Mendel and his laws of segregation and independent assortment S.C.912.L.16.1.
How do we account for genetic variation? *Independent assortment *Crossing over *Random...
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Transcript of How do we account for genetic variation? *Independent assortment *Crossing over *Random...
![Page 1: How do we account for genetic variation? *Independent assortment *Crossing over *Random fertilization Independent Assortment: Cross over:](https://reader035.fdocuments.us/reader035/viewer/2022062519/5697bfe91a28abf838cb6a56/html5/thumbnails/1.jpg)
How do we account for genetic variation?
*Independent assortment *Crossing over *Random fertilization
Independent Assortment:
Cross over:
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Chi-Square Analysis:
Mendel’s monohybrid and dihybrid ratios are predictions based on the following assumptions:
1. Each allele is dominant or recessive
2. Random segregation of alleles
3. Independent assortment
4. Fertilization is random
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Chi-Square Analysis:p value (probability): consider as a percentage (i.e. 0.05 = 5%)
Example from Table 3.1:
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Human PedigreesPedigree
= Female
= Male
=Unknown
*Proband (p)
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Chapter 4: Modification of Mendelian RatiosAllele
*Wild-type allele
*Mutant allele
Conventional symbols for alleles:
recessive allele- initial letter of the name of the recessive trait, lowercased and italicized
dominant allele- same letter in uppercase
Genetic nomenclature is extremely diverse!
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Incomplete or Partial DominanceCross between parents
with contrasting traits: Red flowers or white
flowers
Offspring with an intermediate
phenotype: pink flowers
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Codominance:
Example:
MN Blood group- red blood cells contain a transmembrane glycoprotein (glycophorin); two different forms of this protein exist, M and N
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Examples:
*Table 4.1: over 100 alleles at a given locus in Drosophila
*ABO Blood group in humans
Multiple Alleles:
*Characterized by the presence of glycoprotein antigens on the surface of red blood cells
*Distinct from the M and N antigens
*Also exhibits codomiance
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Lethal Alleles:
Example: Coat color in mice
*A = agouti = wild-type allele
*AY = yellow = mutant allele
Dominant Lethal: Huntington’s disease (H); heterozygous individuals (Hh) have late onset
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Combining modified modes of inheritance:
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Gene interaction:
*Epistasis
Example:
In Drosophila, the recessive gene eyeless (when homozygous) prevents the expression of eye color genes present in genome
*Novel phenotypes due to gene interaction
Example:
disc-shaped fruit (AABB) X long fruit (aabb)
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Genes on the X Chromosome:
*Sex chromosomes*Autosomes
Example: In Drosophila and all mammals
sex chromosomes designated as X and Y
XX=female
XY=male
*1909 Thomas Hunt Morgan
II III IV
XX
XY
or
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*X-linkage
X-linkage in Drosophila: white mutation (eyes)
Genes on the X Chromosome con’t:
½ red
½ white
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Genes on the X Chromosome con’t:
½ red
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Sex-limited Inheritance:*Sex-limited trait
*holandric genes: genes on the Y chromsome
Example: hypertrichosis (ear hair)
*autosomal genes
Example: milk production in mammals; L=lots, l=little
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*Sex-influenced trait
Sex-influenced Inheritance:
Examples:
*cleft palate in humans*horns in sheep *pattern baldness in
humans
Summary:
Sex-linked on X or Y sex-chromosome
Sex-limited all or none expression by sex
Sex-influenced genotype + sex determines phenotype
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Phenotypic Expression:
*Penetrance
*Expressivity
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
*Temperature