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5.3 Mendelian Genetics

4 min readdecember 28, 2022

Caroline Koffke

Caroline Koffke

Haseung Jun

Haseung Jun

Caroline Koffke

Caroline Koffke

Haseung Jun

Haseung Jun

The Father of Modern Genetics

, the father of modern genetics, came up with some really important laws, including the , that allows for scientists to determine how genes are inherited from generation to generation.

DNA

and are carriers of genetic information. This is used to create proteins, so ribosomes (protein factory) is found in all forms of life.

Law of Segregation

The states that the two alleles from each parent are segregated during gamete formation. Essentially, each gamete gets only one of the two copies of the gene.

Law of Independent Assortment

The states that the two alleles get split up without regard to how the other alleles get split up. This means that you can get your father's copy of genes for eye color, but that doesn't mean you'll also get your father's copy of genes of hair color; you might get your mother's.

Essential Vocabulary

- the physical appearance of an organism, or the actual depiction of a trait (think: , PHYSICAL). Ex. red, purple, white, sparkly, spiky.

- the alleles that make up an individual trait (think: , GENES). Ex. AA, Aa, aa OR , , .

- a version of a gene. Usually an can be or . For Mendelian genetics, all genes have two alleles. - an organism that has two alleles. The organism will have the .

- a trait that produces enough protein or product in order to overtake another trait.

- a trait that does not produce enough protein or product and is overpowered by traits.

- an organism that has two alleles. The organism will have the .

- an organism that has one and one . The organism will have the .

Punnett Squares

Note that only the leads to the . All of the vocabulary above is used frequently and should be memorized and thoroughly understood. 

Because of the rules that Mendel created, the frequency of inheritance can be determined when two individuals are crossed. This can be shown with a Punnett Square.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F700px-Punnett_Square.png?alt=media&token=ef27fdeb-fbfa-4e82-97ac-49aa12afd5a5

Image courtesy of WikiMedia Commons.

As shown in the Punnett square above, when a (Yy) and (yy) individual is crossed, there is a 50% chance that the offspring will show the (yellow) and a 50% chance that the offspring will show the (green) . This can be done for any trait that has a simple inheritance pattern. By knowing the of the parents, the various possible offspring can be calculated with their frequencies.

The probability of having children with a certain trait can be calculated by the laws of probability. In most cases, you'll multiply the probability of having a certain trait with another trait. For example, the probability of having a child with brown eyes or blue eyes will be just multiplying the probability of a child having brown eyes times the probability of a child having blue eyes.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-EHdFrzzoNI9J.png?alt=media&token=80ba73eb-8f4d-451e-92a2-95d5c4d4643a

Image Courtesy of College Board

Unfortunately, most traits do not have a simple / inheritance pattern and, therefore, do not fit Mendel’s rules. These traits, referred to as , are explained next.

Pattern of Inheritance

Like regular Punnett square inheritance, these other patterns can be found using . The example shown above is an example of .

Dihybrid

This inheritance pattern is just like monohybrid, except two genes are looked at. When you look at the Punnett square, you'll get 16 offspring.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-MDf13pYo2GkT.png?alt=media&token=54ccd1d8-1ba9-4660-bc2f-d97b04095405

Image Courtesy of Expii

If both genes are crossed, you'll actually get a magic ratio of 9:3:3:1 for phenotypes (beware, this is not !), which can be useful when you don't really want to actually calculate out the full Punnett square.

Sex-linked

involve genes that are linked to our X and Y chromosomes instead of our other chromosomes. Traits such as and are sex-linked traits. With these sex-linked traits, you are affected if all your X chromosomes have the sex-linked gene. Since males only have one X chromosome, they are more likely to be affected. This is why men are more likely to be color-blind as opposed to women. If a female has a X chromosome that is affected by the gene, she won't express it because it'll be . In order to be color-blind, she would have to have two X chromosomes that are both affected by the gene. A female with only one color blind X chromosome is called a carrier, because though she herself is not color-blind, she can still pass it onto her children.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-B6j7G76o3lih.png?alt=media&token=bc8cb3f4-fca8-42c7-9b47-76d4608aa54d

Image Courtesy of Shemmassian

In the above Punnett square, you can tell a carrier female and normal male have a 50% chance of having a non-color-blind child. They have a 25% chance of having a carrier daughter and 25% chance of having a color-blind son.

Key Terms to Review (21)

Allele

: An allele is one of two or more versions of a gene that are found at the same place on a chromosome.

Color Blindness

: A genetic condition where an individual cannot distinguish certain colors. It's often due to defective color-detecting molecules in cone cells of the retina, which are responsible for color vision.

Dihybrid Inheritance

: The simultaneous inheritance of two independent traits, each controlled by different pairs of alleles.

DNA

: DNA (Deoxyribonucleic Acid) is a molecule that carries most of the genetic instructions used in development, functioning and reproduction of all known living organisms.

