Morpho & Physio of Chromosomes

Explanation of X and Y Chromosomes and Their Regions

1. In human beings, there are two types of sex chromosomes – X and Y.

2. These chromosomes decide whether a person is male or female.

3. A female has two X chromosomes (XX), and a male has one X and one Y chromosome (XY).

4. The X and Y chromosomes are not exactly the same, but they do have some parts that are similar and some parts that are different.

5. The similar parts are called homologous regions.

6. The different parts are called non-homologous regions.

7. Homologous regions of X and Y chromosomes contain similar genes.

8. These similar genes help the X and Y chromosomes to pair up during cell division in males.

9. Crossing over is a process where chromosomes exchange pieces of their genetic material.

10. This crossing over happens only in the homologous regions of the X and Y chromosomes.

11. This means only the similar parts of the X and Y chromosomes take part in the exchange.

12. The non-homologous region of the X chromosome is larger than that of the Y chromosome.

13. This means that the X chromosome carries more genes in its different (non-homologous) region than the Y chromosome does.

14. The genes that are present only on the X chromosome and not on the Y chromosome are called X-linked genes.

15. These X-linked genes are found in the non-homologous region of the X chromosome.

16. Since females have two X chromosomes, they have two copies of these X-linked genes.

17. Males have only one X chromosome, so they have only one copy of these X-linked genes.

18. This is why some genetic conditions caused by X-linked genes are more common in males.

19. The Y chromosome also has a non-homologous region, but it is shorter and has fewer genes.

20. The genes that are found only on the Y chromosome and not on the X chromosome are called Y-linked genes.

21. These Y-linked genes are present in the non-homologous region of the Y chromosome.

22. Y-linked genes are passed only from father to son, since only males have Y chromosomes.

23. These genes mostly control male-specific functions, like sperm production.

24. In summary, homologous regions allow pairing and crossing over, while non-homologous regions carry genes specific to each chromosome.

25. This difference plays a big role in inheritance, especially in traits passed through X-linked or Y-linked genes.

Linkage Overview

• Linked Genes: Genes closely located on a chromosome, often inherited together.

• Inheritance Tendency: Genes on the same chromosome show a tendency to be inherited together.

• Discovery:

  • Plants: Discovered by Bateson and Punnett

  • Animals: Discovered by T. H. Morgan

• Types: Linkage is of two kinds.

-Complete Linkage

• Completely Linked Genes: Genes closely located on a chromosome, not separating (no crossing over), and inheriting together.

• Inheritance: These genes inherit together, leading to parental traits being passed to offspring.

• Example: X chromosome of Drosophila males exhibits complete linkage.

Incomplete Linkage

• Incompletely Linked Genes: Genes located farther apart on the same chromosome, with chances of separation by crossing over.

• Inheritance: These genes may separate, leading to the appearance of new traits in offspring.

• Example: In Zea mays, the color and shape of grains show incomplete linkage.

Linkage Groups

• Definition: All linked genes on a specific chromosome form a linkage group.

• Correspondence: The number of linkage groups in a species equals its haploid number of chromosomes.

• Examples:

• Drosophila melanogaster: 4 linkage groups corresponding to 4 pairs of chromosomes.

• Garden Pea: 7 linkage groups corresponding to 7 pairs of chromosomes.

Explanation of X and Y Chromosomes and Their Regions

1. In human beings, there are two types of sex chromosomes – X and Y.

2. These chromosomes decide whether a person is male or female.

3. A female has two X chromosomes (XX), and a male has one X and one Y chromosome (XY).

4. The X and Y chromosomes are not exactly the same, but they do have some parts that are similar and some parts that are different.

5. The similar parts are called homologous regions.

6. The different parts are called non-homologous regions.

7. Homologous regions of X and Y chromosomes contain similar genes.

8. These similar genes help the X and Y chromosomes to pair up during cell division in males.

9. Crossing over is a process where chromosomes exchange pieces of their genetic material.

10. This crossing over happens only in the homologous regions of the X and Y chromosomes.

11. This means only the similar parts of the X and Y chromosomes take part in the exchange.

12. The non-homologous region of the X chromosome is larger than that of the Y chromosome.

13. This means that the X chromosome carries more genes in its different (non-homologous) region than the Y chromosome does.

