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Std.8 Genetics Review Summarize Mendel’s laws of genetics. Create and analyze a punnett square to demonstrate these laws. Explain how phenotype is related to genotype. Distinguish between homozygous and heterozygous. (terms: alleles, dominant, recessive) _________________ alleles need only 1 allele to be expressed; ____________________ alleles must have two alleles to be expressed. Define genotype: ____________________________________________________________________________ Define phenotype: ___________________________________________________________________________ Identify sex chromosomes for female: ______________ ; sex chromosomes for male: _____________ Question Analysis If a corn plant has a genotype of Ttyy, what are the possible genetic combinations that could be present in a single grain of pollen from this plant? (show your work) The following genotypes were found in a male cat and a female cat. BbSs (male) bbSS (female) Which one of the following choices is true of the phenotype of offspring from these parents? a. All offspring will have black fur. b. All offspring will have white fur. c. All offspring will have long-haird fur. d. offspring will have short-haired fur. Based only on the sex chromosomes in typical human egg and sperm cells at fertilization, what is the probability of producing a female? Prove your answer with a punnett square. In fruit flies, the gene for red eyes (R) is dominant and the gene for sepia eyes (r) is recessive. What are the possible combination of genes in the offspring of two red-eyed heterozygous flies (Rr) In certain breeds of dogs, deafness is due to a recessive allele (d) of a particular gene, and normal hearing is due to its dominant allele (D). What percentage of the offspring of a normal heterozygous (Dd) dog and a deaf dog (dd) would be expected to have normal hearing? A genetic disorder due to a recessive allele (a) is lethal in homozygous individuals (aa), whereas heterozygous individuals (Aa) have no symptoms. Based on this information, which of the following is likely to result? a. The disorder will quickly be eliminated since no recessive homozygotes will survive to reproduce. b. The disorder will be maintained in the population through the reproduction of heterozygotes. c. Only homozygous dominant (AA) individuals will survive. d. The prevalence of the disorder will increase over time. Why? Std.8 Genetics Study Guide I. Genetics * Gregor Mendel - studied plants & discovered principles of inheritance → rules of genetics Dominance – can be complete, incomplete or codominant Segregation – alleles segregate (separate) during gamete formation → meiosis Independent assortment – alleles line up and separate independently during gamete formation. Alleles – single form of a gene; come in pairs (Bb, Tt) II. Genes and dominance P-generation = parents (parental generation) F1 generation = first filial generation → offspring F2 generation = second filial→ second generation *Dominant alleles are expressed over recessive alleles (masked) *Allele pairs segregate (separate) during gamete formation Dominant traits are expressed with a capital letter (T) (R) (S) while recessive alleles are expressed with lower case letters (t) (r) (s). Phenotype – physical characteristics (color, height) Genotype – genes/alleles → BB, Tt, Ss 2 recessive alleles must be present in order for the trait to be expressed phenotypically. III. Dihybrid Crosses Crosses involving 2 traits at once (height & color) Foil parents genotype to find gametes o TtRr gametes = TR, Tr, tR, tr Standard dihybrid cross F2 generation 9:3:3:1 ratio. IV. Human Chromosomes Humans = 46 chromosomes o 2 sex chromosomes → X and Y o 44 autosomes * sex – male (XY) & female (XX) is determined by father Karyotype → photos of chromosomes cut & grouped together; shows abnormalities → extra/missing chroms. * Human genome is made of thousands of genes V. Genetic Disorders * Many genetic disorders are caused by recessive alleles * A small change in DNA (mutation) of a single gene can affect the structure of a protein causing a serious genetic disorder Nondisjunction = failure of homologous chromosomes to separate during meiosis (Down’s Syndrome) Sex chromosome disorders – result from nondisjunction of sex chromosomes o Turners Syndrome – XO in females o Kleinfelter’s Syndrome – XXY in males Sex-Linked Genes – o Genes located on the X or Y chromosome o Many genes are located on the X-chromosome * Since males only have a single X-chromosome, all X-linked traits will be expressed even if recessive o Examples = color blindness & hemophilia