Lecture for Thursday September 25, 2003
What’s due?
CH3 problem set
Assignments:
Study for Exam!
Today’s lecture:
Review material from 9/23
Genes on the X Chromsome
Sex-linked, -limited, -influenced Inheritance
Review:*Pedigree- a family tree that shows the phenotype of a particular trait for each family member
*Modification of Mendelian Ratios:
Incomplete dominance- expression of a heterozygous phenotype which is distinct from, and often intermediate to, that of either parent
P: Red Flower CRCR
White Flower CWCW
X
F1: Pink Flower CRCW
F2:¼ Red CRCR ½ Pink CRCW ¼ White CWCW
Review con’t:*Codominance- The effects of two alleles are equally and simultaneously expressed in the heterozygote
*Multiple Alleles- three or more alleles of the same gene
*ABO Blood group in humans
Genotype IAIA IAIO IBIB IBIO IAIB
IOIO
Antigen A A B B A,B
Neither
Phenotype A A
B B AB
O
simple dominancesimple dominancecodominance
A=B>O
*Lethal Allele- an allele whose expression results in the death of the individual expressing it
*Can be either dominant or recessive
Review con’t:Gene Interactions:
*Epistasis: situation in which one gene masks the expression of another*Novel phenotypes: interaction between genes yields new phenotype
Example: shape of summer squash
In the F2 genotypes: A_B_ disc phenotype A_bb or aaB_ sphere phenotype aabb long phenotype
Genes on the X Chromosome:
Sex chromosomes- chromosomes involved in sex determinationAutosomes- non sex determining chromosomes
Example: In Drosophila and all mammals
sex chromosomes designated as X and Y
XX=female
XY=male
*1909 Thomas Hunt Morgan determined that there were differences in chromosomes of male and female Drosophila
II III IV
XX
XY
or
X-linkage- transmission and expression of genes located on the X chromosome
X-linkage in Drosophila: white mutation (eyes)
Genes on the X Chromosome con’t:
½ red
½ white
Genes on the X Chromosome con’t:
*Morgan proposed that the white eyed mutation was X-linked
*Reciprocal cross did not yield identical results
*Males were XY and females were XX
*Since females could be white eyed, the trait was not determined by a locus on Y
½ red
Symbols:
red eye = w+
white eye = w
w+w+ X wY
w+w
w+Y
Gametes from mother
:½ w+ ½ w
Gametes from father:
½ w+ ½ Y
¼ w+w+ ¼ w+Y ¼ ww+ ¼ wY
½ red female ¼ red male ¼ white male
Sex-limited Inheritance:
Sex-limited trait: A trait that is expressed in only one sex even though the trait may not be X-linked
*holandric genes: genes on the Y chromsome
Example: hypertrichosis (ear hair)
P: XX x XYh note: h is only on Y
F1: ½ XX + ½ XYh
*autosomal genes
Example: milk production in mammals; L=lots, l=little
Genotype Phenotypefemale
male LL lotsnone Lllots
none ll littlenone
Sex-influenced trait: Phenotypic expression that is conditioned by the sex of the individual
Sex-influenced Inheritance:
Examples:
*cleft palate in humans*horns in sheep *pattern baldness in
humansGenotype Phenotype
female male BB bald bald Bb not bald bald bbnot bald not bald
B=bald b=not bald
Summary:
Sex-linked on X or Y sex-chromosome
Sex-limited all or none expression by sex
Sex-influenced genotype + sex determines phenotype
Phenotypic Expression:
Penetrance: (quantitative term) the frequency with which individuals with a given genotype, exhibit some degree of a phenotype associated with that trait
Expressivity: (qualitative term) the degree or range in which a phenotype for a given trait is expressed
*Gene expression often governed by genotype and environment
Example:
if 9/10 of individuals carrying an allele express the trait, the trait is said to be 90% penetrant
*Onset of genetic expression (when is a gene expressed?)
Tay-Sachs autosomal rec. lipid metab death by 3 yearsLesch-Nyhan X-linked rec. nucleic acid metab onset >6mo.
DMD X-linked rec. muscular degeneration diagnosed at 3-5yrsHuntingtons autosomal dom. brain deterioration onset >30yrs
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