The Chromosomal Basis of Inheritance Chapter 15 Chromosomal

The Chromosomal Basis of Inheritance Chapter 15

Chromosomal Theory of Inheritance § Genes have specific loci (locations) on chromosomes and it is the chromosomes that undergo segregation and independent assortment

Sex-linked Genes § A gene located on a sex chromosome (typically the X chromosome) § Fathers pass sex-linked alleles to all of their daughters but not to their sons § Mothers can pass sex-linked alleles to both sons and daughters § Females rarely show a recessive sex-linked trait because they need two of the alleles § If the mother passes the sex-linked allele to her son, he WILL express it

Discovery of Sex-linked Traits § Thomas Hunt Morgan § Wild-type (w+) – what is normally observed; ex red eyes for fruit flies § Mutant (w) – alternatives to the wild-type; ex - white eyes § Crossed a white eyed male with a purebred eyed female § F 1 was all red eyed § F 2 showed the expected 3: 1 ratio but only the males were white eyed

Morgan’s Conclusions § The gene must be linked to gender; otherwise the results would have also shown female white eyed flies § w+ is dominant over w because the F 1 all had red eyes § The gene must be on the X chromosome because males only have one § A female fly must be ww to have white eyes

Human Sex-Linked Disorders § Color-blindness § Duchenne Muscular Dystrophy § Hemophilia

X Inactivation in Females § The inactive X condenses into a compact object called a Barr body § The X chromosome that becomes the Barr body occurs randomly and independently in each cell § Females that are heterozygous for a character on the X chromosomes consist of a mosaic of two types of cells: those with the active X derived from the father and those with the active X derived from the mother (usually 50/50) - This means that half of their cells are expressing the dominant trait and the other half are expressing the recessive trait

Linked Genes § Genes located on the same chromosome tend to be inherited together and are called linked genes - Crossing-over may separate the genes if they are not near each other on the chromosome; therefore, the closer the genes are to each other, the greater the chance they will be inherited together § Genes that are not located on the same chromosome are generally not inherited together because of the independent assortment of chromosomes during gamete formation

Morgan & Linked Genes § Morgan’s fruit fly experiments led him to understand linked genes § When studying the characters for body color and wing size he noticed there were generally only two phenotypes: 1. 2. Wild-type – gray body with normal wings Mutants – black bodies with vestigial wings § The offspring with a phenotype identical to a parent = parental type § The offspring with a phenotype different from either parent = recombinants Parental types Recombina nts

Morgan & Linked Genes § The 4 possible phenotypes were observed in the F 2 generation; however, most of the offspring were identical in phenotype to the parents § If more than 50% of the offspring are parental type, the genes are assumed to be linked § If the genes are linked, why were recombinants Numbers of flies for each phenotype 965 observed? Predicted ratio if genes were on different chromosomes Predicted ratio if linked genes & always inherited together 944 206 185 1 1 1 0 0

Genetic Mapping § The discovery of linked genes and recombination due to crossing over led one of Morgan’s students (Alfred Sturtevant) to create a method for mapping the sequence of genes along a particular chromosome § When doing a testcross, if 50% of the offspring are recombinants, then geneticists say there is a 50% frequency of recombination - A 50 % frequency indicates the genes are on different chromosomes § The farther apart two genes are, the higher the probability that a cross over will occur between them and therefore the higher the recombination frequency

Linkage Maps § A genetic map based on recombination frequencies § Sturtevant did a series of experiments studying 3 genes: body color (b), wing-size (vg), cinnabar (cn) § The recombination frequencies are as follows: - cn and b = 9% - cn and vg = 9. 5% - b and vg = 17% • therefore b and vg must be farther apart than cn is from b or vg § The distance between genes is expressed as map units - 1 map unit = 1% recombination frequency - ex) b and vg are 17 map units from each other

Nondisjunction § Occurs when tetrads fail to separate during meiosis I or when sister chromatids fail to separate during meiosis II § Leads to an abnormal number of chromosomes in a gamete § May be missing a chromosome or have an extra one = aneuploidy § Types of aneuploidy: monosomy and trisomy

Human Disorders Caused by Nondisjunction § § § Down syndrome (trisomy 21) Klinefelter syndrome (XXY) XYY syndrome Trisomy X (XXX) Turner syndrome (X)

Chromosome Alterations

Disorders Caused by Chromosomal Alterations § Cri du chat – deletion of a gene on chromosome 5; child’s cry sounds like the meowing of a distressed cat § Chronic myelogenous leukemia – reciprocal translocation; effects white blood cells

Genetic Imprinting § Occurs on autosomes § When the phenotype depends on whether the allele was inherited from the mother or father § Occurs during the formation of gametes and results in the silencing of one allele of certain genes

Genetic Imprinting