14. The genes that are present only on the X chromosome and not on the Y chromosome are called X-linked genes.

15. These X-linked genes are found in the non-homologous region of the X chromosome.

16. Since females have two X chromosomes, they have two copies of these X-linked genes.

17. Males have only one X chromosome, so they have only one copy of these X-linked genes.

18. This is why some genetic conditions caused by X-linked genes are more common in males.

19. The Y chromosome also has a non-homologous region, but it is shorter and has fewer genes.

20. The genes that are found only on the Y chromosome and not on the X chromosome are called Y-linked genes.

21. These Y-linked genes are present in the non-homologous region of the Y chromosome.

22. Y-linked genes are passed only from father to son, since only males have Y chromosomes.

23. These genes mostly control male-specific functions, like sperm production.

24. In summary, homologous regions allow pairing and crossing over, while non-homologous regions carry genes specific to each chromosome.

25. This difference plays a big role in inheritance, especially in traits passed through X-linked or Y-linked genes.

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Sex-linkage (Sex-linked Inheritance)

1. Sex-linkage means the passing on (inheritance) of genes that are found on the sex chromosomes – the X chromosome and the Y chromosome.

2. Normally, genes are passed through all chromosomes, but when genes are located on the X or Y chromosome, the pattern of inheritance is different.

3. These special patterns are called sex-linked inheritance.

4. In humans, there are 23 pairs of chromosomes, out of which one pair is the sex chromosomes – X and Y.

5. Females have two X chromosomes (XX), and males have one X and one Y chromosome (XY).

6. Some genes are found only on the X chromosome and not on the Y chromosome.

7. These are called X-linked genes, and they follow the pattern of X-linked inheritance.

8. Some genes are found only on the Y chromosome, and these follow the pattern of Y-linked inheritance.

9. Together, the passing of X-linked and Y-linked genes from parents to children is called sex-linked inheritance.

10. There are three types of sex-linked inheritance:

11. (1) X-linked inheritance – When genes present only on the X chromosome are passed from parent to child.

12. These genes can be passed from mother to both son and daughter, and also from father to daughter, but not from father to son (since sons get Y from father).

13. Examples of X-linked inheritance include haemophilia and colour blindness.

14. These conditions are more common in males, because they have only one X chromosome.

15. (2) Y-linked inheritance – When genes present only on the Y chromosome are passed from father to son.

16. These genes are never found in females because females do not have a Y chromosome.

17. Y-linked genes usually control male traits, like male fertility or development of testes.

18. This type of inheritance is also called holandric inheritance.

19. (3) XY-linked inheritance – This is a special term sometimes used to describe genes that are found in the homologous regions of both X and Y chromosomes.

20. These regions are similar, so the genes can be passed from either parent to either child.

21. XY-linked inheritance is very rare, and not many genes follow this pattern.

22. In summary, sex-linked inheritance is different from other types of inheritance because it depends on which sex chromosome carries the gene.

23. Understanding sex-linked inheritance helps us know why some genetic disorders appear more in one sex than the other.

24. It also explains how some traits or conditions are passed only from fathers to sons, or only through mothers.

25. That’s why studying X-linked, Y-linked, and XY-linked inheritance is important in genetics and biology.

Complete X-Linkage (X-linked traits)

1. These are genes found on the non-homologous region of the X chromosome only.

2. Since males have only one X chromosome, if the gene is defective, the male will definitely show the trait or disorder.

3. Females have two X chromosomes, so even if one has a defect, the other may cover up (this is called being a carrier).

4. X-linked traits are passed from mother to son, because mothers pass one of their X chromosomes to their children.

5. Examples of completely X-linked traits include:

• Haemophilia A: A bleeding disorder caused by the lack of clotting factor VIII.

