Mendelian Inheritance

 

Prepared by Dr Bhabesh Nath

Assistant Professor

Department of Zoology

B N College (Autonomous) Dhubri 

1. Introduction to Heredity and Inheritance

Heredity is the biological process by which characteristics are transmitted from parents to offspring across generations. Variation refers to the differences in traits that exist among individuals of the same species. Apart from identical twins or clones, no two organisms are genetically identical. Inheritance is the mechanism through which hereditary information, encoded in genes, passes from one generation to the next.

Long before DNA was discovered, naturalists observed that offspring resemble their parents, but the rules governing this resemblance remained mysterious. Early theories such as blending inheritance proposed that offspring traits were simply an average of parental traits. This idea failed to explain why traits could reappear unchanged after skipping a generation. It took a systematic, quantitative experimental approach to uncover the true rules of heredity.

Gregor Johann Mendel and His Contribution

Gregor Johann Mendel (1822 to 1884), an Augustinian friar and scientist working in Brno, in the present day Czech Republic, conducted meticulous breeding experiments on garden pea plants between 1856 and 1863. Unlike earlier naturalists, Mendel approached the problem with unusual rigor.

•       Studied one or a few traits at a time rather than the whole organism

•       Used large sample sizes and counted offspring precisely

•       Applied mathematical and statistical reasoning to his results

•       Designed controlled crosses with clearly defined parental generations

 

His work, published in 1866 as “Versuche über Pflanzen Hybriden” (Experiments on Plant Hybrids), went largely unrecognized during his lifetime. It was independently rediscovered in 1900 by three botanists, Hugo de Vries, Carl Correns, and Erich von Tschermak, after which Mendel was posthumously recognized as the Father of Genetics.

2. Mendel's Experimental Organism

Why Mendel Selected Pisum sativum

Mendel's choice of the garden pea, Pisum sativum, was central to his success. Peas offered several practical and biological advantages that made the inheritance patterns clear and reproducible.

Key Characteristics That Made Peas Ideal

Feature	Advantage to Mendel's Research
Short generation time	Multiple generations could be studied within a few years
Easily cultivated	Grown cheaply in large numbers in a monastery garden
Naturally self pollinating	Ensured true breeding, or pure, parental lines
Can be artificially cross pollinated	Allowed controlled crosses between chosen parents
Many distinct, easily scored traits	Clear cut, non overlapping phenotypes with no ambiguity
Produces many offspring per cross	Enabled statistically meaningful ratios
Available in pure breeding varieties	Provided a reliable genetic starting point

 

The Seven Contrasting Characters Studied by Mendel

Mendel examined seven traits, each with two clearly distinguishable forms.

Character	Dominant Form	Recessive Form
Seed shape	Round	Wrinkled
Seed color	Yellow	Green
Flower color	Purple (violet)	White
Pod shape	Inflated	Constricted
Pod color	Green	Yellow
Flower position	Axial	Terminal
Stem height	Tall	Dwarf

 

Each of these traits is controlled by a single gene with two alternative forms, a simplicity that made the underlying pattern of inheritance possible to detect.

3. Important Genetic Terminology

A firm grasp of vocabulary is essential before analyzing crosses.

Term	Definition
Gene	A unit of heredity. A segment of DNA that codes for a particular trait or product
Allele	An alternative form of a gene, occupying the same locus on homologous chromosomes, for example the allele for tallness versus the allele for dwarfism
Character	A heritable feature of an organism, for example flower color
Trait	A specific form or variant of a character, for example purple flower color
Dominant allele	An allele that expresses its phenotype even in the presence of a different, recessive, allele. Conventionally symbolized with an uppercase letter such as T
Recessive allele	An allele whose phenotype is masked when paired with a dominant allele, expressed only in the homozygous state, symbolized with a lowercase letter such as t
Homozygous	Having two identical alleles for a gene, such as TT or tt
Heterozygous	Having two different alleles for a gene, such as Tt
Genotype	The genetic constitution of an organism for a given trait, such as Tt
Phenotype	The observable physical or biochemical expression of a genotype, such as a tall plant
Pure line, true breeding	A homozygous lineage that produces offspring identical to the parents when self fertilized, generation after generation
Hybrid	The offspring produced by crossing two genetically different, usually pure breeding, parents
Gamete	A haploid reproductive cell such as sperm or egg in animals, or pollen or ovule in plants, that carries one allele of each gene
P generation	The parental generation, the true breeding individuals originally crossed
F1 generation	The first filial generation, offspring of the P cross
F2 generation	The second filial generation, offspring produced by self or inter crossing the F1 individuals

 

Exam Tip: Genotype is written in letters, such as Tt, while phenotype is described in words, such as tall. Students frequently confuse the two. Remember that phenotype is what you see, and genotype is what is encoded.