Dominant

: A dominant trait or gene is one that will appear in the offspring if one of the parents contributes it.

Genotype

: A genotype refers to the genetic makeup of an organism, specifically the set of genes that it carries.

Gregor Mendel

: Gregor Mendel was a 19th-century scientist and Augustinian friar who is best known for his experiments with pea plants that laid the foundation for the science of genetics.

Hemophilia

: Hemophilia is a genetic disorder where blood doesn't clot normally because it lacks sufficient blood-clotting proteins (clotting factors).

Heterozygous

: A term used to describe a pair of genes where one is dominant and one is recessive. They're different forms of the same gene.

Homozygous Dominant

: Homozygous dominant refers to an individual with two dominant alleles for a certain trait. This results in the expression of the dominant phenotype.

Homozygous Recessive

: Homozygous recessive refers to having inherited two copies of the same recessive allele for a particular gene locus.

Law of Independent Assortment

: The Law of Independent Assortment states that genes for different traits can segregate independently during the formation of gametes.

Law of Segregation

: The Law of Segregation states that during the formation of sex cells (gametes), the pairs of hereditary traits (alleles) become separated, so that each sex cell carries only one kind of allele.

Monohybrid Inheritance

: The inheritance pattern of a single trait controlled by two alleles, one dominant and one recessive.

Non-Mendelian Traits

: These are traits that don't follow Mendel's laws and are not controlled by single genes with dominant and recessive alleles. Examples include multiple allele traits, polygenic traits, and codominance.

Pattern of Inheritance

: The manner in which a particular genetic trait or disorder is passed from one generation to the next.

Phenotype

: Phenotype refers to an individual's observable physical properties such as appearance, development and behavior—the outcomes influenced by its genotype interacting with the environment.

Punnett Squares

: A diagram used by biologists to predict the outcome of a genetic cross. It shows all possible combinations of alleles that can result from a genetic cross.

Recessive

: A recessive trait or gene is one that can be carried in a person's genes without appearing in that person. It can only be expressed (appear) when two copies are present (one from each parent).

RNA

: RNA (Ribonucleic Acid) is a molecule similar to DNA that plays a crucial role in protein synthesis and other chemical activities of the cell.

Sex-linked Genes

: These are genes that are located on the sex chromosomes (X and Y in humans). Their expression can result in traits that differ between sexes.

5.3 Mendelian Genetics

4 min readdecember 28, 2022

Caroline Koffke

Caroline Koffke

Haseung Jun

Haseung Jun

Caroline Koffke

Caroline Koffke

Haseung Jun

Haseung Jun

The Father of Modern Genetics

, the father of modern genetics, came up with some really important laws, including the , that allows for scientists to determine how genes are inherited from generation to generation.

DNA

and are carriers of genetic information. This is used to create proteins, so ribosomes (protein factory) is found in all forms of life.

Law of Segregation

The states that the two alleles from each parent are segregated during gamete formation. Essentially, each gamete gets only one of the two copies of the gene.

Law of Independent Assortment

The states that the two alleles get split up without regard to how the other alleles get split up. This means that you can get your father's copy of genes for eye color, but that doesn't mean you'll also get your father's copy of genes of hair color; you might get your mother's.

Essential Vocabulary

- the physical appearance of an organism, or the actual depiction of a trait (think: , PHYSICAL). Ex. red, purple, white, sparkly, spiky.

- the alleles that make up an individual trait (think: , GENES). Ex. AA, Aa, aa OR , , .

- a version of a gene. Usually an can be or . For Mendelian genetics, all genes have two alleles. - an organism that has two alleles. The organism will have the .

- a trait that produces enough protein or product in order to overtake another trait.

- a trait that does not produce enough protein or product and is overpowered by traits.

- an organism that has two alleles. The organism will have the .

- an organism that has one and one . The organism will have the .

Punnett Squares

Note that only the leads to the . All of the vocabulary above is used frequently and should be memorized and thoroughly understood. 

Because of the rules that Mendel created, the frequency of inheritance can be determined when two individuals are crossed. This can be shown with a Punnett Square.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F700px-Punnett_Square.png?alt=media&token=ef27fdeb-fbfa-4e82-97ac-49aa12afd5a5

Image courtesy of WikiMedia Commons.

As shown in the Punnett square above, when a (Yy) and (yy) individual is crossed, there is a 50% chance that the offspring will show the (yellow) and a 50% chance that the offspring will show the (green) . This can be done for any trait that has a simple inheritance pattern. By knowing the of the parents, the various possible offspring can be calculated with their frequencies.

