Modes of Inheritance – Complete Genetics Notes

Genetics Autosomal Dominant Autosomal Recessive X-linked Recessive X-linked Dominant Y-linked Sex Influenced Sex Limited Genomic Imprinting CSIR NET GATE

1. Introduction to Modes of Inheritance

Inheritance is the transmission of genetic information from parents to their offspring. The pattern by which a particular genetic trait is transmitted through generations is called its mode of inheritance. Understanding inheritance patterns is fundamental to genetics, molecular biology, evolution, medical genetics, and genetic counseling.

The inheritance pattern of a trait depends on several factors, including the location of the gene, the relationship between alleles, whether the gene is located on an autosome or sex chromosome, whether expression differs between males and females, and whether the parent from whom an allele was inherited affects its expression.

For competitive examinations such as CSIR-NET, GATE, DBT, ICMR and other Life Science examinations, questions frequently ask students to identify a mode of inheritance from a pedigree or predict the offspring produced from a particular parental cross.

Most important concept: The first question to ask in an inheritance problem is:

Where is the gene located?
1. Autosome → Autosomal inheritance
2. X chromosome → X-linked inheritance
3. Y chromosome → Y-linked inheritance
4. Parent-of-origin dependent expression → Genomic imprinting

2. Index / Table of Contents

3. Basic Genetic Terminology

Before studying different inheritance patterns, it is important to understand some basic genetic terms.

Gene: A hereditary unit containing information that contributes to a functional product or biological trait.
Allele: An alternative form of a gene found at the same locus.
Locus: The physical location of a gene on a chromosome.
Autosome: Any chromosome other than the sex chromosomes.
Sex chromosome: A chromosome involved in sex determination and carrying many genes unrelated to sex determination as well.
Homozygous: Having two identical alleles at a locus, such as AA or aa.
Heterozygous: Having two different alleles, such as Aa.
Dominant allele: An allele whose effect is expressed in the classical heterozygous state.
Recessive allele: An allele whose classical phenotype is expressed when present in the appropriate homozygous state.
Carrier: An individual who carries a recessive allele but generally does not show the associated recessive phenotype.
Hemizygous: Having only one copy of a gene, as occurs for many X-linked genes in XY males.
Pedigree: A diagram representing the transmission of a trait through multiple generations of a family.

Exam tip: For an X-linked gene, a typical XY male has only one X chromosome and is therefore hemizygous for most X-linked genes.

4. Autosomal Dominant Inheritance

In autosomal dominant inheritance, the gene responsible for the trait is located on an autosome and one copy of the dominant disease-associated allele is sufficient to produce the phenotype in the classical complete penetrance model.

Let the dominant allele be represented by A and the recessive allele by a.

The typical genotypes are:

AA → affected

Aa → affected

aa → unaffected

An important feature is that the trait can generally occur in both males and females at similar frequencies, because the gene is located on an autosome. Father-to-son transmission is possible.

Major characteristics

The responsible gene is located on an autosome.
One copy of the dominant allele can be sufficient for the phenotype.
Males and females can be affected.
The trait often appears in successive generations in a typical pedigree.
An affected heterozygous individual can transmit the allele to approximately 50% of offspring when crossed with an unaffected homozygous individual.
Father-to-son transmission is possible.
Unaffected individuals generally do not transmit a fully penetrant dominant allele under the simplest model.

5. Autosomal Dominant Cross

Consider an affected heterozygous individual crossed with an unaffected individual:

Aa × aa

The gametes are:

Aa parent → A or a
aa parent → a only

The offspring are:

1/2 Aa → affected

1/2 aa → unaffected

Therefore, the expected phenotypic ratio is:

1 affected : 1 unaffected

Pedigree clue: An autosomal dominant trait commonly shows a vertical pattern, meaning affected individuals may be present in successive generations. However, real pedigrees can deviate because of incomplete penetrance, variable expressivity, new mutations, or other factors.

6. Autosomal Recessive Inheritance

In autosomal recessive inheritance, the gene is located on an autosome, and the classical phenotype requires two copies of the recessive allele.

Using A and a:

AA → unaffected

Aa → unaffected carrier

aa → affected

Autosomal recessive traits can occur in both males and females. Affected individuals may be born to phenotypically unaffected carrier parents. Consequently, the trait may appear to skip generations in a pedigree.

