Chromosomal Inheritance Chapter 15

Morgan and Fruit Flies Thomas Hunt Morgan – experimental embryologist Evaluated Mendel’s findings using fruit flies (Drosphila melongaster) Fast reproduction Large offspring production 4 pairs of chromosomes (1 sex, 3 autosomes) Wild type – phenotype found most commonly observed in natural populations (w+) Mutant type – versions due to assumed allele changes due to mutations (w)

Morgan and Sex-linked Traits Morgan mated white eyed (w) males with red eyed (w+) All red eyes – red is dominant Bred F1 – classical 3:1 ratio Finding: All the females had red eyes while half males had white and half had red eyes Assumption: Eye color is linked to its sex Females have 2 XX chromosomes, males have XY Since white eyes is the mutant (w+) type and is recessive, and only females had the red eyes (w) because two versions can exist there was no second allele on an X chromosome to override the recessive allele

Chromosomes and Sex Inheritance Males develop from zygotes containing one X and one Y Y chromosome contains the SRY region – sex determining region Required for development of the testes Genes located on sex chromosomes are called sex linked genes Y chromosomes are passed on virtually intact from father to son Very few genes on the Y chromosomes-means very few disorders transferred on Y chromosome to son Anatomical signs begin to develop at about 2 months in the embryo Other types of sex determination Grasshoppers, cockroaches and some other insects-only one sex chromosome X – females XX, males XO Birds, fish, some insects – The sex chromosome in the egg determines the sex-ZW female, ZZ male Bees and ants – haplo-diploid – no sex chromosomes, females develop fertilized eggs (diploid) and males non-fertilized (haploid)

X-Linked Inheritance X chromosome contains ~1,100 genes Fathers pass all of their X alleles to their daughters Mothers can pass X-linked alleles to sons or daughters More males than females will exhibit X-linked disorders than females Ex. Duchenne muscular dystrophy, Hemophilia, color blindness If trait is sex linked recessive, female will only express if she is homozygous for the allele Males can not be homozygous since they lack the other allele – they are therefore hemizygous – any male receiving a recessive allele will express the disorder

X Chromosome Inactivation Early in development, one of the female chromosomes becomes inactivated Insures the same dose of X-linked genes in males and females Results in a Barr body Selection of which X chromosome is completely random and independent during development Barr body is a condensed object which lies along the inside of the nuclear envelope – most of the genes in the barr body are not expressed After X chromosome inactivation all cells mitotically derived from that cell will have that same X chromosome inactivated – thus is she heterozygous for a trait, about half her cells will express one allele and half will express the other – mosaicism Some human females will have patches of sweat glands and patches lacking

Gene Linkage and Recombination Certain genes tend to be inherited together – linked genes Some offspring inherit matches either of the parent phenotypes – parental types Some will inherit traits that are new – recombinant types/recombinants If two genes have a recombinance of 50% they are said to be on separate chromosomes Crossing over leads to recombinants The further the distance between two genes the more likely for recombinance

Mapping Genes Alfred Sturtevant created an ordered list of the genes on a chromosome – genetic map Used the recombinant frequency calculated from experiments using fruit flies to determine distance between them The further the distance the greater the chance for crossing over between 2 genes-high recombination frequency Created a linkage map that showed the position of the gene on the chromosome – expressed in map units Different than a cytogenic map which has the actual position based on staining patterns of chromsomes

Abnormal Chromosome Numbers Chromosomes fail separate during meiosis – nondisjunction event Results in a gamete with either an extra or missing chromosome If fertilization occurs the zygote will exhibit aneuploidy – abnormal chromosome number in a zygote Monosomic – the zygote is missing a chromosome Trisomic – the zygote has an extra chromosome Some organisms will have extra complete chromosome sets – polyploidy Ex. Plants, some fish and amphibians Down syndrome Trisomy 21 Characteristic facial feature, short stature, correctable heart defects, developmental delays Shorter life span than most Frequency increases with age - .04 under 30 increase .92 over 40 and continues to increase Monosomy X Turners syndrome 1 in 2500 births Only known viable monosomy in humans Sterile and do not mature Normal intelligence With estrogen, can develop seconday sex characterisitics XXX 1 in 1000 births Healthy, no unusual appearance Slightly taller than average At risk for learning disabilities XYY Below normal intelligence Normal sexual development Somewhat taller than average XXY Klinfelters syndrome 1:500-100 births Testes are small, male is sterile Even though X chromosome is inactivated, male will exhibit some female characteristics

Chromosome Mutations Problems during crossing over or radiation can lead to chromosome damage Deletion - Chromosome fragment lost Duplication – detached fragment attaches to non-sister chromatid of a homologous chromosome Inversion – chromosome reattaches but in the reverse order Translocation – the fragment attaches to a nonhomologous chromosome