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Sex-linked inheritance is a term used in genetics which is defined as the transmission of traits or characteristics from parents to offspring. In such a type of inheritance, the characters are influenced by genes present on the X chromosome. This is because more number of genes are found in X chromosome than the Y chromosome.
- Males have a single X chromosome, while two copies of X chromosomes are found in females.
- Sex-linked Inheritance examples include hemophilia, colorblindness, Duchenne muscular dystrophy, etc.
What is Sex-linked Inheritance?
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The biological process of transmission of characters from parents to offspring is called sex-linked inheritance. Such a type of inheritance is determined by a gene located on one of the sex chromosomes, i.e., the X chromosome and the Y chromosome.
- Females are called homogametic as they have two X chromosomes (XX). They give one X chromosome to their child.
- Males have one X and one Y chromosome (XY) are called heterogametic and are responsible for determining the sex of the child.
- Sex-linked inheritance is called criss-cross inheritance as the traits are transferred from father to daughter and then to her son. (grandchild).
- Apart from sex-linked inheritance, sex-limited traits sex-influenced characters, and sex-influenced characters were also observed.
Sex-linked Inheritance
Sex-Influenced Characters
Sex-limited characters are those that are controlled by non-sex genes (autosomal).
- One gene is dominant in one sex while in another sex it is recessive.
- Examples of sex-influenced characters are spina bifida is dominant in females and in males white forelock and harelip are dominant.
Sex-limited Characters
Sex-linked Characters are controlled by non-sex genes.
- In this type, the gene expression is only limited to only one sex because of the sex hormones.
- Examples of sex-limited characters include milk production and the production of eggs in chicken.
Types of Sex-Linked Inheritance
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Genes located on the X chromosome are termed X-linked genes, while those located on the Y chromosome are called Y-linked genes. Based on their existence, there are two types of sex-linked inheritance: X-linked Inheritance and Y-linked Inheritance.
X-linked Inheritance
In X-linked Inheritance, the genes that are responsible for certain types of traits or disorders are located in the X chromosomes.
- X chromosomes carry a huge number of genes because of their large size.
- X-linked inheritance can be either recessive or dominant.
The table below shows the difference between X-linked recessive inheritance and X-linked dominant inheritance:
| X-linked recessive Inheritance | X-linked dominant Inheritance |
|---|---|
| Two mutated X chromosomes cause X-linked recessive disorders. | Single mutated X chromosomes cause X-linked dominant disorders. |
| The X-linked recessive disorders affect only males. | The X-linked dominant disorders affect both males and females. |
| Passed from carrier mother to sons. | Passed from the affected father to all his daughters. |
| Examples: Hemophilia and Colour Blindness. | Examples: Rett Syndrome, Incontinentia Pigmenti. |
Y-linked Inheritance
In Y-linked Inheritance, the genes that cause certain types of disorders are located in the Y chromosomes.
- Y chromosomes are small and carry a smaller number of genes.
- This type of Inheritance is found in males.
- Disorders related to Y-linked inheritance are passed down from father to son.
- Example: Retinitis pigmentos
Disorders of Sex-linked Inheritance
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Disorders of sex-linked inheritance is caused by the mutation of genes present on the X and Y chromosomes. Males are susceptible to X-linked disorders when compared with females because of the presence of a single X chromosome.
Here are some of the examples of sex-linked disorders:
Things to Remember
- Sex-linked inheritance is the inheritance of characteristics determined by a gene present on the sex chromosomes.
- Any sudden change in DNA sequence may cause sex-linked disorders.
- Genes not located on the sex chromosomes are called autosomal genes.
- Examples of autosomal disorders are widows peak and freckles.
- Hemophilia is the most common example of X-linked recessive inheritance.
Sample Questions
Ques. Name the scientist who discovered a sex-linked inheritance disorder. (1 mark)
Ans. Thomas Morgan was the scientist who first discovered sex-linked disorder in fruit flies in the year 1910.