4. Mendel's Monohybrid Cross

A monohybrid cross tracks the inheritance of a single gene or character between two parents differing in that one trait.

Experimental Design, Using Stem Height as an Example

1.   Mendel began with two true breeding, pure, parental lines: one homozygous tall (TT) and one homozygous dwarf (tt).

2.   He cross pollinated them by hand, producing the F1 generation.

3.   All F1 plants were tall, despite one parent being dwarf. The dwarf trait seemed to disappear.

4.   Mendel then allowed the F1 plants to self pollinate, producing the F2 generation.

5.   In the F2 generation, the dwarf trait reappeared, in a ratio of approximately 3 tall to 1 dwarf.

 

Punnett Square: P Generation Cross (TT x tt)

	T	T
t	Tt	Tt
t	Tt	Tt

 

All F1 offspring are Tt, heterozygous and tall. This is why the F1 generation is phenotypically uniform.

Punnett Square: F1 x F1 Cross (Tt x Tt)

	T	t
T	TT	Tt
t	Tt	tt

 

F2 Genotypic Ratio: 1 TT to 2 Tt to 1 tt

F2 Phenotypic Ratio: 3 Tall to 1 Dwarf

Why the 3 to 1 Ratio Occurs

Each F1 parent (Tt) produces two types of gametes, T and t, in equal proportion, because of the Law of Segregation described below. When gametes combine randomly at fertilization, the outcome can be broken down as follows.

•       One quarter of offspring receive T from both parents, producing TT, which is tall

•       Two quarters of offspring receive T from one parent and t from the other, producing Tt, which is tall since T is dominant

•       One quarter of offspring receive t from both parents, producing tt, which is dwarf

 

Because both TT and Tt plants are phenotypically tall, the 1 to 2 to 1 genotypic ratio collapses into a 3 to 1 phenotypic ratio, that is 3 tall to 1 dwarf.

Law of Dominance

When two alleles of a contrasting pair are present in a heterozygote, only one, the dominant allele, is phenotypically expressed, while the effect of the other, the recessive allele, remains masked.

This law explains why the F1 generation showed only the tall phenotype, even though it carried both the T and t alleles.

Law of Segregation

The two alleles of a gene, present together in a diploid individual, separate from each other during gamete formation, so that each gamete receives only one allele. Upon fertilization, the full complement is restored.

This is often called Mendel's First Law, and it directly explains the reappearance of the recessive trait in the F2 generation. Segregation ensures both T and t gametes are produced by a Tt plant, allowing tt offspring to arise.

5. Mendel's Dihybrid Cross

A dihybrid cross tracks the simultaneous inheritance of two genes or characters, such as seed shape and seed color.

Experimental Design

6.   Mendel crossed a plant that was true breeding for round, yellow seeds (RRYY) with one true breeding for wrinkled, green seeds (rryy).

7.   All F1 offspring were round and yellow (RrYy), showing that round and yellow are dominant.

8.   F1 plants were self pollinated to produce the F2 generation.

9.   The F2 generation displayed four phenotypic classes in a ratio of 9 to 3 to 3 to 1.

•       9 Round, Yellow

•       3 Round, Green

•       3 Wrinkled, Yellow

•       1 Wrinkled, Green

 

Gamete Formation

A heterozygous RrYy individual produces four types of gametes in equal frequency: RY, Ry, rY, and ry. This occurs because the alleles for seed shape (R and r) and seed color (Y and y) assort into gametes independently of one another.

Four by Four Punnett Square (RrYy x RrYy)

	RY	Ry	rY	ry
RY	RRYY	RRYy	RrYY	RrYy
Ry	RRYy	RRyy	RrYy	Rryy
rY	RrYY	RrYy	rrYY	rrYy
ry	RrYy	Rryy	rrYy	rryy

 

Deriving the 9 to 3 to 3 to 1 Ratio

Grouping the sixteen boxes by phenotype gives the following distribution.

