Patterns of inheritance
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Patterns of Inheritance. 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.

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Patterns of Inheritance

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Patterns of Inheritance


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.


Genotypes and phenotypes of pigeons


III. Codominance

  • Both alleles are equally dominant so they are both expressed


Example: Codominance in Roan cattle


A cattle that is (R1R1) is red


A cattle that is (R2R2) is white


A heterozygous cattle (R1R2) is roan (both red and white)


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.


A white snapdragon (R’R’)


A red snapdragon (RR)


A pink snapdraon (RR’)


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.


Example of a sex linked trait in humans.


  • 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.


Use a punnet square to solve the following problem:


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.


Start by showing the sex chromosomes that mom could give

X

X


Then show the alleles that are found on mom’s sex chromosomes

XH

Xh


Then show the sex chromosomes that dad could give.

X

Y

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


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