The probability of having children with a certain trait can be calculated by the laws of probability. In most cases, you'll multiply the probability of having a certain trait with another trait. For example, the probability of having a child with brown eyes or blue eyes will be just multiplying the probability of a child having brown eyes times the probability of a child having blue eyes.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-EHdFrzzoNI9J.png?alt=media&token=80ba73eb-8f4d-451e-92a2-95d5c4d4643a

Image Courtesy of College Board

Unfortunately, most traits do not have a simple / inheritance pattern and, therefore, do not fit Mendel’s rules. These traits, referred to as , are explained next.

Pattern of Inheritance

Like regular Punnett square inheritance, these other patterns can be found using . The example shown above is an example of .

Dihybrid

This inheritance pattern is just like monohybrid, except two genes are looked at. When you look at the Punnett square, you'll get 16 offspring.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-MDf13pYo2GkT.png?alt=media&token=54ccd1d8-1ba9-4660-bc2f-d97b04095405

Image Courtesy of Expii

If both genes are crossed, you'll actually get a magic ratio of 9:3:3:1 for phenotypes (beware, this is not !), which can be useful when you don't really want to actually calculate out the full Punnett square.

Sex-linked

involve genes that are linked to our X and Y chromosomes instead of our other chromosomes. Traits such as and are sex-linked traits. With these sex-linked traits, you are affected if all your X chromosomes have the sex-linked gene. Since males only have one X chromosome, they are more likely to be affected. This is why men are more likely to be color-blind as opposed to women. If a female has a X chromosome that is affected by the gene, she won't express it because it'll be . In order to be color-blind, she would have to have two X chromosomes that are both affected by the gene. A female with only one color blind X chromosome is called a carrier, because though she herself is not color-blind, she can still pass it onto her children.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-B6j7G76o3lih.png?alt=media&token=bc8cb3f4-fca8-42c7-9b47-76d4608aa54d

Image Courtesy of Shemmassian

In the above Punnett square, you can tell a carrier female and normal male have a 50% chance of having a non-color-blind child. They have a 25% chance of having a carrier daughter and 25% chance of having a color-blind son.

Key Terms to Review (21)

Allele

: An allele is one of two or more versions of a gene that are found at the same place on a chromosome.

Color Blindness

: A genetic condition where an individual cannot distinguish certain colors. It's often due to defective color-detecting molecules in cone cells of the retina, which are responsible for color vision.

Dihybrid Inheritance

: The simultaneous inheritance of two independent traits, each controlled by different pairs of alleles.

DNA

: DNA (Deoxyribonucleic Acid) is a molecule that carries most of the genetic instructions used in development, functioning and reproduction of all known living organisms.

Dominant

: A dominant trait or gene is one that will appear in the offspring if one of the parents contributes it.

Genotype

: A genotype refers to the genetic makeup of an organism, specifically the set of genes that it carries.

Gregor Mendel

: Gregor Mendel was a 19th-century scientist and Augustinian friar who is best known for his experiments with pea plants that laid the foundation for the science of genetics.

Hemophilia

: Hemophilia is a genetic disorder where blood doesn't clot normally because it lacks sufficient blood-clotting proteins (clotting factors).

Heterozygous

: A term used to describe a pair of genes where one is dominant and one is recessive. They're different forms of the same gene.

Homozygous Dominant

: Homozygous dominant refers to an individual with two dominant alleles for a certain trait. This results in the expression of the dominant phenotype.

Homozygous Recessive

: Homozygous recessive refers to having inherited two copies of the same recessive allele for a particular gene locus.

Law of Independent Assortment

: The Law of Independent Assortment states that genes for different traits can segregate independently during the formation of gametes.

Law of Segregation

: The Law of Segregation states that during the formation of sex cells (gametes), the pairs of hereditary traits (alleles) become separated, so that each sex cell carries only one kind of allele.

Monohybrid Inheritance

: The inheritance pattern of a single trait controlled by two alleles, one dominant and one recessive.

Non-Mendelian Traits

: These are traits that don't follow Mendel's laws and are not controlled by single genes with dominant and recessive alleles. Examples include multiple allele traits, polygenic traits, and codominance.

Pattern of Inheritance

: The manner in which a particular genetic trait or disorder is passed from one generation to the next.

Phenotype

: Phenotype refers to an individual's observable physical properties such as appearance, development and behavior—the outcomes influenced by its genotype interacting with the environment.

Punnett Squares

: A diagram used by biologists to predict the outcome of a genetic cross. It shows all possible combinations of alleles that can result from a genetic cross.

Recessive

: A recessive trait or gene is one that can be carried in a person's genes without appearing in that person. It can only be expressed (appear) when two copies are present (one from each parent).

RNA

: RNA (Ribonucleic Acid) is a molecule similar to DNA that plays a crucial role in protein synthesis and other chemical activities of the cell.

Sex-linked Genes

: These are genes that are located on the sex chromosomes (X and Y in humans). Their expression can result in traits that differ between sexes.


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AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.


© 2024 Fiveable Inc. All rights reserved.

AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.