Phenotype	Genotypes Included	Count	Fraction
Round, Yellow	RRYY, RRYy, RrYY, RrYy	9	9 out of 16
Round, Green	RRyy, Rryy	3	3 out of 16
Wrinkled, Yellow	rrYY, rrYy	3	3 out of 16
Wrinkled, Green	rryy	1	1 out of 16

 

This ratio can also be derived by multiplying the two independent monohybrid ratios: three Round to one Wrinkled, multiplied by three Yellow to one Green, gives nine to three to three to one.

Law of Independent Assortment

Alleles of different genes, located on different chromosome pairs, or far apart on the same chromosome, assort independently of one another during gamete formation. The inheritance of one trait does not influence the inheritance of another.

This is Mendel's Second Law. It holds strictly true only for genes that are unlinked, meaning they are on separate chromosomes, or so far apart on the same chromosome that recombination effectively separates them. Genes that are physically close together on the same chromosome tend to be inherited together, a phenomenon called linkage, discussed in Section 8.

Monohybrid Versus Dihybrid Inheritance: A Comparison

Feature	Monohybrid Cross	Dihybrid Cross
Number of traits tracked	1	2
Gamete types from a heterozygote	2	4
Punnett square size	2 by 2	4 by 4
F2 phenotypic ratio	3 to 1	9 to 3 to 3 to 1
F2 genotypic ratio	1 to 2 to 1	1:2:1:2:4:2:1:2:1
Law demonstrated	Segregation, Dominance	Independent Assortment

6. Mendel's Laws of Inheritance: Summary

Law	Statement	Key Evidence
Law of Dominance	In a heterozygote, one allele (dominant) masks the expression of the other (recessive)	Uniform tall F1 from Tt
Law of Segregation	The two alleles of a gene separate during gamete formation. Each gamete carries only one allele	3 to 1 ratio in F2 of monohybrid cross
Law of Independent Assortment	Alleles of different genes on different chromosomes segregate independently of each other during gamete formation	9 to 3 to 3 to 1 ratio in F2 of dihybrid cross

7. Back Cross and Test Cross

These two cross types are often confused but serve distinct purposes.

Test Cross

A test cross is a cross between an individual of unknown genotype, showing the dominant phenotype, and a homozygous recessive individual. It is used to determine whether the unknown individual is homozygous dominant or heterozygous.

•       If the unknown parent is homozygous dominant (TT): TT crossed with tt produces all offspring that are Tt and tall, a uniform result

•       If the unknown parent is heterozygous (Tt): Tt crossed with tt produces approximately half tall (Tt) and half dwarf (tt), a mixed result

 

The appearance of any recessive phenotype offspring proves the unknown parent was heterozygous.

Back Cross

A back cross is a broader term. It refers to a cross between an F1 hybrid and either of its original parental genotypes, dominant or recessive. Its main uses are to reintroduce or reinforce a desirable parental trait in breeding programs, and to analyze the genetic composition of the F1 hybrid.

Difference Between Back Cross and Test Cross

Feature	Back Cross	Test Cross
Definition	F1 hybrid crossed with either parent	Unknown genotype crossed with homozygous recessive individual
Purpose	Trait reinforcement or breeding	Determining an unknown genotype
Parent used	Can be dominant or recessive parent	Always the homozygous recessive individual
Relationship	General category	A specific type of back cross

 

Key takeaway: every test cross is technically a back cross, but not every back cross is a test cross.

8. Extensions and Limitations of Mendelian Inheritance

Mendel's laws describe idealized inheritance for genes with simple dominant or recessive relationships on separate chromosomes. Many real inheritance patterns are more complex.

Incomplete Dominance

Neither allele is fully dominant. The heterozygote shows an intermediate, blended, phenotype. In four o'clock plants, Mirabilis jalapa, a cross between red flowered (RR) and white flowered (rr) plants produces pink flowered (Rr) offspring, an intermediate blend. The F2 ratio is 1 Red to 2 Pink to 1 White, where the genotypic ratio equals the phenotypic ratio, unlike simple dominance.

Codominance

Both alleles are fully and simultaneously expressed in the heterozygote, without blending. Examples include the human ABO blood group system, and roan coat color in cattle, where red and white hairs are both distinctly visible.

Multiple Alleles

Some genes have more than two allelic forms in the popula
tion, though any individual carries only two. Human ABO blood type is controlled by three alleles, I superscript A, I superscript B, and i, where I superscript A and I superscript B are codominant to each other and both dominant to i.