• Red-green Colour Blindness: A condition where a person cannot clearly distinguish red and green colours.

• Myopia (Near-sightedness): A vision problem where a person can see near objects clearly, but distant objects are blurry.

• Ichthyosis: A skin disorder where the skin becomes dry, scaly, and rough like fish scales.

Complete Y-Linkage (Y-linked traits)

1. These are genes located on the non-homologous region of the Y chromosome.

2. Since only males have a Y chromosome, these traits are passed only from father to son.

3. Females do not inherit these genes because they do not have a Y chromosome.

4. Y-linked traits are usually related to male-specific functions, especially those related to sex determination or fertility.

5. Examples of completely Y-linked traits include:

• Hypertrichosis of the Ear: Excessive hair growth on the outer ear, passed from father to son.

• H-Y Antigen Gene: This gene is involved in organ rejection and male-specific immune response.

• Other Y-linked traits include genes for spermatogenesis (sperm production) and testis development.

IMPS(KEY TAKE AWAYS):

• Complete sex linkage means the gene is located only on one specific sex chromosome and not the other.

• X-linked traits can affect both males and females but are more commonly expressed in males.

• Y-linked traits affect only males and are passed directly from father to son.

• Since crossing over does not happen in these non-homologous regions, these traits are inherited in a fixed pattern
  

Incomplete Sex Linkage – Explanation and Examples

What is Incomplete Sex Linkage?

1. Incomplete sex linkage is seen in genes that are present in the homologous regions of both the X and Y chromosomes.

2. The homologous region is the part where the X and Y chromosomes are similar in structure and gene content.

3. These genes can be found on both chromosomes, unlike completely sex-linked genes which are present only on X or only on Y.

4. In this region, crossing over can happen during meiosis (the process of forming sperm or egg cells).

5. Because of this crossing over, the genes do not always get inherited together, leading to what is called incomplete linkage.

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Why is it called ‘Incomplete’ Sex Linkage?
6. It is called incomplete because the genes are not fixed only to one chromosome (X or Y), and their inheritance pattern can change due to crossing over.
7. Unlike complete sex linkage, these genes can sometimes shuffle places during the formation of reproductive cells.
8. As a result, the expected pattern of inheritance can be altered, which makes it more unpredictable.
9. Traits controlled by such genes can be seen in both males and females but may appear differently based on recombination.
10. These traits are also called XY-linked traits because they can be carried on either X or Y chromosome within the homologous region.

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Examples of Incomplete Sex-Linked Traits
Here are some traits that are inherited due to incomplete sex linkage:

1. Total Colour Blindness
11. This is a rare condition where a person cannot see any colours at all.
12. It is different from red-green colour blindness (which is completely X-linked).
13. This type of colour blindness is linked to a gene in the homologous region, so it may appear in both sexes in complex patterns.

2. Nephritis (Hereditary Nephritis or Alport Syndrome)
14. This is a kidney disease that may also affect hearing and vision.
15. The defective gene is found in the homologous region of the sex chromosomes, leading to variable inheritance.
16. Males usually show more serious symptoms, but females can also be affected if crossing over occurs.

3. Retinitis Pigmentosa
17. This is an eye disease where a person slowly loses night vision and peripheral vision.
18. It is caused by a gene located in the homologous part of the sex chromosomes.
19. Due to incomplete linkage, the trait may pass through unexpected combinations across generations.

IMPS(KEY TAKE AWAYS):
20. In incomplete sex linkage, the gene is shared by both X and Y chromosomes in their homologous parts.
21. Crossing over can happen in this region during the formation of sperm or egg cells.
22. Because of this, the genes do not always stay linked together, and inheritance becomes variable.
23. Both males and females can be affected, depending on how the genes are passed on.
24. This is different from complete sex linkage, where genes are stuck only on X or only on Y and do not shuffle.
25. Some important examples include total colour blindness, nephritis(also known as Alport Syndrome-inflammation (swelling) of the kidneys), and retinitis pigmentosa(progressive vision loss)