Genotype vs. Phenotype
Genotype is
the possible combination of alleles for a particular gene.
Example: Yellow vs. Green seeds
Yellow color is coded by the allele Y, and shortness is coded by the allele y. These two different alleles cause variant forms of the same trait such as the color of the seeds.
Multiple variations of genotypes are possible when there are different combinations of alleles.
For instance, we can have YY, Yy, and yy as possible genotypes.
The ultimate goal of genotypes is to be transcripted and translated into proteins and express the phenotype.
Phenotype is physical result of the genotype. It refers to the physical trait such as the yellow color or green
color resulting from the combination of alleles.
For example, even though the YY and Yy are different genotypes, they both produce same phenotype of yellow seeds.
How is that possible?
This is where Mendelian genetics comes into play!
Here Y is a dominant trait and y is a recessive trait, which means that when both Y and y alleles are present in the genotype, only the phenotype for Y will be expressed. Normally dominant traits are denoted by a capital letter and recessive traits are denoted by a small letter.
Going back to the genotypes again, we can have
YY-homozygous dominant
Yy-hybrid or heterozygous
yy- homozygous recessive
Out of these possible combinations, both homozygous dominant and heterozygous genotype give out the phenotype for yellow seeds while homozygous recessive can only produce the green seeds.
While passing on the traits, Mendelian traits follow both law of segregation and Independent Assortment.
So if we do a monohybrid cross with Yy x Yy:
Genotype: YY (homozygous dominant)- 25% Phenotype: Yellow Seeds-75%
Yy (heterozygous) – 50% Green Seeds -50%
yy (homozygous recessive)- 25%
A cross between two heterozygous genes always gives us a genotype ratio of 1:2:1 and phenotype ratio of 3:1 in the next generation.
Similarly a dihybrid cross between two heterozygous genes will give a phenotype ratio of 9:3:3:1 in the next generation.
There are several diseases and disorders resulting from Dominant / Recessive relationship:
Recessive Dominant
No freckles Freckles
5 fingers 6 fingers
No widow’s peak Widow’s peak
Albinism Achondroplasia
Cystic fibrosis Alzheimer’s disease
Tay Sachs disease Huntington’s disease
Sickle Cell Anemia Hypercholesterolemia
Phenylketonuria
Galactosemia
Sathasivan, K. “Introduction to Genetics.” Cell & Molecular Biology: An Introduction. Dubuque, IA: Kendall Hunt, 2013. 111. Print.
This simple dominant and recessive relationship only exists between traits that follow Mendelian Inheritance while more complicated relationships are observed between alleles in traits that fall under non-Mendelian inheritance.
Examples: Incomplete Dominance, Codominance, Pleiotropy, Epistasis, Multiple Genes, Sex Linked Inheritance, Maternal Inheritance, Environmental Effect on Gene Expression (Sathasivan).
Time for Practice:
1) If a mother who is a carrier for cystic fibrosis and a father who has cystic fibrosis has an offspring, what are the offspring’s chances of having a heterozygous genotype? A normal phenotype?
2) Give an example of a non-Mendelian inheritance?
Answers:
1) Cystic fibrosis is a recessive disorder. The mother’s genotype is Cc, and father’s genotype is cc.
C c
c Cc cc
c Cc cc
Genotype: CC- 0% Phenotype: Cystic Fibrosis-50 %
Cc- 50% Normal-50% cc- 50%
There are 50% chance of having a heterozygous genotype and equal chance having a normal phenotype.
2) Codominance can be observed in blood groups as the genotype AB will express both proteins for types A &B.
Example: Yellow vs. Green seeds
Yellow color is coded by the allele Y, and shortness is coded by the allele y. These two different alleles cause variant forms of the same trait such as the color of the seeds.
Multiple variations of genotypes are possible when there are different combinations of alleles.
For instance, we can have YY, Yy, and yy as possible genotypes.