Major characteristics

Gene is located on an autosome.
Usually two disease-associated recessive alleles are required for the classical phenotype.
Males and females are affected.
Parents of an affected individual can be unaffected carriers.
The trait may appear among siblings even when it is not present in every generation.
Consanguinity can increase the probability of some rare recessive conditions because related individuals have a greater chance of sharing ancestral alleles.
Father-to-son transmission is possible.

7. Autosomal Recessive Cross

Consider two heterozygous carriers:

Aa × Aa

The possible genotypes are:

1 AA : 2 Aa : 1 aa

Phenotypically:

3 unaffected : 1 affected

Therefore, for two carrier parents under a complete-penetrance simple model:

25% → AA
50% → Aa carrier
25% → aa affected

Important: The 3:1 ratio is a phenotypic ratio, whereas 1:2:1 is the genotypic ratio.

8. X-linked Recessive Inheritance

In X-linked recessive inheritance, the responsible gene is located on the X chromosome and the phenotype is classically expressed in individuals who lack another functional X-linked allele that could mask the recessive variant.

Because typical XY males have only one X chromosome, a recessive allele on their X chromosome is often expressed. Such males are often described as hemizygous for X-linked genes.

For notation:

X^A = normal allele

X^a = recessive disease-associated allele

Typical genotypes include:

X^AX^A → unaffected female
X^AX^a → carrier female in the classical model
X^aX^a → affected female
X^AY → unaffected male
X^aY → affected male

Major characteristics

Gene is located on the X chromosome.
Males are more frequently affected in many X-linked recessive disorders.
There is no father-to-son transmission of an X-linked allele because fathers transmit their Y chromosome to sons.
An affected father passes his X chromosome to all daughters.
A carrier mother can transmit the allele to sons and daughters.
Affected females generally require an appropriate combination of recessive alleles, although real biology can involve mechanisms such as skewed X-inactivation and other factors.

9. X-linked Recessive Crosses

Carrier female × normal male

X^AX^a × X^AY

Possible offspring:

X^AX^A → unaffected daughter
X^AX^a → carrier daughter
X^AY → unaffected son
X^aY → affected son

Thus, under the classical model:

50% of sons affected

50% of daughters carriers

Do not say that all sons are affected. Only the sons who inherit the disease-associated X chromosome from a carrier mother are affected in this example.

Affected father × normal homozygous mother

X^aY × X^AX^A

All daughters receive the affected father's X chromosome and the normal mother's X chromosome:

All daughters → X^AX^a carriers

All sons → X^AY unaffected

Classic exam clue: Affected father + normal homozygous mother in an X-linked recessive trait: all daughters are carriers and all sons are unaffected, under the classical model.

10. X-linked Dominant Inheritance

In X-linked dominant inheritance, the responsible gene is located on the X chromosome and one copy of the dominant allele can be sufficient for the phenotype.

Both males and females can be affected. However, the transmission pattern is distinct because fathers transmit their X chromosome to all daughters and their Y chromosome to all sons.

Important characteristics

The responsible gene is located on the X chromosome.
Both males and females can be affected.
There is no father-to-son transmission.
An affected father transmits his affected X chromosome to all daughters.
An affected father transmits his Y chromosome to all sons.
An affected heterozygous mother can transmit the allele to approximately 50% of sons and 50% of daughters in a simple model.

11. X-linked Dominant Crosses

Affected father × unaffected mother

X^AY × X^aX^a

The father provides:

X^A to every daughter
Y to every son

Therefore:

All daughters → X^AX^a affected

All sons → X^aY unaffected

Highly important: In a classical X-linked dominant pedigree, affected fathers transmit the trait to all daughters but none of their sons.

Heterozygous affected mother × unaffected father

X^AX^a × X^aY

The expected inheritance is approximately:

50% daughters affected
50% daughters unaffected
50% sons affected
50% sons unaffected

12. Y-linked Inheritance

In Y-linked inheritance, the gene is located on the Y chromosome. Because the Y chromosome is normally transmitted from father to son, a Y-linked trait follows a highly characteristic paternal transmission pattern.

Major characteristics

Gene is located on the Y chromosome.
Only individuals with a Y chromosome can inherit a Y-linked trait in the classical model.
Females do not inherit Y-linked genes.
An affected father transmits the Y chromosome to all biological sons.
All sons of an affected father can inherit the Y-linked allele, assuming the relevant Y-linked locus is transmitted normally.
There is no father-to-daughter transmission of a Y-linked allele.
Y-linked traits can show a strictly paternal lineage pattern.

Affected father ↓ All sons ↓ Their sons ↓ Paternal lineage

Memory trick: Y-linked = Father → Son → Son → Son

Y-linked genes are not limited to genes determining male sex. The Y chromosome contains genes involved in sex determination, development, fertility, and other biological functions.