Ques. Mention any two characteristics of sex-linked inheritance. (2 marks)
Ans. There are various characteristics of sex-linked inheritance, but two of them include:
- Most of the sex-linked disorders are shown in males as they are heterozygous (having two different alleles).
- Females transfer the alleles to both their daughters and sons as they are homozygous (having two similar alleles).
Ques. Why does x-linked disorder affect only males? (2 marks)
Ans. X-linked disorders are common among males because they require only one copy of the X chromosome to cause the disease. On the other hand, two copies of X-linked genes are required to cause the same in females.
Ques. Define Y-linked Inheritance. State the reason why this type of inheritance occurs in males alone (2 marks)
Ans. Y-linked inheritance is defined as the gene inheritance that is located on the Y chromosome. This type of inheritance occurs in males, as females do not have a Y chromosome.
Ques. Why is Hemophilia more prevalent in males than females? (5 marks)
Ans. Hemophilia is a sex-linked disorder. The genes responsible for causing the disease are present on the X chromosome. The reasons behind the prevalence of the disease in males are as follows:
- Both X-chromosomes must possess the gene to be affected by the disease.
- If a female has one of the X-chromosomes affected, the female becomes a carrier of the disease.
- Males, on the other hand, have only one X chromosome that carries the gene for hemophilia disease.
Ques. If a trait is passed from a father to all his daughters, what kind of inheritance do they possess? (2 marks)
Ans. The trait passed from the father to all his daughters is called X-linked dominant inheritance.
- This is because daughters inherit one of the X chromosomes from their fathers.
- On the other hand, the sons get the Y chromosome from their affected fathers so they will not inherit the trait.
Ques. What is a gene mutation, and why does it occur? (2 marks)
Ans. Change in our body’s genetic material (DNA) is called mutation or gene mutation.
- Several factors cause gene mutations, such as exposure to chemicals, UV rays, errors in DNA replication, etc.
- An example of gene mutation is sickle-cell anemia, which leads to abnormal production of hemoglobin molecules.
Ques. Is Duchenne muscular dystrophy a sex-linked disorder? (3 marks)
Ans. Yes, Duchenne muscular dystrophy is a sex-linked disorder that causes muscle weakness and degradation of muscle fibers.
- This occurs due to a change or absence of a protein called dystrophin.
- Muscular dystrophy is passed from carrier mothers to their sons and is rarely passed on to daughters.
Ques. What is a carrier? (2 marks)
Ans. A carrier refers to a person who carries an affected gene of hemophilia. The most common carriers of hemophilia are females, as the genes are carried on the X chromosome and have one affected X chromosome, which can be passed on to their children.
Ques. Is sex-linked Inheritance similar in both Drosophila and humans? (1 mark)
Ans. Yes, the sex-linked inheritance is similar in both Drosophila (fruit fly) and humans. In both species, females have two X chromosomes, while males have one X and one Y chromosome.
Ques. What do you understand by criss-cross inheritance? Give examples (5 marks)
Ans. Criss-cross inheritance is an inheritance where the transmission of a trait is from the father to his daughters and from a daughter to her son.
- This type of inheritance is also called skip inheritance because the trait is passed from the first generation to the second generation.
- Color blindness is one such example of a crisscross inheritance.
- If a father is colorblind, he will transmit the trait to his grandson via his daughter, who is a carrier.
Ques. What type of sex-linked inheritance disorder is hypertrichosis? (2 marks)
Ans. Hypertrichosis is a Y-linked disorder that results in excessive hair growth all over the body of a child at the time of birth. This type of disorder may occur in both males and females but is more prevalent in males.
Ques. Why are males referred to as hemizygous? (2 marks)
Ans. Males are referred to as hemizygous because they have only one copy of a gene, i.e., one X chromosome. For example, a man will be affected by color blindness since the gene responsible for causing it is present on the X chromosome, and unlike females, males do not have a second copy of the X chromosome to reduce the effect of the disease.
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