mendel’s hypotheses (to explain his results) 1. alternative versions of genes (alleles) account...
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Mendel’s hypotheses (to explain his results)1. Alternative versions of genes (alleles) account for variation in inherited characters
2. For each character, an organism inherits two alleles, one from each parent
3. If two alleles differ, one is dominant, the other recessive4. The two alleles for each character segregate (separate) during gamete production.
Modifications of Mendelian Ratios*Incomplete dominance *Codominance
Example:
MN Blood group
*We can predict genotypic and
phenotypic ratios
*Multiple Alleles
Examples:
*100 alleles at a given locus in Drosophila *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
Modifications of Mendelian Ratios
*Epistasis
Example:
In Drosophila, gene:eyeless
Modifications of Mendelian Ratios
*Black (B) is dominant to brown (b)*Second gene responsible for allowing pigment to be deposited in hair
C = presence, c = absence (colorless)
*Quantitative characters
*often due to polygenic inheritance
Example-
*skin color – controlled by 3 genes
*each gene with two alleles
light and dark
incomplete dominance
Modifications of Mendelian Ratios
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