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Mendelian Genetics

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Biology

The principles of Mendelian genetics are the cornerstone of all that we know about genetics and heredity. Three laws, coined centuries ago by scientist Gregor Mendel, help us understand how genes are passed down from generation to generation and how these genes determine what an organism looks like.

Introduction to Mendelian Genetics

Mendelian genetics, also called classical genetics, are principles of biology created in the 19th Century by The Father of Genetics, Austrian monk Gregor Mendel. Mendel examined the humble garden pea and discovered three principles of inheritance that apply not just to peas but to all living organisms.

Mendelian Genetics Overview

Before Mendelian Inheritance was commonly recognized, many people believed that heredity was akin to mixing two paint buckets, creating an intermediate color. For example, a black-haired parent and a blond-haired parent would give birth to a child with brown hair.

Mendel demonstrated that inheritance is not based on this blending concept. Instead, individuals have discreet units of heredity, which we now know as genes, and these genes are passed down to offspring. The characteristics the offspring display are based on the alleles they inherit and the dominance of those alleles.

Mendel began his experiments using peas that were pure-bred for specific traits. For example, he knew which of his plants were pure-bred for purple flowers because he self-pollinated them for years, over and over, and the flowers they produced were always purple. He eventually cross-pollinated these purple pure-breeds with white pure-breeds, creating a hybrid. The pure breeds were called the parent generation (P), and the hybrids were called the first filial generation (F1). He saw that the F1 flowers were all purple!

P = this is the parental generation. These are pure-bred plants (or animals or whatever organism you're studying) that are homozygous for whatever allele they display.

F1 = this is the first filial generation. When you cross-pollinate two different P plants, their offspring are F1. F1 plants always have one allele from each P parent; they are heterozygotes.

F2 = this is the second filial generation. When you self-pollinate two F1 plants, their offspring are F2. You can self-pollinate F2 plants to get F3 (third generation), and self-pollinate F3 plants to get F4 (fourth generation), and so on.

Now Mendel took two F1s and crossed them together to produce the second filial generation (F2). This F2 generation appeared different: most of its flowers were purple, yes, but some were white again! In fact, after performing this F1 x F1 cross time and time again, Mendel noticed a consistent ratio of purple to white flowers in the F2 generation. Purple flowers were consistently 3/4 of the crop, while white flowers were 1/4 (Fig. 1). These findings helped consolidate Mendel's Theory of Inheritance.

Mendelian Genetics Definitions

Before we go on, it's important to define some terms in Mendelian genetics.

  • What is a gene? A gene is the basic unit of heredity.
    • For each trait, organisms get one gene from each parent, so there are two genes per trait.
  • What is an allele? An allele is a variant of a gene.
    • In Mendel's pea plants, some peas were wrinkly, and others were round. These are two variants, or two different alleles, of the gene deciding pea shape. If an organism's two alleles are the same, it is homozygous (AA or aa) for that trait. If the two alleles are different, it is heterozygous (Aa). (Homo - the same, Hetero - different).
  • What is a phenotype? Phenotype refers to how an organism looks, regardless of its alleles.
  • What is a genotype? Genotype refers to the exact allelic makeup of an organism, regardless of how the organism looks.
  • What is a dominant allele? A dominant allele is an allele that shows up in the phenotype of a heterozygote.
    • The Round (R) allele is dominant in peas over the Wrinkled (r) allele. So in a plant heterozygous for pea shape, with one copy of the round allele and one copy of the wrinkled allele, the plant would have the Rr genotype, and its peas would appear completely round, just as if it were a RR homozygote with two copies of the round allele (Fig. 2).
  • What is a recessive allele? A recessive allele is an allele that does not show up in the phenotype of a heterozygote.
    • An organism must be homozygous for a recessive allele for it to be observed in its phenotype. Because wrinkled peas are recessive, we need an rr genotype to observe a wrinkled pea.

Basic Principles of Mendelian Genetics

Three principles make up the Mendelian Theory of Inheritance. These principles are the cornerstone of the entire field of genetics. To understand the exceptions to these laws and the more complex concepts that build on them, we must first understand each of the three in detail.

1) The Law of Dominance

2) The Law of Segregation (read more about this in the article "Mendel's Law of Segregation")

3) The Law of Independent Assortment

Mendelian Theory of Inheritance

The Law of Dominance

The Law of Dominance states that, in a heterozygote, the dominant allele is expressed exclusively.

