biol139-lecture16-2011 - Next topic Next Validation of...

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Unformatted text preview: Next topic Next Validation of Chromosome Theory Validation Sex-linked Traits Sex-linked pp 105 - 113 pp pp 52 - 70 iGenetics Validation of the chromosomal theory of inheritance -heredity information is on genes and -heredity -genes are located on chromosomes -egg and sperm contribute equally to the -egg genetic endowment of offspring through genetic their nuclei How do we know this is true? Circumstantial Evidence so far… Sutton and others demonstrated that the phenotype of sexual identity is associated with the inheritance of particular chromosomes (X, Y) • suggests specific traits carried by specific chromosomes Events of mitosis and meiosis ensure a constant number of chromosomes in the somatic cells of a species: • no other molecule (DNA) is so conserved in each cell • suggests chromosomes are the agent of inheritance Need evidence that… Need 1. The inheritance of genes corresponds to the inheritance of chromosomes The transmission of particular chromosomes coincides with the transmission of specific traits (other than just sex) Need to find gene that is unambiguously linked to chromosome! • mutant allele (distinct phenotype) • on a chromosome morphologically distinct (easy to follow) • transmission of gene must reflect chromosome behaviour during reproduction (segregation) Do chromosomes carry genes that determine traits? Thomas Hunt Morgan Thomas 1910, work with the fruit fly, Drosophila melanogaster to 1910, Drosophila firmly establish the chromosomal theory of inheritance Discovered a gene determining eye colour resides on the X chromosome on Found a mutant male fruit fly with white eye amongst thousands of wild-type red-eyed flies. thousands decided to trace its inheritance Drosophila: Males XY and Females XX Drosophila: XWY XW+ XW+ w+ = wild-type allele (seen in > 1% of population) female male White gene w = mutant allele (seen in <1% of population) Nomenclature: • white eye allele = w + • red eye allele = w (+ signifies wild-type) • sex chromosomes as: Y and X • lower case indicates w recessive to w+ Morgan Crossed pure breeding pure red-eyed female to white-eyed male heterozygote F1 = all red-eyed F1 • red is dominant to white • mutation results in an alternate allele which leads to white eye • both males and females had red eyes. 3:1 ratio of red eyed flies to white eye flies. 2:1 red females:red males Notice: all white-eyed flies are male, all also males are 50:50 red: white also F2 =no white-eyed females ever produced F2 F1 females are heterozygous for Xw+ and Xw Gametes passed to males will be 1:1 red:white Fathers pass only Xw+ to females so females will be either homo or heterozygous for trait Since red is dominant to white, all females are red F2 F2 heterozygous F3 = 1:1:1:1 1:1:1:1 F3 genotypes F3 phenotypes Obtained white-eyed females in F3 generation generation Morgan crossed white eyed females to Red-eyed Males White-eyed females x red-eyed males White-eyed = Red-eyed females + white-eyed males = crisscross inheritance ended up with all white-eyed males ended Gene for eye colour is located on X chromosome white eye male red eye male white eye male red eye male Can observe a pattern of crissCan crisscross inheritance -females inherit -females one X from father, males only inherit X from mother X-linked traits X-linked Results of reciprocal crosses are not identical, Results not in contrast to what Mendel had observed Can observe a pattern of criss-cross inheritance Can criss-cross -females inherit trait from father, males inherit from mother mother Males are said to be hemizygous for genes Males hemizygous on the X chromosome, i.e. their diploid cells have only half the number of alleles carried by have the female on her two X chromosomes …back to theory of chromosomal inheritance 1) the inheritance of genes corresponds to the inheritance of chromosomes chromosomes 2) the transmission of particular chromosomes coincides with the transmission of specific traits (other than just sex) -inheritance of the gene for eye colour corresponds to the inheritance of a particular chromosome (the X chromosome; not just looking at sex phenotype) -tracking X and Y chromosomes allows tracking of the gene for eye colour through simple breeding experiments, without high-tech methodologies Further proof came from extended experiments Bridges allowed his homozygous white-eyed females to cross with the hemizygous red-eyed males every now and then (1 in every 2000), produced white-eyed females and red-eyed males normally; X