13. Sex-Influenced Inheritance

A sex-influenced trait is generally controlled by an autosomal gene, but its phenotypic expression differs between males and females.

The key point is that the gene is not necessarily located on a sex chromosome. Instead, the hormonal or physiological environment associated with sex changes the expression of the allele.

A classical example used in genetics teaching is pattern baldness. In simplified textbook models, the same allele combination can behave as dominant in males but recessive in females.

Suppose allele B contributes to the trait and b is the alternative allele. In a simplified model:

Genotype	Male	Female
BB	Trait expressed	Trait expressed
Bb	Trait expressed	Trait not expressed
bb	Trait not expressed	Trait not expressed

This illustrates how the same autosomal genotype can produce different phenotypes in males and females.

Important distinction: Sex-influenced inheritance does not mean that the gene is on the X or Y chromosome. The gene can be autosomal, but its expression is influenced by sex-related biological factors.

14. Sex-Limited Inheritance

A sex-limited trait is a trait whose phenotypic expression is restricted to one sex, even though the gene may be present in both males and females.

The gene can be carried by both sexes, but the anatomical, physiological, or hormonal conditions necessary for expression occur only in one sex.

Examples

Milk production in female mammals.
Egg production in females.
Some male-specific secondary sexual characteristics.
Some reproductive traits expressed only in one sex.

For example, genes influencing milk production can be inherited by both male and female offspring, but normal lactation is expressed physiologically in females.

Sex-limited ≠ sex-linked

Sex-linked: Gene is physically located on a sex chromosome.

Sex-limited: Gene may be autosomal, but phenotype is expressed only in one sex.

15. Sex-Influenced vs Sex-Limited Inheritance

Feature	Sex-Influenced	Sex-Limited
Gene location	Often autosomal	Often autosomal
Expression	Differs between sexes	Restricted mainly to one sex
Can both sexes carry the allele?	Yes	Yes
Example	Classical pattern baldness model	Milk production
Main clue	Same genotype may behave differently in sexes	Only one sex expresses phenotype

16. Genomic Imprinting

Genomic imprinting is an epigenetic phenomenon in which the expression of a gene depends on whether the allele was inherited from the mother or from the father.

This is known as a parent-of-origin effect.

The DNA sequence of the maternal and paternal alleles may be essentially the same at a locus, but epigenetic marks can cause one allele to be expressed while the other is silenced.

Maternal allele → expression pattern may differ

Paternal allele → expression pattern may differ

The important point is that genomic imprinting is not simply classical dominance or recessiveness. Instead, epigenetic regulation causes parent-specific expression.

Important features

It is an epigenetic phenomenon.
Expression depends on parental origin.
DNA methylation is an important mechanism involved in many imprinted regions.
Histone modifications and chromatin organization can also contribute.
Some genes are preferentially expressed from the paternal allele.
Other genes are preferentially expressed from the maternal allele.
Imprinting marks are established or reset during germ-cell development.
The phenomenon is reversible at the appropriate stage of the germline cycle rather than being a permanent mutation of the DNA sequence.

17. Mechanism of Genomic Imprinting

Genomic imprinting is primarily an epigenetic process. Epigenetic regulation changes gene activity without changing the underlying DNA sequence.

DNA methylation

DNA methylation, particularly at CpG-rich regulatory regions, can contribute to transcriptional silencing. In many imprinted genes, differentially methylated regions are important regulatory elements.

Histone modification

Histone proteins package DNA into chromatin. Chemical modifications of histones can influence whether chromatin is accessible for transcription.

Chromatin structure

Changes in chromatin organization can promote an active or inactive transcriptional state.

Non-coding RNA

Some imprinted genomic regions are regulated by long non-coding RNAs and other regulatory elements.

DNA sequence unchanged

↓

Epigenetic marking

↓

Differential gene expression

↓

Parent-of-origin effect

18. Important Examples of Genomic Imprinting

One of the most frequently discussed examples of genomic imprinting involves the 15q11-q13 region of the human genome. The phenotype resulting from certain abnormalities in this region can depend on whether the affected chromosomal segment is inherited from the father or the mother.

Prader-Willi syndrome

Prader-Willi syndrome is associated with loss of expression of specific paternally expressed genes in the 15q11-q13 region. A common mechanism is deletion of the paternal region, although other molecular mechanisms can produce the condition.

