CHAPTER 15.2A Few Outcomes of Gene Controls

AP BiologyFall 2010

X CHROMOSOME INACTIVATION In mammalian females, the gene products of

only one X chromosome are needed The other is condensed and inactive Called a Barr body

X CHROMOSOME INACTIVATION Where does a female get each of her X

chromosomes from? One from: _____________ One from: _____________

X CHROMOSOME INACTIVATION In some cells the parental X chromosome is

inactivated In other cells the maternal X chromosome is

inactivated Each adult female is a mosaic of X-linked

traits Mosaic tissue effect

X CHROMOSOME INACTIVATION When alleles on two homologous X

chromosomes are not identical, patches of tissues through the body often show variation

This mosaic effect is seen in human females affected by anhidrotic ectodermal dysplasia In which a mutant gene on one X chromosome

results in patches of skin with no sweat glands When sweat glands are absent, mutant allele is

on the active X chromosome

X CHROMOSOME INACTIVATION Calico Cat

Colouration is contributed to by X inactivation in select cells

Heterozygous for a certain coat colour allele on their X chromosomes

The Calico females have one X that bears a dominant allele for black fur pigment and another X that bears an allele for orange fur pigment

Various cells have one or the other X chromosome inactivated during embryogenesis, contributing to a random colour pattern in the felines

X CHROMOSOME INACTIVATION Dosage compensation: name for the

process that shuts down one X of the female so that the overall expression of the remaining X of the female is equivalent to the lone X of the male Gene control mechanism Nomal development of females depends on this

control

X CHROMOSOME INACTIVATION How does one of two X chromosomes get

shut down? Methylation of histones and action of XIST (X

linked gene) XIST product, large RNA molecule, sticks like

masking paint to chromosomal DNA Do not know why, but the XIST gene on only one

of the two chromosomes is active Chromosome and genes get painted with RNA,

other one remains paint-free and its genes remain available for transcription Critical thinking #9: puts twist on generalized picture

GENE CONTROL OF FLOWER FORMATION Plants have gene controls too Studies of plant mutations in Arabidopsis

thaliana support an ABC model of floral development and specialization

A = wildB = mutant

C = wildD = wild

E = mutantF = mutant

G = mutant

GENE CONTROL OF FLOWER FORMATION Three sets of master genes (A, B, C) guide

flower development in a particular pattern Cell differentiation in the plant depends upon

which genes of the ABC group are activated Support how all the specialized parts of a

flower develop in a predictable pattern Whorls of new tissues become sepals, petals,

stamens, and carpels

Page 235