Genotype	Phenotype, Blood Group
IA IA or IA i	A
IB IB or IB i	B
IA IB	AB
ii	O

 

Lethal Alleles

Certain alleles, usually in the homozygous state, cause death of the organism before or shortly after birth, distorting expected Mendelian ratios. The yellow coat color allele in mice is dominant for coat color but lethal when homozygous, since YY dies, producing a 2 to 1 ratio of yellow to agouti instead of 3 to 1 among survivors.

Epistasis

One gene's expression masks or modifies the phenotypic expression of another, non allelic gene. In Labrador retrievers, the E gene determines whether pigment is deposited in the coat at all. If a dog is homozygous recessive (ee), it appears yellow regardless of its genotype at the separate B gene for black or brown color.

Pleiotropy

A single gene influences multiple, seemingly unrelated phenotypic traits. The allele causing sickle cell anemia in humans affects red blood cell shape, causes anemia, and also affects organs such as the spleen and kidneys, all from one mutation.

Polygenic Inheritance

A single trait is controlled by the cumulative, additive effects of multiple genes, producing continuous variation rather than discrete categories. Human skin color and height are polygenic traits, showing a continuous range rather than distinct classes.

Linkage and Recombination

Genes located close together on the same chromosome tend to be inherited together rather than assorting independently, which violates the Law of Independent Assortment. Crossing over during meiosis can separate linked genes, producing recombinant gamete types, though at a lower frequency than parental types.

Sex Linked Inheritance

Genes located on the sex chromosomes, usually the X chromosome, show inheritance patterns tied to the sex of the offspring, since males (XY) have only one X chromosome and are hemizygous for X linked genes. Human red green color blindness and hemophilia are X linked recessive conditions, appearing far more frequently in males than females, since a single copy of the recessive allele is sufficient to produce the phenotype in males.

Quick Reference: Extensions to Mendelian Ratios

Phenomenon	Effect on Classic Ratio	Example
Incomplete dominance	1 to 2 to 1 phenotypic, not 3 to 1	Four o'clock flower color
Codominance	Both traits visible together	ABO blood groups, roan cattle
Multiple alleles	More than two alleles in population	ABO blood groups
Lethal alleles	Ratio distorted, for example 2 to 1	Yellow coat color in mice
Epistasis	9 to 3 to 3 to 1 modified, for example 9 to 3 to 4	Labrador coat color
Pleiotropy	One gene, multiple traits	Sickle cell allele
Polygenic inheritance	Continuous variation, not discrete classes	Human height, skin color
Linkage	Deviation from independent assortment	Genes on same chromosome
Sex linkage	Trait frequency differs by sex	Color blindness, hemophilia

9. Applications of Mendelian Genetics

•       Human genetics: predicting inheritance patterns of traits and genetic disorders, such as cystic fibrosis and sickle cell anemia, within families

•       Medical genetics: understanding recessive and dominant genetic diseases to guide diagnosis and management

•       Plant and animal breeding: selecting for desirable dominant or recessive traits such as disease resistance, yield, and coat color, using controlled crosses, test crosses, and back crosses

•       Agriculture: development of hybrid crop varieties with superior traits, known as hybrid vigor

•       Evolutionary biology: Mendelian ratios underpin population genetics models, such as Hardy Weinberg equilibrium, that describe allele frequency changes over time

•       Genetic counselling: estimating the probability that prospective parents will have a child with a particular inherited condition, based on known genotypes and pedigrees

•       Biotechnology and modern genetics: Mendelian principles inform techniques such as marker assisted selection, genetic engineering, and CRISPR based gene editing, which build upon a molecular understanding of what Mendel called hereditary factors

10. Common Misconceptions

Misconception	Correction
Dominant traits are more common in a population	Dominance refers to phenotypic expression in a heterozygote, not the trait's frequency in a population. A dominant allele can be rare
Recessive traits are weaker or less important biologically	Recessive simply means the trait is masked when a dominant allele is present. It has no bearing on the trait's biological significance
Genotype and phenotype are interchangeable terms	Genotype is the genetic makeup, such as Tt. Phenotype is the observable outward expression, such as tall
Blending inheritance explains offspring traits	Except in cases of incomplete dominance, alleles do not blend. They remain discrete units that segregate unchanged across generations, which is precisely what Mendel's work disproved
All traits follow simple 3 to 1 or 9 to 3 to 3 to 1 ratios	Many traits show more complex inheritance due to incomplete dominance, codominance, epistasis, linkage, or polygenic control, see Section 8
A dominant allele is always better or normal, and recessive is always abnormal	Either allele can be associated with a disease or an advantageous trait. Dominance is a molecular and functional relationship, not a value judgment

11. Worked Genetic Problems

Problem 1: Monohybrid Cross, Basic

Question: In pea plants, purple flower color (P) is dominant over white (p). A heterozygous purple flowered plant (Pp) is self pollinated. What are the expected genotypic and phenotypic ratios in the offspring?