Angelman syndrome

Angelman syndrome is associated with loss of function of the maternally expressed UBE3A contribution in relevant brain tissues. A common mechanism is maternal deletion of the corresponding region, although other mechanisms can also occur.

High-yield memory:

Prader-Willi → paternal contribution affected

Angelman → maternal contribution affected

Remember that these are simplified memory rules; the underlying molecular mechanisms can include deletions, uniparental disomy, imprinting defects, and other genetic alterations.

19. Uniparental Disomy and Imprinting

Uniparental disomy (UPD) occurs when both copies of a chromosome or chromosomal region are inherited from one parent rather than receiving one copy from each parent.

UPD becomes especially important when the chromosome contains imprinted genes because the two parental copies are not necessarily functionally equivalent.

For example, if a chromosome region normally requires expression from the paternal copy, inheriting both copies from the mother can cause a deficiency of the required paternal expression.

Exam concept: Uniparental disomy can produce disease not because the DNA sequence of every gene is necessarily abnormal, but because the parental origin of genomic material matters for imprinted loci.

20. How to Identify Modes of Inheritance from a Pedigree

Pedigree analysis is one of the most important applications of inheritance patterns. When given a pedigree, do not immediately memorize the shape. Instead, systematically eliminate possibilities.

Step 1: Are males and females affected?

If both males and females are affected at similar frequencies, an autosomal mode becomes a strong possibility. However, sex chromosome inheritance can also affect both sexes, so additional clues are needed.

Step 2: Is father-to-son transmission present?

If yes → autosomal or Y-linked inheritance may be possible.
If no → X-linked inheritance becomes possible, but absence alone is not enough to prove it.

Step 3: Does an affected father transmit the trait to all daughters?

This is a classic clue for an X-linked dominant trait when the mother is unaffected and the affected father carries the relevant allele.

Step 4: Does an affected father transmit the trait to all sons?

This strongly suggests Y-linked inheritance in a classical pedigree.

Step 5: Does the trait skip generations?

A recessive inheritance pattern may appear to skip generations, especially when unaffected carrier parents have affected offspring.

21. Pedigree Clues for Major Modes

Mode	Major Pedigree Clue
Autosomal dominant	Often vertical; males and females affected; father-to-son possible
Autosomal recessive	May skip generations; unaffected carrier parents can have affected child
X-linked recessive	More affected males; no father-to-son transmission
X-linked dominant	Affected father transmits to all daughters, not sons
Y-linked	Father → all sons in classical transmission
Sex-influenced	Phenotypic expression differs between sexes
Sex-limited	Phenotype expressed only in one sex
Genomic imprinting	Expression depends on parent of origin

22. Probability in Inheritance Problems

Probability is frequently used in genetic inheritance questions. Two basic rules are particularly important.

Multiplication rule

When two independent events must both occur, their probabilities are multiplied.

P(A and B) = P(A) × P(B)

For example, if the probability of inheriting allele A is 1/2 and the probability of inheriting allele B is 1/2, then the probability of inheriting both is:

1/2 × 1/2 = 1/4

Addition rule

When either of two mutually exclusive events can produce the desired outcome, their probabilities are added.

P(A or B) = P(A) + P(B)

These principles become especially useful when analyzing sex-linked inheritance and pedigree probability.

23. Comparison of Major Modes of Inheritance

Mode	Gene Location	Main Feature	Father-to-Son?
Autosomal Dominant	Autosome	One dominant allele sufficient	Yes
Autosomal Recessive	Autosome	Two recessive alleles generally required	Yes
X-linked Recessive	X chromosome	Often more males affected	No
X-linked Dominant	X chromosome	Affected father → all daughters	No
Y-linked	Y chromosome	Father → sons	Yes, paternal-line transmission
Sex-influenced	Often autosome	Expression differs by sex	Depends on gene
Sex-limited	Often autosome	Expression restricted to one sex	Depends on gene
Genomic imprinting	Specific imprinted regions	Parent-of-origin dependent expression	Depends on locus

24. Sex-Linked vs Sex-Influenced vs Sex-Limited

These three terms are commonly confused in examinations. They must be clearly distinguished.

Sex-linked

The gene is physically located on a sex chromosome, such as X or Y.

Sex-influenced

The gene can be autosomal, but its phenotypic expression differs between males and females.

Sex-limited

The gene can be inherited by both sexes, but the phenotype is expressed primarily or exclusively in one sex.

