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How many pairs of autosomes does a human have
how many pairs of sex chromosomes do humans have??
What do women's chromosomes look like ??
What do men chromosomes look like ??
What are some characteristics of the Y chromosome??
- 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"
Humans are basd on the presence of what chromosome??
Drosophila is based on ____ of ______ chromosomes to autosome sets??
Ratio of X chromosome
- 2X chromosomes + 2autosomes per pair = female (raito=1)
- 1X chromosome + 2 autosomes per pair = male (raito=0.5)
What are human X-linked Recessive Diseases/disorders???
- 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
Can Females every get hemophilia??
Yes but it is rare. females can get X-linked diseases
Red-Green Color Blindness
- 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
Duchenne Muscular Dystrophy
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
Males and Sex-Linked Genes
- 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
X-inactivation -----> Dosage compensation
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
In each female cell ......???
1X chromosome is inactivated (highly condensed into Barr body by Xist RNA)
Females heterozygous for genes on X chromosome are ....??
two genes on the same chromosome; recombination occurs btw genes ---> recombinant gametes
Predicting phenotypes involving two characters (genes)
Dihybrid: GgLl * GgLl
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])
With genes on the same chromosome ---> ______....very different ratios of offspring
Linked Genes DO NOT assort _________!!
Do Not assort independently
Linked Genes are ...??
Two Characters (encoded by two genes) on the same chromosome
Linked genes for two given characters are???
are located on the same chromosome
may be inherited together
Morgan observed that ...
recombinant progeny reflected relevant gene position on chromosome
What did Alfred Sturtevant find??
Quantitation of recombinants---> distance btw genes
increase physical distance btw genes on chromosome, increase probabiliy of recombination (crossover) btw the gene loci
How close are the genes in linked genes???
- Crossing over can..
- -separate linked alleles
- -porduce gemetes w/ recombinant chromosomes
- -produce offspring w/recombnant phenotypes
Percentage of recombinant offspring among total
The distance btw genes is proportional to
frequency of recombination events
recombination frequency= recombinant progeny/ total progeny
- 1% recombination= 1 map unit (m.u)
- 1 map unit = 1 centimorgan (cM)
Three Characters with recessive traits in flies
- 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