We can observe this when we cross two homozygous parent organisms for different alleles, and see that their offspring is heterozygous for both alleles but has the same phenotype as the parent with the dominant allele.

Let's use the wrinkly and round peas again to examine this. Also, we will use a Punnett Square, a tool used in genetics to determine the possible genotypes of future offspring made by crossing two parent organisms (Fig. 3).

Mendelian Genetics Punnett Square of Peas Homozygous for their shape StudySmarterFigure 3: A Punnett Square of Peas Homozygous for their Shape. Chisom Ani, StudySmarter Originals.

The Law of Segregation

The Law of Segregation states that when an organism is making gametes, it separates its gene pair, or alleles so that each one is individually packaged. Then, during reproduction, one maternal and one paternal gamete will fuse so that their offspring will get one random allele from each parent for two alleles.

The Law of Independent Assortment

The Law of Independent Assortment states that alleles of different genes are inherited independently of one another. Thus, an allele inherited for one gene doesn't influence or affect the ability to inherit an allele of a different gene.

For example, a parent plant with purple flowers and wrinkly peas passes down their wrinkled shape and purple flower alleles independently and equally.

Exceptions to Mendelian genetics

It's important to note that while Mendelian genetics is foundational, not every trait fits neatly into these three laws, and we do see exceptions.

Exception 1: Multiple Genes

Multiple genes control some characteristics. These are called polygenic traits. An example of this is your height, which is influenced by over 50 genes!

Exception 2: Multiple Alleles

Even if a trait is controlled by just one gene, there may be more than two alleles for that gene. In Mendel's pea plants, every trait he studied had only two possible alleles (wrinkled or round, green or yellow, normal-sized or dwarf, purple or white flowers, etc.) But the gene determining human blood types, for example, has three possible alleles A, B, and O.

Exception 3: Codominance

When Mendel crossed purple flowers and white flowers, he didn't get light-purple flowers, so he postulated that all traits have an all-or-nothing, dominant or recessive phenotype. However, we have discovered some traits in some animals where both alleles can be expressed together, called codominance. An example of this is speckled chickens, which have both white and black feathers from their pure white and pure black parents (Fig. 4).

Exception 4: Incomplete Dominance

Sometimes, an offspring's phenotype is the intermediate of its two parents; thus, neither allele is completely dominant. This blending form of inheritance is reminiscent of the popularly held concepts in Mendel's era. We can see this form of inheritance in Palomino-colored horses, whose tan coat color is in between their brown and white parent's coats (Fig. 5).

Exception 5: Pleiotropy

If a gene is pleiotropic, it has multiple effects on the phenotype. Unlike the allele for wrinkled peas, which didn't affect height or flower color, or anything other than pea shape, some genes in higher organisms have multiple effects. For example, PKU, a disease in humans due to an altered gene, causes features like slow growth, reduced skin pigment, and intellectual disability. One gene alteration has multiple effects.

Exception 6: Gene Linkage

Gene linkage means that a gene at a particular spot on a chromosome influences the ability to inherit a different gene on the same or different chromosomes. Two linked genes tend to assort together, and inheriting one would increase the likelihood that you inherit another. In humans, genes for hair color and eye color exhibit some gene linkage, which you may have noticed if you've thought of how often blonde hair and blue eyes occur together.

Mendelian Genetics - Key Takeaways

  • Mendelian genetics is based on three laws: The Law of Dominance, The Law of Segregation, and The Law of Independent Assortment.
  • The Law of Dominance states that the dominant allele is the only allele on display in the phenotype of a heterozygote.
  • The Law of Segregation states that alleles separate independently into gametes.
  • The Law of Independent Assortment states that alleles of different genes are inherited independently without affecting each other.
  • Pure-bred plants are always homozygous, and they are called the P or parental generation.
  • F1 plants are the offspring produced from crossing two different P plants.
  • F2 plants are the offspring produced from crossing two of the same F1 plants.
  • Mendel's Laws have several exceptions, including gene linkage, polygenic traits, codominance, incomplete dominance, and more.

Mendelian Genetics

Mendelian Genetics refers to a pattern of inheritance in traits controlled by a single gene, with dominant and recessive alleles.

