ch13

22 pairs

1 pair

XX

XY

• Y chromosome is highly condensed
• Most alleles on male's X lack counterpart on Y
• PARS1 and PARS2
• Human "default" is female
•     -requires SRY gene on Y for "maleness"

Presence of Y chromosome 

• female=2X
• male=XY

Ratio of X chromosome

• 2X chromosomes + 2autosomes per pair = female (raito=1)
• 1X chromosome + 2 autosomes per pair = male (raito=0.5)

• Hemophilia
• Colorblindness
• Duschene's Muscular Dystrophy (DMD)

inability to clot blood normally due to nonfunctional clotting factor protein on X

• Female carriers are unaffected
• Hemophiliacs bleed excessively when injured
• Most seriously affected individuals may bleed to death after relatively minor bruises or buts

Yes but it is rare. females can get X-linked diseases

• X-linked Recessive Disorder
• malfunction of light-sensitive cells in eye

complex of disorders, involving several X-linked genes

• normal color vision: 150 visible colors
• red-green color blindness: < 25 colors

mostly males are affected, but heterozygous females have some defects

X-Linked Recessive Disorder

• progressive weakening and loss of musce tissue
• mostly affect males

• eventually wheelchair bound by 12
• death usually occurs by age 20

• Only non-lethal alleles exhibit inheritance
• Males pass to all daughters
• Rare diseases 
•     -X-linked vitamin d-resistant rickets (hypophosphatemia)
•     -CHILD sydrome: congenital hemidysplasia with ichthyosiform erythroderma and limb defects
•     -incontinentia pigmenti

• Male inherit X from mother
• Males are hemizygous for all X-linked genes
• Males are more likely to express recessive X-linked traits than females (hemizygous)
• Only females can be carriers of X-linked recessive traits
• X-linked dominant is rare

Early female embryos randomly inactivate one X chromosome in each cell

Inactivation is irreversible and inherited during cell division

X-ist RNA wraps around inactive X chromosome

patches of tissue in adult female w/ different active X chromosome active 

different colors activate diff chromosomes

Ensures equal expression of genes from sex chromosomes even though females have two X chromosomes and males have one

1X chromosome is inactivated (highly condensed into Barr body by Xist RNA)

genetic mosaics

two genes on the same chromosome; recombination occurs btw genes ---> recombinant gametes

9:3:3:1 ratio of offspring phenotypes if and only if, two genes are on separate chromosome

(1:1:1:1 ratio if testcross [GgLl * ggll])

LINKAGE

Do Not assort independently

Two Characters (encoded by two genes) on the same chromosome

are located on the same chromosome

may be inherited together

recombinant progeny reflected relevant gene position on chromosome

Quantitation of recombinants---> distance btw genes

increase physical distance btw genes on chromosome, increase probabiliy of recombination (crossover) btw the gene loci

• Crossing over can..
• -separate linked alleles
• -porduce gemetes w/ recombinant chromosomes
• -produce offspring w/recombnant phenotypes

Percentage of recombinant offspring among total

frequency of recombination events 

recombination frequency= recombinant progeny/ total progeny

• 1% recombination= 1 map unit (m.u)
• 1 map unit = 1 centimorgan (cM)

• black body
• vestigial wings
• cinnabar eye color

if homologues undergo two crossovers btw loci, parental combination is restored

Recombination frequency is underestimate of true genetic distance

Relationship btw true distance on a chromosome and recombination frequency is not linear