Asexual Reproduction Vegetative propagation Binary Fission Budding Sporogenesis

Chromosomes and InheritanceChapter 15

Meiosis Oogenisis Spermatogenisis http://highered.mcgraw-hill.com/olcweb/cgi/p

luginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120074/bio19.swf::Stages%20of%20Meiosis

Chromosome Theory of Inheritance Mendelian genes have

specific loci along chromosomes, and it’s the chromosomes that undergo segregation and independent assortment.

Thomas Hunt Morgan First to associate a specific gene with a

specific chromosome Drosophila melanogaster = fruit fly

Fruit Flies Wild type = normal character phenotype Mutant = alternative traits

Morgan’s cross: White-eyed male with a Red-eyed female All the F1 offspring had red eyes

What would you conclude? The red allele is dominant to the white allele

But then… Crosses between F1 offspring produced 3:1

phenotypic ratio in F2 offspring The white-eyed trait appeared only in males Morgan concluded that a fly’s eye color was

linked to its sex

sex-linked gene

The gene with the white-eyed mutation is on the X chromosome

linked genes Tend to be inherited together

the chromosome is passed along as a unit

Genetic recombination Offspring with new combinations of traits

inherited from two parents

Result from: independent assortment of genes located on

nonhomologous chromosomes crossing over of genes located on homologous

chromosomes

SRY Gene Anatomical signs of sex first appear when the

embryo is about 2 months old Presence of SRY gene - sex determining

region of the Y chromosome) generic embryonic gonads are modified into

testes

Sex Linked Genes

Heterozygous females will be carriers

Any male receiving the recessive allele from his mother will express the trait

Examples Duchenne muscular dystrophy

Absence of an X-linked gene for a key muscle protein, called dystrophin

Characterized by a progressive weakening of the muscles and loss of coordination

Hemophilia Absence of one or more clotting factors Prolonged bleeding because clots form slowly

Barr body During female development, one X

chromosome per cell condenses into a compact barr body

This inactivates most of its genes Reactivated in ovarian cells that produce ova

Females consist of a mosaic of cells some with an active paternal X others with an active maternal X

Alfred Sturtevant Chromosome map

Constructed using crossing over of linked genes Ordered list of the genetic loci along a particular

chromosome

body color and wing shape are usually inherited together because their genes are on the same chromosome

linkage map used recombination frequencies from fruit fly

crosses to map the relative position of genes along chromosomes

frequency of recombinant offspring reflected the distances between genes on a chromosome

Genetic recombination Parental types = phenotypes that match the original

parents Recombinants = new combination of parental traits

dihybrid cross combination of traits that did not match either parent crosses between hybrid plants produces four phenotypes

map units One map unit is equivalent to a 1%

recombination frequency

map units Some genes on a chromosome are so far apart

that a crossover between them is virtually certain.

In this case, the frequency of recombination reaches is its maximum value of 50% the genes act as if found on separate

chromosomes and are inherited independently

Nondisjunction Problems with the meiotic spindle cause

errors in daughter cells

Aneuploidy Trisomic cells three copies of a particular

chromosome type 2n + 1 total chromosomes

Monosomic cells only one copy of a particular chromosome type 2n - 1 chromosomes

Polyploidy Organisms with more than two complete sets

of chromosomes Relatively common among plants and much

less common among animals fishes and amphibians have polyploid species

Aneuploidy vs. Polypoidy One extra or missing chromosome upsets the

genetic balance during development more than does an entire extra set of chromosomes

changes in chromosome structure deletion a chromosome fragment lacking a

centromere is lost during cell division duplication a fragment becomes attached

as an extra segment to a sister chromatid

changes in chromosome structure inversion a chromosomal fragment

reattaches to the original chromosome in the reverse orientation

translocation a chromosomal fragment joins a nonhomologous chromosome

Outcome Most of these alterations are so disastrous that

the embryos are spontaneously aborted long before birth

Down syndrome 3 copies of chromosome 21 affects 1 in 700 children born in the US

Sex Chromosome Disorders Klinefelter’s syndrome

XXY male Occurs 1 in every 2000 live births have male sex organs, but are sterile

Turner’s syndrome XO female (monosomy) occurs 1 in every 5000 births produces phenotypic, but immature females

Genomic Imprinting A gene on one homologous chromosome is

silenced, while its allele on the homologous chromosome is expressed

Depends on whether the gene resides in a female or a male

Genomic Imprinting

The same alleles may have different effects on offspring, depending on whether they arrive in the zygote via the ovum or via the sperm