ap bio ch. 15

20th century, Thomas Hunt Morgan

first solid evidence associating specific gene w/ specific chromosome; prov. evidence that chromosomes r location of Mendel’s heritable factors

Drosophila melanogaster

convenient b/c: produce many offspring, generation can be bred every 2 wks, only 4 pairs of chromosomes

wild type

normal, phenotypes that were common in the fly populations

mutant phenotypes

alternative to wild eyes (recessive); only males had white eyed mutant allele (must be on X chromosome) which supported chromosome theory of inheritance

human sex chromosomes

larger X & smaller Y; only ends of Y chromosome have regions that are homologous w/ corresponding regions of the X chromosome (allows them to behave like homologues during meiosis in males)

SRY/sex-determining region on the Y chromosome

gene responsible for dev. of testes in an embryo

sex-linked gene

located on either sex chromosome

Y-linked genes

genes on the Y chromosome; few of these; most r related to sex determination

X-linked genes

genes on the X chromosome; have genes for many characters unrelate dto tex

recessive X-linked trait inheritance

female needs 2 copies of allele (homozygous), male needs only 1 copy of allele (hemizygous) → X-linked recessive disorders r much more common in males than in females

recessive X-linked disorders

color blindness (mostly X-linked), duchenne muscular dystrophy, hemophilia

X inactivation in female mammals

one of the two X chromosomes in each cell is randomly inactivated during embryonic dev; the inactive X condenses into a Barr body. If a female is heterozygous for a gene located on the X chromosome, she will be a mosaic for that character

Barr body

when one of the two X chromosomes in each cell (of female mammal) is randomly inactivated during embryonic dev; the inactive X condenses into a Barr body

each chromosome has:

hundreds or thousands of genes (except Y chromosome)

linked genes

located on the same chromosome, tend to be inherited tgt

Morgan crossed flies

that differed in traits of body color & wing size (to see how linkage affects inheritance of two characters) → found that body color & wing size r usu inherited tgt in specific combinations (parental phenotypes, on the same chromosome), also produced nonparental phenotypes

genetic recombination

production of offspring w/ combinations of traits differing from either parent

parental types

offspring w/ a phenotype matching one of the parental phenotypes

recombinant types (recombinants)

offspring w/ nonparental phenotypes (new combinations of traits); 50% frequency of recombination is observed for any two genes on diff chromosomes

crossing over of homologous chromosomes

Morgan discovered that genes can be linked, but the linkage was incomplete b/c some recombinant phenotypes were observed → proposed the same process must ocassionaly break the physical connection btwn genes on the same chromosome

recombination frequency

# of recombinants/total offspring

recombinant chromosomes

bring alleles together in new combinations in gametes; random fertilization even further inc. # of variant combinations that can be produced; raw material upon which natural selection works

genetic map

ordered list of the genetic loci along a particular chromosome; Alfred Sturtevant (one of Morgan’s students) constructed this

Sturtevant predicted:

he farther apart two genes are, the higher the probability that a crossover will occur between them & thus the higher the recombination frequency

linkage map

genetic map of a chromosome based on recombination frequencies

map units

dist btwn genes; one map unit (or centimorgan) represents 1% recombination frequency. Indicate relative distance & order, not precise locations of genes

genes far apart on the same chromosome

have a recombination frequency near 50%; physically linked but genetically unliked & behave as if found on diff chromosomes

Sturtevant used recombination frequencies

to make linkage maps of fruit fly genes; he & his colleagues found that the genes clustered into 4 groups of linked genes (linkage groups). The linkage maps + 4 chromosomes in Drosophila prov. additional evidence that genes r located on chromosomes

large-scale chromosomal alterations in humans & mammals

often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders; plants tolerate such genetic changes better than animals do

nondisjunction

pairs of homologous chromosomes do not separate normally during meiosis. One gamete receives two of the same type of chromosome, and another gamete receives no copy

aneuploidy

results from the fertilization of gametes in which nondisjunction occurred; offspring have abnormal # of a particular chromosome

monosomic zygote

only one copy of a particular chromosome

trisomic zygote

three copies of a particular chromosome

polyploidy

organism has more than two complete sets of chromosomes; common in plants but not animal; more normal in appearance than aneuploids

triploidy (3n)

3 sets of chromosomes

tetraploidy (4n)

4 sets of chromosomes

breakage of chromosome can leda to 4 types of changes in chromosome structure: deletion, duplication, inversion, translocation

deletion

removes a chromosomal segment

duplication

repeats a chromosomal segment

inversion

reverses orientation of a segment within a chromosome

translocation

moves a segment from one chromosome to another

human disorders

alterations of chromosome # & structure are asso. w/ some serious disorders. Some types of aneuploidy appear to upset the genetic balance less than others, resulting in individuals surviving to birth and beyond. These surviving individuals have a set of symptoms, or syndrome, characteristic of the type of aneuploidy

Down syndrome

aneuploid condition that results from three copies of chromosome 21; affects 1-700 children in the US; frequency inc. w/ age of mother

aneuploidy of sex chromosomes

produced in nondisjunction of sex chromosomes; XXX females are healthy, with no unusual physical features

Klinfelter syndrome

result of an extra chromosome in a male, producing XXY individuals

monosomy X/Turner syndrome

produces X0 females (sterile); only known viable monosomy humans

syndrome cri du chat (“cry of the chat”)

results from a specific deletion in chromosome 5. Child born w/ this syndrome is severely intellectually disabled and has a catlike cry; individuals usually die in infancy or early childhood

certain cancers, including chronic myelogenous leukemia (CML)

are causes by translocations of chromosomes

2 normal exceptions to Mendelian genetics:

(1) genes located in the nucleus, (2) genes located outside the nucleus. In both cases, the sex of the parent contributing an allele is a factor in the pattern of inheritance

genomic imprinting

for a few mammalian traits, the phenotype depends on which parent passed along the alleles for those traits; the variation in the phenotype is called genomic imprinting (inv. silencing of certain genes depending on which parent passes them on)

genomic imprinting is the result of

methylation (addition of -CH3) of cysteine nucleotides; thought to only affect a small fraction of mammalian genes; most critical for embryonic dev.