Solution. Cross: Pp with Pp.

	P	p
P	PP	Pp
p	Pp	pp

 

Genotypic ratio: 1 PP to 2 Pp to 1 pp. Phenotypic ratio: 3 Purple to 1 White.

Problem 2: Test Cross

Question: A tall pea plant of unknown genotype is crossed with a dwarf plant. The offspring consist of 48 tall and 52 dwarf plants. What is the genotype of the tall parent?

Solution. Since roughly half the offspring are dwarf, a recessive phenotype, the tall parent must be heterozygous. Cross: Tt with tt produces 1 Tt (tall) to 1 tt (dwarf), matching the observed near 1 to 1 ratio. The genotype of the tall parent is Tt.

Problem 3: Dihybrid Cross

Question: A pea plant heterozygous for seed shape and seed color (RrYy) is self pollinated. What fraction of the offspring is expected to be round and green?

Solution. Treat each gene separately. Probability of round (R underscore) is 3 out of 4. Probability of green (yy) is 1 out of 4. Round and green together equals 3 out of 4 multiplied by 1 out of 4, which is 3 out of 16.

Problem 4: Dihybrid Test Cross

Question: A plant with genotype RrYy is test crossed with a homozygous recessive plant (rryy). What phenotypic ratio is expected in the offspring?

Solution. The RrYy parent produces four gamete types in equal proportion: RY, Ry, rY, and ry. The rryy parent contributes only ry gametes. Offspring genotypes are RrYy, Rryy, rrYy, and rryy, each in equal, one quarter, proportion.

Phenotypic ratio: 1 Round Yellow to 1 Round Green to 1 Wrinkled Yellow to 1 Wrinkled Green, that is 1 to 1 to 1 to 1.

Problem 5: Probability, Multiple Offspring

Question: Two heterozygous (Aa) parents plan to have four children. What is the probability that exactly one of the four children shows the recessive phenotype (aa)?

Solution. Probability of a single child being aa is one quarter. Probability of not being aa is three quarters. Using the binomial probability formula for exactly one success in four trials: the number of ways to choose 1 outcome from 4 trials, multiplied by one quarter to the first power, multiplied by three quarters to the third power, equals 4 multiplied by one quarter multiplied by twenty seven sixty fourths, which equals 27 out of 64.

Problem 6: Incomplete Dominance

Question: In four o'clock plants, red (RR) is crossed with white (rr), producing pink (Rr) F1 offspring. If two pink F1 plants are crossed, what genotypic and phenotypic ratios are expected in the F2?

Solution. Cross: Rr with Rr produces 1 RR (red) to 2 Rr (pink) to 1 rr (white). Since there is no dominance, the genotypic ratio equals the phenotypic ratio: 1 Red to 2 Pink to 1 White.

Problem 7: Multiple Alleles, ABO Blood Type

Question: A woman with blood type A (genotype IA i) and a man with blood type B (genotype IB i) have children. What blood types, and in what ratio, are possible?

Solution. Mother's gametes: IA or i. Father's gametes: IB or i.

	IB	i
IA	IA IB, type AB	IA i, type A
i	IB i, type B	ii, type O

 

Expected ratio: 1 AB to 1 A to 1 B to 1 O.

Problem 8: Sex Linked Inheritance

Question: A carrier woman for hemophilia (XH Xh) marries an unaffected man (XH Y). What proportion of their sons and daughters are expected to be affected?

Solution.

	XH	Y
XH	XH XH	XH Y
Xh	XH Xh	Xh Y

 

Daughters: one half XH XH, unaffected, and one half XH Xh, carrier and unaffected. No affected daughters are expected.

Sons: one half XH Y, unaffected, and one half Xh Y, affected. Fifty percent of sons are expected to be affected.

