I. Simple inheritance (Mendelian inheritance) • One gene controls the trait • There are two versions (alleles) of the gene • One allele is completely dominant over the other
II. Multiple allele inheritance • The trait is still controlled by only one gene. • There are more than two versions (alleles) for the one gene
Example: Multiple allele inheritance of feather color in pigeons • BA allele produces red feathers. It is dominant to all other alleles • B allele produces blue feathers. It is dominant to b but recessive to BA. • b allele produces chocolate colored feathers. It is recessive to all other alleles.
III. Codominance • Both alleles are equally dominant so they are both expressed
IV. Incomplete dominance • Both alleles are equally dominant so they are both expressed
Example: Incomplete dominance in snapdragons • Snapdragons that are RR are red. • Snapdragons that are R’R’ are white. • If a snapdragon is heterozygous for these two alleles (RR’) then it is pink.
V. Polygenic inheritance • The trait is controlled by more than one gene.
Example: Eye color in humans • Eye color is controlled by genes found in two different spots on chromosome number 15 and also by a gene found on chromosome number 19.
VI. Sex linked traits • The trait is controlled by a gene that is found on the sex chromosome (the 23rd pair in humans) • Because males only have one X and females have two X’s some unique inheritance patterns emerge.
The gene that controls your blood clotting factors is found on the X chromosome.
There are two alleles for this gene a normal, dominant H allele that clots blood, and an abnormal, recessive h allele that doesn’t clot blood. If a person doesn’t have the normal H allele then they will have the disease hemophilia.
Since a man only has one X chromosome, and therefore only one gene for blood clotting, he is more likely to get hemophilia.
A man that does not have hemophilia and a woman that is heterozygous for hemophilia have a child. What is the probability that their child will have hemophilia.
Then show the alleles that are found on mom’s sex chromosomes XH Xh
Then show the alleles that are found on dad’s sex chromosomes XH Y XH Xh
Notice that there is no allele shown on the Y chromosome because it doesn’t have the same genes as the X XH Y XH Xh
Finally, show the different possibilities for the offspring. XH Y XH XHXH XHY Xh XHXh XhY
Only one of the four children would have hemophilia. XH Y XH XHXH XHY Xh XHXh XhY