The ultimate goal of genotypes is to be transcripted and translated into proteins and express the phenotype.
Phenotype is physical result of the genotype. It refers to the physical trait such as the yellow color or green
color resulting from the combination of alleles.
For example, even though the YY and Yy are different genotypes, they both produce same phenotype of yellow seeds.
How is that possible?
This is where Mendelian genetics comes into play!
Here Y is a dominant trait and y is a recessive trait, which means that when both Y and y alleles are present in the genotype, only the phenotype for Y will be expressed. Normally dominant traits are denoted by a capital letter and recessive traits are denoted by a small letter.
Going back to the genotypes again, we can have
YY-homozygous dominant
Yy-hybrid or heterozygous
yy- homozygous recessive
Out of these possible combinations, both homozygous dominant and heterozygous genotype give out the phenotype for yellow seeds while homozygous recessive can only produce the green seeds.
While passing on the traits, Mendelian traits follow both law of segregation and Independent Assortment.
So if we do a monohybrid cross with Yy x Yy:
Genotype: YY (homozygous dominant)- 25% Phenotype: Yellow Seeds-75%
Yy (heterozygous) – 50% Green Seeds -50%
yy (homozygous recessive)- 25%
A cross between two heterozygous genes always gives us a genotype ratio of 1:2:1 and phenotype ratio of 3:1 in the next generation.
Similarly a dihybrid cross between two heterozygous genes will give a phenotype ratio of 9:3:3:1 in the next generation.
There are several diseases and disorders resulting from Dominant / Recessive relationship:
Recessive Dominant
No freckles Freckles
5 fingers 6 fingers
No widow’s peak Widow’s peak
Albinism Achondroplasia
Cystic fibrosis Alzheimer’s disease
Tay Sachs disease Huntington’s disease
Sickle Cell Anemia Hypercholesterolemia
Phenylketonuria
Galactosemia
Sathasivan, K. “Introduction to Genetics.” Cell & Molecular Biology: An Introduction. Dubuque, IA: Kendall Hunt, 2013. 111. Print.
This simple dominant and recessive relationship only exists between traits that follow Mendelian Inheritance while more complicated relationships are observed between alleles in traits that fall under non-Mendelian inheritance.
Examples: Incomplete Dominance, Codominance, Pleiotropy, Epistasis, Multiple Genes, Sex Linked Inheritance, Maternal Inheritance, Environmental Effect on Gene Expression (Sathasivan).
Time for Practice:
1) If a mother who is a carrier for cystic fibrosis and a father who has cystic fibrosis has an offspring, what are the offspring’s chances of having a heterozygous genotype? A normal phenotype?
2) Give an example of a non-Mendelian inheritance?
Answers:
1) Cystic fibrosis is a recessive disorder. The mother’s genotype is Cc, and father’s genotype is cc.
C c
c Cc cc
c Cc cc
Genotype: CC- 0% Phenotype: Cystic Fibrosis-50 %
Cc- 50% Normal-50% cc- 50%
There are 50% chance of having a heterozygous genotype and equal chance having a normal phenotype.
2) Codominance can be observed in blood groups as the genotype AB will express both proteins for types A &B.
Links for more information:
For more information on phenotypes and genotypes: http://evolution.berkeley.edu/evosite/evo101/IIIA1Genotypevsphenotype.shtml
For more information on non-Mendelian Inheritance: http://anthro.palomar.edu/mendel/mendel_3.htm
Links to Practice Problems:
1) http://anthro.palomar.edu/mendel/quizzes/mendqui1.htm
2) http://biology.clc.uc.edu/courses/bio105/geneprob.htm
For more information on non-Mendelian Inheritance: http://anthro.palomar.edu/mendel/mendel_3.htm
Links to Practice Problems:
1) http://anthro.palomar.edu/mendel/quizzes/mendqui1.htm
2) http://biology.clc.uc.edu/courses/bio105/geneprob.htm