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

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