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AP Biology Chromosomal Basis of Inheritance Chapter 15
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AP Biology Morgan & Drosophila Thomas Hunt Morgan - first to associate a specific gene with specific chromosome in early 20th century Drosophila breeding good genetic subject prolific 2 week generations 4 pairs of chromosomes 3 pair autosomes, 1 pair sex chromosome XX=female, XY=male 1910 | 1933
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AP Biology autosomal chromosomes sex chromosomes Classes of chromosomes
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AP Biology Wild Type Morgan looked for varied traits Discovered a single male fly with white eyes instead of the usual red The normal character phenotype is wild type Alt traits are mutant phenotypes
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AP Biology Huh! Sex matters?! F 2 generation 100% red-eye female 50% red-eye male 50% white eye male Discovery of sex linkage P X F 1 generation (hybrids) 100% red eye offspring true-breeding white-eye male true-breeding red-eye female
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AP Biology RRrr What’s up with Morgan’s flies? x rr R R Rr 100% red eyes Rr x Rr R r RR Rrrr Rr 3 red : 1 white Doesn’t work that way!
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AP Biology In humans & other mammals, there are 2 sex chromosomes: X & Y 2 X chromosomes develop as a female: XX gene redundancy, like autosomal chromosomes an X & Y chromosome develop as a male: XY no redundancy Genetics of Sex XY X X XX XY 50% female : 50% male XX
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AP Biology XRXRXRXR XrYXrY Let’s reconsider Morgan’s flies… x XrXr Y XRXR 100% red eyes XRXR XRXrXRXr XRYXRY XRYXRYXRXrXRXr x XRXrXRXr XRYXRY XRXR Y XRXR XrXr XRXrXRXr XRYXRYXRXRXRXR XrYXrY 100% red females 50% red males; 50% white males BINGO!
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AP Biology Sex-linked Gene Sex-linked genes appear on X chrom Females (XX) may have 2 red-eyed alleles or may be heterozygous (all have red eyes) Males (XY) have only 1 allele and will be red eyed ONLY if they have red-eyed allele (otherwise white allele – white)
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AP Biology Genes on sex chromosomes Y chromosome few genes other than SRY sex-determining region master regulator for maleness turns on genes for production of male hormones many effects = pleiotropy! X chromosome other genes/traits beyond sex determination mutations: hemophilia Duchenne muscular dystrophy color-blindness
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AP Biology Sex-linked usually means “X-linked” more than 60 diseases traced to genes on X chromosome Human X chromosome
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AP Biology Map of Human Y chromosome? < 30 genes on Y chromosome Sex-determining Region Y ( SRY )
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AP Biology Why is crossing over not as common with the sex chromosomes? What types of genes are on the sex chromosomes? If there is a gene at the top of the X chromosome, how many alleles does a man have for that gene? Sex- linked gene
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AP Biology
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Hemophilia Hh x HH XHYXHY XHXhXHXh XHXhXHXh XHXH XhXh XHYXHY Y XHXH sex-linked recessive XHXH Y male / sperm XHXH XhXh female / eggs XHXHXHXH XHXhXHXh XHYXHYXhYXhY XHXHXHXH XHYXHY XHXhXHXh XhYXhY carrierdisease
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AP Biology X-inactivation Female mammals inherit 2 X chromosomes one X becomes inactivated during embryonic development condenses into compact object = Barr body which X becomes Barr body is random patchwork trait = “mosaic” XHXH XhXh XHXhXHXh patches of black patches of orange tricolor cats can only be female
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AP Biology Male pattern baldness Sex influenced trait autosomal trait influenced by sex hormones age effect as well = onset after 30 years old dominant in males & recessive in females B_ = bald in males; bb = bald in females
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AP Biology 1. If a gene for male pattern baldness is carried on the X chromosome, who in the family would pass the trait to their son? 2. Since females have two X chromosomes and males only have one, women end up with excess genetic info. What does the cell do to make up for this extra amount of DNA? 3. So, if one X chromosome is turned off, why are women not as susceptible as men to sex-linked traits such as color-blindness, balding, MD, and hemophilia? Learning Check
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AP Biology Cell 1- Diploid cell w/ homologous chromosomes. traits A and B are on different chromosomes. Heterozygous. Chance that they will produce gamete with genotype AB? aB? Ab? Or ab? (50%) Cell 2- genes A and B are on opposite ends of the same chromosome Chance that they will produce gamete with genotype AB? aB? Ab? Or ab? (recombination frequency will be 50% due to crossing over) Cell 3- genes are on the same chromosome close together Probability of recombination decreases because the genes are less likely to cross over, segregate and independently assort. They will be “linked”. Gene linkage
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AP Biology
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As genes are located closer to one another, what happens to their recombination frequency? Learning Check
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AP Biology If gene A recombines 12% of the time with gene B, but 16% of the time with gene C, which gene is closer to gene A? If gene B and C recombine with one another 28% of the time, what is the order of genes A, B, and C along the chromosome? Recombination frequencies BAC 1612 28
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AP Biology The following is a map of four genes on a chromosome: Between which two genes would you expect the highest frequency of recombination? A) A and W B) W and E C) E and G D) A and E E) A and G Learning Check
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AP Biology Lab 7: Genetics (Fly Lab)
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AP Biology Lab 7: Genetics (Fly Lab) Description given fly of unknown genotype use crosses to determine mode of inheritance of trait
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AP Biology Lab 7: Genetics (Fly Lab) Concepts phenotype vs. genotype dominant vs. recessive P, F1, F2 generations sex-linked monohybrid cross dihybrid cross test cross chi square
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AP Biology Lab 7: Genetics (Fly Lab) Conclusions: What is the genotype of the parent generation? Case 1 Case 2
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