12.Exam Oriented Section

Key Points to Remember

•       Mendel worked with Pisum sativum and studied seven contrasting traits

•       The monohybrid F2 ratio is 3 to 1 phenotypic and 1 to 2 to 1 genotypic

•       The dihybrid F2 ratio is 9 to 3 to 3 to 1

•       The Law of Segregation applies to a single gene pair. The Law of Independent Assortment applies to two or more gene pairs on different chromosomes

•       A test cross always uses a homozygous recessive individual to reveal an unknown genotype

•       Not all traits show simple dominant or recessive inheritance. Incomplete dominance, codominance, multiple alleles, epistasis, and polygenic inheritance are important extensions

 

Frequently Asked Conceptual Questions

10.        Why did the recessive trait disappear in the F1 generation but reappear in the F2 generation?

11.        How does the Law of Segregation explain the 3 to 1 ratio?

12.        Why does a dihybrid cross require a four by four Punnett square rather than a two by two square?

13.        What is the biological basis, in terms of meiosis, for independent assortment?

14.        Why is a test cross always performed with a homozygous recessive individual rather than a homozygous dominant one?

 

Short Answer Questions

15.        Define genotype and phenotype with an example.

16.        Differentiate between homozygous and heterozygous genotypes.

17.        State the Law of Dominance.

18.        What is a pure line? Why was it important to Mendel's experiments?

19.        Differentiate between a test cross and a back cross.

 

Long Answer Questions

20.        Describe Mendel's monohybrid cross experiment in detail, explaining the genetic basis of the 3 to 1 F2 ratio.

21.        Explain Mendel's dihybrid cross experiment and derive the 9 to 3 to 3 to 1 ratio using a Punnett square.

22.        Discuss the extensions of Mendelian inheritance, with suitable examples for each.

23.        Explain sex linked inheritance in humans, using color blindness or hemophilia as an example.

24.        Compare and contrast incomplete dominance and codominance with examples.

 

Multiple Choice Questions with Answer Key

1. Who is known as the Father of Genetics?

a) Charles Darwin   b) Gregor Mendel   c) Thomas Morgan   d) James Watson

Answer: b 

2. Mendel conducted his experiments primarily on which organism?

a) Drosophila melanogaster   b) Pisum sativum   c) Zea mays   d) Escherichia coli

Answer: b 

3. The genotypic ratio in the F2 generation of a monohybrid cross is:

a) 3:1   b) 9:3:3 :1   c) 1:2:1   d) 1:1

Answer: c 

4. Which law explains the separation of allele pairs during gamete formation?

a) Law of Dominance   b) Law of Segregation   c) Law of Independent Assortment   d) Law of Codominance

Answer: b 

5. A cross between an individual of unknown genotype and a homozygous recessive individual is called a:

a) Back cross   b) Dihybrid cross   c) Test cross   d) Reciprocal cross

Answer: c 

6. Roan coat color in cattle is an example of:

a) Incomplete dominance   b) Codominance   c) Epistasis   d) Polygenic inheritance

Answer: b 

7. How many phenotypic classes appear in a typical dihybrid F2 generation?

a) 2   b) 3   c) 4   d) 16

Answer: c 

8. The ABO blood group system in humans is an example of:

a) Simple dominance only   b) Multiple alleles with codominance   c) Polygenic inheritance   d) Sex linked inheritance

Answer: b 

9. In pea plants, a heterozygous tall plant (Tt) is test crossed. What phenotypic ratio is expected in the offspring?

a) 3 to 1   b) 1 to 1   c) 1 to 2 to 1   d) 9 to 3 to 3 to 1

Answer: b 

10. Genes located close together on the same chromosome and inherited together demonstrate:

a) Independent assortment   b) Codominance   c) Linkage   d) Polygenic inheritance

Answer: c 

Numerical and Genetic Cross Practice Questions

25.        In pea plants, tall (T) is dominant over dwarf (t), and round seeds (R) are dominant over wrinkled seeds (r). A plant heterozygous for both traits (TtRr) is self pollinated. What fraction of the offspring is expected to be dwarf with round seeds?

26.        A man with blood type O and a woman with blood type AB have children. What blood types are possible in their offspring, and in what proportion?

27.        Two pea plants, both heterozygous for flower color, with purple dominant over white, are crossed. Out of 200 offspring, how many are expected to have white flowers?