WX W x X W+Y X WX W+ and X WY white red red female white male BUT 1 in 2000; white female and red male Analysis of rare mistakes in meiosis further supported the chromosome theory of inheritance the Calvin Bridges : Studies involving nondisjunction Calvin nondisjunction (failures in chromosome segregation during meiosis) (failures Can occur in sex or autosomal white female and red male -hypothesized that these were the result of non-disjunction. In other words, a failure of the X chromosomes to segregate during meiosis XWXWY and XW+O white female red male (XXX and OY die) Meiotic Nondisjunction Meiotic Remember: X essential for survival Balance of sex to autosomal important 4 possible zygotes formed if there is nondisjunction of X chromosomes in female gametes: XXX, XXY, OX, and OY sterile • white eyed females are XXY • received both Xw ’s from mother • red eyed male Xw+ (father) + 0 mother -confimed hypothesis by microscopically examining and linking eye colour and sex chromosomes for each of the aberrant females and males -transmission of the white allele for eye colour followed the predicted behaviour of X chromosomes during rare meiotic mistakes White-eyed females were White-eyed iindeed XXY; ndeed Compelling evidence that specific genes lie on specific chromosomes X-linked and Y-linked Traits in Humans are Identified by Pedigree Analysis (also referred to as sex-linked traits) Pedigree Most are recessive - called X-linked recessive trait Females usually must be homozygous for trait homozygous Males only need one recessive copy due to hemizygousity hemizygousity X - Linked Traits First human gene to be assigned to a particular chromosome was the gene for red-green colour-blindness Affected mother II -All females are carriers II -All males affected -All Biggest indicator of Biggest X-linked recessive X-linked All normal Both normal and affected males Females carriers carrier Hemophilia Disease Hemophilia • disease where blood fails to clot properly • rare, recessive mutation on X-chromosome Pedigree of Hemophilia Hh HY HH, HY, Hh, hY died @ 31 yr Hh, HY carrier, non affected hY HH Hh HY hY hh hY affected HH normal carrier female Rare X-linked traits exhibit basic properties: Rare Trait appears in more males than females Trait • since males hemizygous need only 1 copy • females must be homozygous recessive to express Mutation and trait never pass from father to son • fathers only pass Y to sons • father to son inheritance of a rare trait would rule out X-linked recessive Affected male who mates with homozygous normal female will have normal children, but ALL his daughters will become heterozygous carriers heterozygous • daughters normal, but will be carriers x+/x+ x x/Y = x+/x females and x+/Y (normal males) Sons of heterozygous (carrier) mothers should Sons show an approximately 1:1 ratio of normal/affected • X+/X x X+/Y = 1/2 X+/Y and 1/2 X/Y sons • thus trait often skips a generation as passes from grandfather to carrier daughter to sons Mating of a carrier female with a normal male • all daughters will be normal, but 1/2 carriers • X+/X x X+/Y = 1/2 X+/X+ and 1/2 X+/X females •1/2 sons of carrier females will show trait All sons of a homozygous mutant mother should show trait All since males receive their only X from mother since Pedigree of Dominant X-linked trait Pedigree Hypophosphatemia (vitamin D-resistant rickets) Dominant X-linked (very rare) • more females than males show phenotype • all daughters of affected male, but no sons will show trait H + + i.e. affected male give 1/2 affected female X X+ = X X Affected female willXHY x unaffected sons and+daughters, X Y Autosomal genes can also determine Autosomal differences between the sexes differences Sex-limited traits – affect a structure or process that is found in one sex but not the other e.g. bright plumage on male birds egg development or sperm production milk production, horns/antlers Sex-influenced traits – show up in both sexes but their expression may differ between the two sexes because of hormonal differences sexes e.g. patterned baldness e.g. Autosomal gene 007 -premature loss of hair from top of head, but not sides -begins in late 20s in heterozygous men, but heterozygous women show no effect -in homozygous men and women, baldness results in both, but much earlier in men -influenced by sex hormones -dominant in men, recessive in women ...
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This note was uploaded on 10/04/2011 for the course BIOL 139 taught by Professor Christinedupont during the Spring '10 term at Waterloo.

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