25. CSIR-NET / GATE High-Yield Points

Autosomal genes are located on non-sex chromosomes.
Autosomal dominant traits can show vertical inheritance.
Autosomal recessive traits can appear in children of unaffected carrier parents.
Father-to-son transmission is possible for autosomal traits.
X-linked genes are located on the X chromosome.
A typical XY male is hemizygous for many X-linked genes.
There is no father-to-son transmission of an X-linked allele.
An affected father transmits his X chromosome to all daughters.
In X-linked dominant inheritance, an affected father with an unaffected mother transmits the phenotype to all daughters and none of the sons, under the classical model.
In X-linked recessive inheritance, affected males can inherit the allele from carrier mothers.
Y-linked traits are transmitted through the paternal lineage.
A Y-linked allele is transmitted from father to son.
Sex-influenced traits are generally not necessarily sex-linked.
Sex-limited traits may involve autosomal genes whose expression is restricted by biological sex.
Genomic imprinting is an epigenetic parent-of-origin effect.
DNA methylation is an important mechanism associated with genomic imprinting.
Prader-Willi and Angelman syndromes are classic examples associated with parent-of-origin effects at chromosome 15q11-q13.
Uniparental disomy can have major consequences at imprinted loci.
Do not confuse genotype ratio with phenotype ratio.
Always examine whether the gene is autosomal, X-linked or Y-linked before calculating offspring probabilities.

26. Common Mistakes Students Make

Mistake 1: Calling every sex-related trait X-linked

A trait can be sex-influenced or sex-limited even when its gene is located on an autosome.

Mistake 2: Assuming all X-linked traits are recessive

X-linked genes can show dominant or recessive inheritance.

Mistake 3: Forgetting that fathers give Y to sons

For a typical XY male, the father contributes the Y chromosome to a son and the X chromosome to a daughter. This single fact solves many X-linked and Y-linked problems.

Mistake 4: Assuming affected father means affected son

This is not true for X-linked inheritance. A father gives his Y chromosome to his son, so an X-linked allele from the father does not go directly to the son.

Mistake 5: Confusing Y-linked and X-linked inheritance

Y-linked inheritance is father-to-son through the Y chromosome. X-linked inheritance follows the X chromosome and therefore has a different pattern.

Mistake 6: Confusing genomic imprinting with mutation

Genomic imprinting is primarily an epigenetic parent-of-origin effect. It does not necessarily involve a change in the DNA sequence.

Mistake 7: Forgetting parent of origin

For imprinted genes, knowing that an allele is present may not be enough. You must know whether it was inherited from the mother or father.

27. Easy Memory Tricks

Autosomal Dominant

One dominant allele = phenotype.

Think: AD → Appears in successive generations.

Autosomal Recessive

Two recessive alleles = phenotype.

Think: AR → Can Appear after a generation gap.

X-linked Recessive

Think:

Mother → Son

Carrier mothers are important sources of affected sons in classical X-linked recessive pedigrees.

X-linked Dominant

Affected Father → ALL Daughters

Affected Father → NO Sons

Y-linked

Father → Son → Son → Son

Sex-influenced

Same gene, different expression in males and females.

Sex-limited

Gene in both sexes, phenotype mainly in one sex.

Genomic imprinting

Parent of origin matters.

28. Practice MCQs – 10 Important Questions

Try these questions before revealing the answers. These MCQs are designed around the high-yield concepts of modes of inheritance.

Q1. Which inheritance pattern commonly shows an affected father transmitting the trait to all daughters but none of his sons? A. Autosomal dominant B. X-linked dominant C. Y-linked D. X-linked recessive

Correct Answer: B. X-linked dominant
An affected father transmits his X chromosome to all daughters and his Y chromosome to all sons. Therefore, in a classical X-linked dominant cross with an unaffected mother, all daughters can be affected while sons are not.

Q2. Which mode of inheritance shows father-to-son transmission through the Y chromosome? A. X-linked recessive B. X-linked dominant C. Y-linked D. Autosomal recessive

Correct Answer: C. Y-linked
The Y chromosome is transmitted from father to son. Therefore, a Y-linked trait follows a paternal-line inheritance pattern.

Q3. A carrier female for an X-linked recessive condition has children with an unaffected male. What fraction of sons is expected to be affected under the classical model? A. 0% B. 25% C. 50% D. 100%

Correct Answer: C. 50%
A carrier mother has a 1/2 probability of transmitting the recessive X-linked allele to each son. A son who receives that allele is affected in the classical model.

Q4. Which statement correctly describes autosomal recessive inheritance? A. Only males are affected. B. An affected child can have unaffected carrier parents. C. Father-to-son transmission is impossible. D. The gene is located on the Y chromosome.