Traits that are Mendelian are those determined by a single gene that has only two possible alleles, one dominant, one recessive.

Mendelian genetics is important because it is the foundation of our modern understanding of genetics, and its pattern of inheritance occurs in all living organisms.

Mendelian genetics refers to classical genetics or genetics that follow Mendel's three laws; the Law of Dominance, the Law of Segregation, and the Law of Independent Assortment.

The Three Principles of Mendelian Genetics, also known as Mendel's Laws, are the Law of Dominance, the Law of Segregation, and the Law of Independent Assortment.

Final Mendelian Genetics Quiz

Question

What are the three laws of Mendelian Inheritance?

Show answer

Answer

  1. The Law of Dominance
  2. The Law of Segregation
  3. The Law of Independent Assortment

Show question

Question

Fill in the blank: The Law of Dominance states that the _____ allele is expressed exclusively.

Show answer

Answer

Dominant

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Question

What does the Law of Segregation state in Mendelian genetics?

Show answer

Answer

When an organism is making its gametes, it separates its gene pair so that each gene is individually packaged. 

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Question

Choose the correct answer: The Law of Independent Assortment in Mendelian genetics states that alleles of different genes are inherited _____ of one another.

  1. Indirectly
  2. Independently
  3. Intrinsically
  4. Indomitably

Show answer

Answer

Independently

Show question

Question

Practice Problem: If Mendel self-pollinated peas heterozygous for color (Gg), what percentage of their offspring would be yellow?



Note: Green is the dominant allele G

Yellow is the recessive allele g

Show answer

Answer

Self pollinating two Gg heterozygous = this cross Gg x Gg


The offspring of that cross would be 1/4 gg, 1/2 Gg, and 1/4 GG.


Only gg plants look yellow, so 


1/4 or 25% of the offspring would be yellow.

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Question

If a pea plant has this genotype for height: Tt,  what size will that plant be?


T = dominant tall gene

t = recessive dwarf gene

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Answer

The plant will be tall

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Question

Which law of Mendelian Genetics does the concept of incomplete dominance go against?


Show answer

Answer

The Law of Dominance.

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Question

In Incomplete Dominance, crossing a red flower and a white flower would give rise to what color offspring?

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Answer

Pink flowered offspring.

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Question

What is the genotype of a plant that is pure-bred green?


G - Dominant green allele

g - Recessive yellow allele

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Answer

GG

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Question

What's the scientific term for a pure-bred plant? 

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Answer

A Homozygote.

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Question

What term is used to describe the generation you get when you cross two F1 plants together?



Show answer

Answer

F2.

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Question

What does pleiotropy mean? Is it a rule of Mendelian genetics or an exception?

Show answer

Answer

Pleiotropy means that one gene has multiple effects. It is an exception to Mendelian genetics. 

Show question

Question

If you have two flowers that obey the laws of Mendelian genetics, and they are both purebred for different colors, what can you do to determine which color is dominant?

Show answer

Answer

You can cross-breed them, or cross them, and then examine their offspring. 

Show question

Question

What percentage of offspring produced from a cross between two parents that are homozygous for different colors, will be the color of the dominant parent?

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Answer

100% of the offspring produced in such a cross will be the color of the dominant parent. 

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Question

Define phenotype

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Answer

Phenotype is defined as the observable characteristics of an organism, due to how its genes are expressed in a given environment.

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Question

What's the difference between genotype and phenotype?

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Answer

Genotype is what an organism's genes say, regardless of what it looks like. Phenotype, on the other hand, is what an organism looks like, regardless of what its genes say.

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Question

Give an example of a genetic disease which has little-to-no environmental component

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Answer

Cystic Fibrosis, Sickle Cell Anemia, Tay Sachs Disease, Duchenne Muscular Dystrophy, and more.

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Question

Which two factors affect phenotype

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Answer

Genetics and the Environment

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Question

Practice Problem: Which of these phenotypic factors is decided by a combination of genetics and environment?


Eye Color or Height

Show answer

Answer

Height

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Question

Practice Problem: Which of these two phenotypic factors is decided completely by genetics?


Eye Color or Tooth Shape

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Answer

Eye Color

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Question

Practice Problem: What phenotype would a flower have if its genotype was (Bb)

Note: Assume Mendelian Inheritance.