28.        A carrier mother for red green color blindness and a color blind father have children. What fraction of their daughters is expected to be color blind?

29.        If a heterozygous tall, yellow seeded plant (TtYy) is crossed with a homozygous dwarf, green seeded plant (ttyy), what phenotypic ratio is expected in the offspring?

13. Modern Perspective: Mendel's Principles and Molecular Genetics

Mendel formulated his laws decades before the physical basis of heredity was understood. Today, his hereditary factors are known to be genes, made of DNA, located at specific positions, called loci, on chromosomes.

•       The Law of Segregation has its physical basis in the behavior of homologous chromosomes during meiosis one, when homologous chromosome pairs, each carrying one allele, separate into different gametes

•       The Law of Independent Assortment reflects the random orientation of homologous chromosome pairs on the metaphase plate during meiosis one, since different chromosome pairs align independently of one another

•       Genes are now understood at the molecular level as DNA sequences that are transcribed into RNA and, in many cases, translated into proteins. These proteins ultimately produce the observable phenotype

•       Mendel could not have known about chromosomes, DNA, or meiosis. His laws were derived purely from statistical patterns in breeding data. Later scientists, notably Walter Sutton and Theodor Boveri through the Chromosome Theory of Inheritance, and Thomas Hunt Morgan through his work on linkage and sex linked inheritance in Drosophila, connected Mendel's abstract factors to physical chromosomes

•       Modern molecular genetics has revealed the many exceptions and extensions to simple Mendelian ratios described in Section 8, but the core logic of segregation and independent assortment remains foundational to understanding heredity, genetic mapping, and modern techniques such as CRISPR gene editing and genetic counseling

Quick Revision Summary

•       Mendel's work with garden peas established the fundamental laws of inheritance through careful, quantitative experimentation

•       Traits are controlled by genes with alternative forms, called alleles. Dominant alleles mask recessive ones in heterozygotes

•       The Law of Segregation explains single gene inheritance, the monohybrid cross, and the 3 to 1 ratio

•       The Law of Independent Assortment explains two gene inheritance, the dihybrid cross, and the 9 to 3 to 3 to 1 ratio, when genes are on different chromosomes

•       Test crosses reveal unknown genotypes by crossing with a homozygous recessive individual

•       Real world inheritance often deviates from simple Mendelian ratios due to incomplete dominance, codominance, multiple alleles, lethal alleles, epistasis, pleiotropy, polygenic inheritance, linkage, and sex linkage

•       Mendel's laws find their physical explanation in the behavior of chromosomes during meiosis

 

Key Ratios at a Glance

Cross Type	Genotypic Ratio	Phenotypic Ratio
Monohybrid (Aa by Aa)	1 to 2 to 1	3 to 1
Monohybrid test cross (Aa by aa)	1 to 1	1 to 1
Dihybrid (AaBb by AaBb)	1:2:1:2:4:2:1:2:1	9 to 3 to 3 to 1
Dihybrid test cross (AaBb by aabb)	1 to 1 to 1 to 1	1 to 1 to 1 to 1
Incomplete dominance (Aa by Aa)	1 to 2 to 1	1 to 2 to 1, same as genotypic

 

Important Terms at a Glance

Gene, allele, genotype, phenotype, homozygous, heterozygous, dominant, recessive, pure line, hybrid, gamete, P and F1 and F2 generations, test cross, back cross, incomplete dominance, codominance, multiple alleles, epistasis, pleiotropy, polygenic inheritance, linkage, and sex linked inheritance.

References

1. Klug, W. S., Cummings, M. R., Spencer, C. A., and Palladino, M. A. Concepts of Genetics. Pearson Education.
2. Pierce, B. A. Genetics: A Conceptual Approach. W. H. Freeman.
3. Snustad, D. P., and Simmons, M. J. Principles of Genetics. Wiley.
4. Griffiths, A. J. F., and colleagues. An Introduction to Genetic Analysis. W. H. Freeman.
5. Mendel, G. (1866). Versuche über Pflanzen Hybriden (Experiments on Plant Hybrids), Mendel's original publication, available in translation through various genetics history archives.

Zoology · Genetics Unit

Mendelian
Inheritance

FIELD QUIZ NO. 07
25 QUESTIONS · MIXED DIFFICULTY

Question 1 / 25

Easy Terminology

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Final Tally

0/25

CORRECT ANSWERS

P generation → F1 → F2 · every cross is a hypothesis