Correct Answer: B.
In autosomal recessive inheritance, two unaffected carrier parents can have an affected child.

Q5. A gene is present in both males and females, but the phenotype is expressed only in females. Which term best describes this pattern? A. X-linked dominant B. Y-linked C. Sex-limited D. X-linked recessive

Correct Answer: C. Sex-limited
A sex-limited trait can involve a gene carried by both sexes while the phenotype is expressed only in one sex.

Q6. In a sex-influenced trait, which statement is most appropriate? A. The gene must be on the Y chromosome. B. The gene must be on the X chromosome. C. The same genotype may have different phenotypic expression in males and females. D. Only females can carry the allele.

Correct Answer: C.
Sex-influenced inheritance occurs when sex-related biological factors alter the expression of a gene. The gene is often autosomal.

Q7. Genomic imprinting is best described as: A. A permanent DNA sequence mutation. B. Parent-of-origin dependent gene expression. C. Y chromosome inheritance. D. Mitochondrial DNA replication.

Correct Answer: B. Parent-of-origin dependent gene expression
Genomic imprinting is an epigenetic phenomenon in which gene expression depends on whether the allele was inherited from the mother or father.

Q8. Which of the following is associated with the classical study of genomic imprinting? A. Prader-Willi and Angelman syndromes B. Sickle-cell anemia only C. Hemophilia only D. Y-linked inheritance only

Correct Answer: A.
Prader-Willi and Angelman syndromes are classic examples of disorders associated with parent-of-origin effects and genomic imprinting at chromosome 15q11-q13.

Q9. Which inheritance pattern generally allows father-to-son transmission? A. X-linked recessive B. X-linked dominant C. Autosomal inheritance D. Both B and C

Correct Answer: C. Autosomal inheritance
A father can transmit an autosomal allele to a son. He cannot transmit his X chromosome to a son, so classical X-linked traits do not show father-to-son transmission.

Q10. Which statement is TRUE for a typical Y-linked trait? A. It is transmitted from mother to all daughters. B. It can be transmitted from father to son. C. It affects females through the Y chromosome. D. It must skip every generation.

Correct Answer: B. It can be transmitted from father to son.
Y-linked genes are located on the Y chromosome and are transmitted through the paternal line.

29. Quick Revision Chart

Inheritance	Remember This
Autosomal Dominant	One dominant allele; both sexes; father-to-son possible
Autosomal Recessive	Two recessive alleles; carrier parents possible
X-linked Recessive	More males often affected; no father-to-son transmission
X-linked Dominant	Affected father → all daughters, no sons
Y-linked	Father → son → paternal lineage
Sex-influenced	Same gene, different expression in males and females
Sex-limited	Gene in both sexes, phenotype restricted to one sex
Genomic Imprinting	Parent of origin determines expression

AUTOSOMAL → Both sexes + father-to-son possible

X-LINKED → No father-to-son transmission

Y-LINKED → Father → Son

SEX-INFLUENCED → Expression differs by sex

SEX-LIMITED → Expression restricted to one sex

IMPRINTING → Parent of origin matters

30. Final Exam Strategy

When a genetics question asks you to identify the mode of inheritance, do not try to recognize the answer only from the disease name. Instead, analyze the transmission pattern.

Identify the chromosome. Ask whether the gene is autosomal, X-linked or Y-linked.
Look for father-to-son transmission. If present, X-linked inheritance is generally excluded for that allele.
Check whether an affected father passes the trait to all daughters. This is an important clue for X-linked dominant inheritance.
Check whether all sons of affected fathers are affected. This suggests a classical Y-linked pattern.
Look for unaffected parents producing affected offspring. This can suggest recessive inheritance.
Check whether the expression differs between males and females. This may indicate sex-influenced inheritance.
Check whether only one sex expresses the trait. This suggests a sex-limited trait.
Ask whether maternal versus paternal origin matters. If yes, consider genomic imprinting.

Ultimate Memory Table

Autosomal Dominant: One dominant allele → phenotype.

Autosomal Recessive: Two recessive alleles → phenotype.

X-linked Recessive: Often males affected; carrier mother → affected sons.

X-linked Dominant: Affected father → all daughters, no sons.

Y-linked: Father → sons.

Sex Influenced: Same genotype → different expression according to sex.

Sex Limited: Gene in both sexes → phenotype mainly in one sex.

Genomic Imprinting: Same allele → expression depends on parental origin.

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