B = blue

b = white



Show answer

Answer

It would be blue.

Show question

Question

True or False: We can determine the genotype of an organism that has the dominant phenotype just by looking at it. 

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Answer

False. 

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Question

Practice Problem: Assuming Mendelian Inheritance, what would be the phenotype of two peas with the following genotypes - GG and Gg


Note: 

= dominant green

= recessive yellow

Show answer

Answer

They would both be green

Show question

Question

Are most mental health disorders phenotypes caused by factors:

- entirely genetic 

- mixed

- entirely environmental 

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Answer

Mixed

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Question

Which of the following is NOT a phenotype:

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Answer

homozygous for the sickle cell allele

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Question

Phenotype is best understood as the _____ characteristics of an organism

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Answer

Observable

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Question

A pea plant is observed to be yellow, the recessive color. Do we know its genotype?

Show answer

Answer

Yes, we do. 


It must has two copies of the recessive allele for color, in order to appear yellow.

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Question

Name a phenotypic characteristic you might observe in your dog.

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Answer

Bark sound, bark loudness, size, hair texture, hair color, eye color, degree of shedding, excitableness, friendliness, aggressiveness, calmness, etc.

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Question

How does Mendel's Law of Dominance help us to know the phenotype of a heterozygote?

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Answer

According to the Law of Dominance, all heterozygotes solely express the dominant phenotype. So a heterozygote will look just like a homozygote dominant organism.

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Question

What is the definition of genotype?

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Answer

Genotype is the set of specific alleles an organism has for a given gene

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Question

Which of the following is not a genotype

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Answer

Blue eyes

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Question

How many different types of genotypes are there in Mendelian Genetics?

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Answer

Three

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Question

Which of the following is not a genotype seen in Mendelian Genetics?

Show answer

Answer

heterotropic dominant

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Question

Plant species Mirabilis jalapa exhibits incomplete dominance in terms of its flower color. R - red color, r - white color. What is the genotype of a pink flower?

Show answer

Answer

Rr

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Question

What is the difference between genotype and phenotype

Show answer

Answer

Genotype is what an organism's alleles are, regardless of how it looks. Phenotype is what an organism looks like, regardless of what its alleles are.

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Question

_____ + Environment = Phenotype

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Answer

Genotype

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Question

All genotypes for all traits have two alleles

Show answer

Answer

False

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Question

All genotypes for all alleles follow the principles of Mendelian Inheritance

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Answer

False

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Question

Assuming complete dominance, what possible genotypes can give a dominant phenotype?

Show answer

Answer

Homozygous dominant and Heterozygous

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Question

When not in a laboratory setting, what are two tools we can use to examine genotypes of family members?

Show answer

Answer

Punnett Squares and Pedigrees

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Question

Punnett squares can tell us two things about genotypes of future offspring. What are they?

Show answer

Answer

The possible genotypes and their ratios.

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Question

What are all the possible genotypes that we can see in offspring of this cross? Assume Mendelian Inheritance.


AA x Aa


Show answer

Answer

We can see AA and Aa progeny only.

Show question

Question

What are the ratios of the possible genotypes that we can see in the offspring of this cross? Assume Mendelian Inheritance. 

Show answer

Answer

We will see a 1:1 ratio of AA to Aa. 

50% AA and 50% Aa.

Show question

Question

What is the ratio of AA:Aa:aa progeny in this genetic cross? Assume Mendelian Inheritance.


Aa x Aa

Show answer

Answer

The ratio: 1:2:1 for AA:Aa:aa


AA = 25%

Aa = 50 %

aa = 25%

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Question

Which is NOT one of the laws of Mendelian genetics?

Show answer

Answer

the Law of Persistence

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Question

Fill in the blank: The Law of Segregation states that when a diploid organism is making its gametes, or cells of reproduction during meiosis, it makes them in such a way that each ___  is packaged individually. 

Show answer

Answer

allele

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Question

What is a gamete?

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Answer

A sex cell

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Question

Which of these is aneuploid for the human genome?

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Answer

2n + 1

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Question

Gametogenesis involves....

Show answer

Answer

Meiosis

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Question

If a somatic cell has 2n, what is the ploidy of a gamete?

Show answer

Answer

n

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