Himalayan rabbits and Siamese cats
- 25. Complications..... Incomplete dominance (parental phenotypes “blend”) Pleiotropy (1 locus affects multiple traits) Complex traits (multiple loci encode a single trait) Epistasis (interactions between loci) Linkage (non-independent segregation of loci) Phenotypic plasticity (expression of genotype depends on the environment) & many more......
Editor's Notes
- Chromosomes condense during cell division and can be ordered and counted = a karyotype. Within an individual and within a species, virtually all cells have same karyotype Here is human karyotype- 23 pairs of homologous chromosomes = pair of chromosomes with same sequence of genes, one of each pair from mom & other dad e.g., if gene for eye color is in a sp band on chromosome one, gene for eye color is in same band on other of pair of chromosome one = locus You can have different forms of the gene at these two locations however! = alleles one from Mom & other from Dad! We are diploid organisms because for every chromosome – we have a full set from each parent One pair isn’t entirely homologous = sex chromosomes (notice reduced Y), Remaining chromosomes are autosomes
- DNA allows for genetic information to be passed from one generation to the next! Because DNA replicates and cells divide – information gets passed on and around
- Mendel discovered the basic rules of inheritance while working with peas in his Abbey’s garden in the 1860’s (Brunn, Austria) Published his work in 1866 (just 7yrs after Darwin’s Origin of Species!) which argued: Discrete heritable factors are responsible for inherited traits These factors are passed from parents to offspring Factors do not change identity between generations they are like different colored marbles, marbles keep their color regardless of how they are mixed up or how many there are We know now that Mendel’s ‘factors’ are genes! And that these genes are carried on chromosomes from parents to young! Mendel crossed peas and scored their phenotypes for several discrete traits (e.g. pea color, texture) Consistently found that when you cross a true breeding yellow and a true breeding green (AAxaa) you get all yellow peas (Aa) Then cross two of these F1s (AaxAa) and you get ¼ of the young go back to green!!!! He deduced two fundamentals from his experiments...
- Which of two alleles a parent passes on is random (a 0.5 chance of getting either in a gamete, which gamete you get is random) This process is independent for all unlinked loci – so the allele you get a one locus doesn’t depend on the allele you got at another
- To show the principle of segregation (alleles in to gametes and progeny is random).... For a single locus, cross two heterozygotes, a single progeny is just as likely to get the A allele as the a allele from a parent Intro the Punnett square: one parent on each side, list all possible gametes they can contribute Then at fertilization (when gametes fuse), any progeny is equally likely to get any of the four possible combinations of gametes These are the cells within the square So – alleles segregate independently during gametogenesis and combine at random
- To show the principle of independent assortment (alleles at different loci assort in to progeny randomly).... Now consider two loci – now each parent 4 possible gametic combinations and a cross of two double hets means there are 16 possible genotypes (notice that there are 4 phenotypes) So here, the alleles as one locus are not dependent on the alleles at the other! If they were....say yellow was always smooth (YS) and green was always wrinkled (ys travel together – are linked), then the progeny would all be one of these two phenotypes. You’d never get the two traits mix to form yellow wrinkled for example.
- The trait of sex is determined according to these simple rules...usually! Exception e.g. is temperature dependent sex determination, hermaphroditism, etc.
- We have an XY system - as do other mammals and flies and many other critters Birds also heterogametic but have a ZW system and females are heterogametic (ZW) while males are homogametic
- IN this case, traits with genes that are on the X or Y will not associate randomly with the trait of sex, the two are linked and travel together
- Also dominant – freckles, widow’s peak, free earlobes, extra fingers and toes, webbed fingers and toes, if you fold your hands and left thumb on top – that is dominant (try it!) Many human disorders have simple one gene inheritance – here are examples These can be sorted out by pedigrees (necessary in humans where we cannot control the crosses....) Fewer lethal disorders are dominant than recessive b/c a dominant lethal allele cannot be carried by a het without killing the person. The lethal diseases of this kind that persist usually cant be detected until later in life after people have already had their kids. Eg Huntington’s – degeneration of nervous system begins at middle age (dwarfism isn’t lethal and two dwarfs can have a normal sized child! What’s the probability?)
- The frequency of the dom or rec alleles and thus the disorder changes among populations! This is how inheritance is linked to evolution....
- Certain type of deafness found in high frequency on Martha’s Vineyard (island off our East Coast) One family pedigree... Go over symbols & fill in genotypes
- Here are some examples of recessive human disorders coded for on a single X-linked locus... Women are carriers without knowing, males more likely to have the disorder Hemophilia – blood doesn’t clot, here in Russia’s royal family pedigree Go through pedigree and assign genotypes. Same pattern with red-green color blindness...
- The phenotype does not always mirror the genotype For example, different environments can make indiv with the same genotypes look different or... Can make indiv with different genotypes look the same. In this example, temperature dictates the expression of coat color genes that differ among these critters. Often GenxEnv interactions are very complex.
- In fact, all of this gets really complex quickly! There are many complications beyond the simple concepts we’ve worked with so far. However, Mendelian genetics still largely forms the basis for how we approach the study of genetics or heredity. Next time – Quiz at beginning of class over – this lecture, DNA forensics and blood typing labs Lecture on forces that cause frequencies of alleles to change over time – EVOLUTION!!!