AP Review Chapters 13-15

Fast Facts Heredity: the transition of traits from one generation to the next Along w/ similarities, there is also variation- offspring differ somewhat in appearance from parents to offspring Parents pass information to offspring through coded hereditary units called genes. - 30 to 40 thousand genes in humans - genes are segments of DNA

Fast Facts The human life cycle - each somatic cell has 46 chromosomes (23 pair); 2 copies of each pair are called homologous chromosomes - chromosomes can be displayed through a karyotype - pairs 1-22 are called autosomes - pair 23 are called sex chromosomes

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Fast Facts - gametes, or sex cells, contain 22 autosomes and 1 sex chromosome; a cell with a single chromosome set is called a haploid cell. - haploid cells are abbreviated 1n (n = 23) - the combining of gametes forms a zygote; becomes a diploid cell (2n)

Fast Facts Meiosis, like mitosis, is preceded by the replication of chromosomes; however, the single replication is followed by 2 consecutive divisions (meiosis I and meiosis II)

Fast Facts Meiosis I - during prophase I, chromosomes pair up in synapsis; 4 chromatids form a tetrad - during metaphase I, homologous pairs line up on equator - during anaphase I, chromosomes, not chromatids, separate to poles

Fast Facts Meiosis II - goes through the same steps as meiosis I, but does not replicate DNA Meiosis outcome is 4 1n gametes from a single cell

Fast Facts Crossing over - when chromosomes line up along the equator, parts of chromatids can combine genes from parents

Sample Questions Meiosis reduces chromosome number and rearranges genetic information. a) Explain how the reduction and rearrangement are accomplished in meiosis b) several human disorders occur as a result of defects in meiosis. Identify one and explain how this could have occurred.

Fast Facts Punnett squares are used to predict the results of a genetic cross between individuals of known genotypes.

Fast Facts Homozygous: when the alleles for a gene are identical Heterozygous: when the alleles for a gene are different Phenotype: physical appearance Genotype: genetic makeup

Fast Facts A testcross can be used to determine the genotype of an organism that displays a dominant trait. - ex. Is the purple flower homozygous or heterozygous? Cross it with a recessive (white) flower. The results will tell the genotype.

Fast Facts A cross in which only one character is discussed is a monohybrid cross. If two characters are discussed, the cross is a dihybrid cross Incomplete dominance - when the F1 hybrids have an appearance somewhere in between the phenotypes of the parents

Fast Facts Codominance/Multiple Alleles - the two alleles affect the phenotype in separate distinguishable ways. - ex. A, B, O blood types Pleiotropy - the ability of a gene to affect an organism in many ways - ex. the alleles for sickle-cell disease cause multiple symptoms

Fast Facts Epistasis - a gene at one locus alters the phenotypic expression of a gene at a second locus Polygenic inheritance - the additive effect of two or more genes on a single character; not an “either or” situation - converse of pleiotropy where a single gene affects several phenotypic characters

Fast Facts Pedigree: a family tree that describes the relationships between generations for a particular trait - pedigrees are used to map out specific phenotypic traits

Fast Facts Common recessive genetic disorders - Cystic fibrosis: 1/2500 people of European decent; 1/25(4%) are carriers - Tay-Sachs disease: - sickle-cell disease: 1/400 African-Americans Common dominant genetic disorders - Anchondroplasia: form of dwarfism; - Huntington’s disease: no obvious phenotypic effect until 35 to 45 yrs. old

Fast Facts Chromosomal theory of inheritance: genes have specific loci on chromosomes, and the chromosomes undergo segregation and independent assortment Genes located on sex chromosomes are called sex-linked genes - Morgan’s evidence that a specific gene is carried on the X chromosome helped confirm the chromosomal theory of inheritance.

Fast Facts linked genes: genes that are located on the same chromosomes and tend to be inherited together - linked genes deviate from expected Mendelian ratios -ex. in flies body color and wing shape are inherited together

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Fast Facts Geneticists can use recombination data to map a chromosomes genetic loci Genetic map: an ordered list of the genetic loci along a particular chromosome Linkage map: a genetic map based on recombination frequencies

Fast Facts Map of body-color (b), wing-size (vg) and cinnabar (cn) - cn and b is 9% - cn and vg is 9.5% - b and vg is 17%

Fast Facts Sex-linked disorders in humans - muscular dystrophy: 1/3500 males in the US - hemophilia: absence of proteins for blood clotting Nondisjunction: the members of a pair of homologous chromosomes do not move apart properly during meiosis I or II. - gametes are (n+1) or (n-1) if they have too many or too few chromosomes

Fast Facts Aneuploidy: having an abnormal chromosome number Trisomic: a chromosome in triplicate (2n+1); ex. trisomy 21 (Down’s syndrome) Monosomic: if a chromosome is missing (2n-1)

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Fast Facts Polyploidy: organisms that have more than two complete chromosome sets - triploidy (3n) - tetraploidy (4n) Polyploids are more normal in appearance than aneuploids

Fast Facts Breakage of a chromosome can lead to four types of changes in chromosome structure Deletion: occurs when a chromosomal fragment lacking a centromere is lost during cell division

Fast Facts Duplication: a fragment becomes attached as an extra segment to a sister chromatid Inversion: a chromosomal fragment can reattach to the original chromosome in reverse order Translocation: the fragment joins a nonhomologous chromosome

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Fast Facts Alterations in chromosome number and structure are associated with several human disorders - Down syndrome: 1/700 children born; extra 21st chromosome - Klinefelter syndrome: XXY male - Turner syndrome: XO female

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Sample Question In a laboratory population of diploid, sexually reproducing organisms a certain trait is determined by a single autosomal gene and is expressed as two phenotypes. A new population was created by crossing 51 pure-breeding (homozygous) dominant individuals with 49 pure breeding (homozygous) recessive individuals. Generation Dominant Recessive Total 1 51 49 100 2 280 0 280 3 240 80 320 4 300 100 400 5 360 120 480 a) Identify an organism that might have been used to perform this experiment, and explain why this organism is a good choice for conducting this experiment. b) On the basis of the data, propose a hypothesis that explains the change in the phenotype frequency between generation 1 and generation 3.

In a laboratory population of diploid, sexually reproducing organisms a certain trait is determined by a single autosomal gene and is expressed as two phenotypes. A new population was created by crossing 51 pure-breeding (homozygous) dominant individuals with 49 pure breeding (homozygous) recessive individuals. Generation Dominant Recessive Total 1 51 49 100 2 280 0 280 3 240 80 320 4 300 100 400 5 360 120 480 a) Identify an organism that might have been used to perform this experiment, and explain why this organism is a good choice for conducting this experiment. b) On the basis of the data, propose a hypothesis that explains the change in the phenotype frequency between generation 1 and generation 3.