Mendelian Laws: Sex Chromosomes And Beyond

Mendel's laws of heredity are the 'law of dominance', the 'law of segregation', and the 'law of independent assortment'. Mendel's laws are founded on the formation of gametes through meiosis and inheritance of allelic variants at autosomal loci. Mendel's laws hold broadly true, but there are exceptions, including sex-linked inheritance. Sex chromosomes, such as the X and Y chromosomes, differ between the sexes and show different patterns of inheritance. While loci on autosomes follow the classic Mendelian pattern of inheritance, loci on sex chromosomes do not, as the X-chromosome and Y-chromosome are not homologous. This results in sex-linked inheritance, which violates Mendel's laws but provides a simple extension of Mendelian inheritance principles.

Sex chromosomes follow a sex-linked pattern of inheritance

Mendel's First Law does not apply to sex chromosomes, as they follow a sex-linked pattern of inheritance. Sex chromosomes, X and Y, differ between the sexes and show different patterns of inheritance in pedigrees from other chromosomes.

The inheritance patterns differ for genes on sex chromosomes (X and Y) compared to genes located on autosomes (non-sex chromosomes). This is due to the fact that, in general, females carry two X chromosomes (XX), while males carry one X and one Y chromosome (XY). Therefore, females carry two copies of each X-linked gene, but males carry only one copy of X-linked genes and no copies of Y-linked genes.

X-linked dominant disorders are caused by variants in genes on the X chromosome. In males, a variant in the only copy of the gene in each cell causes the disorder. In females, a variant in one of the two copies of the gene in each cell is sufficient to cause the disorder. Females may experience less severe symptoms than males.

X-linked recessive disorders are also caused by variants in genes on the X chromosome. In males, one altered copy of the gene in each cell is enough to cause the condition. In females, a variant would have to occur in both copies of the gene to cause the disorder. Therefore, males are affected by X-linked recessive disorders much more frequently than females.

A condition is considered Y-linked if the altered gene that causes the disorder is located on the Y chromosome. Because only males have a Y chromosome, in Y-linked inheritance, a variant can only be passed from father to son.

In summary, sex chromosomes follow a sex-linked pattern of inheritance, which differs from Mendel's First Law. The inheritance pattern depends on whether the gene is X-linked or Y-linked, and whether it is dominant or recessive.

X-linked genes in fathers are passed to daughters only

Mendel's laws of inheritance are foundational to transmission genetics. However, they do not apply to sex chromosomes, including X-linked genes, which show a crisscross inheritance pattern. In humans, females carry two X chromosomes (XX), while males carry one X and one Y chromosome (XY). Therefore, females carry two copies of each X-linked gene, but males carry only one copy.

In the case of X-linked recessive inheritance, the sons of a man with an X-linked recessive disorder will not be affected, and his daughters will carry one copy of the altered gene. So, while X-linked genes in fathers are indeed passed to daughters, they are not the only genes passed on by fathers. Fathers also pass Y-linked genes to their sons.

X-linked genes in mothers are passed to sons and daughters equally

Mendel's First Law does not apply to sex chromosomes, as sex-linked genes follow a different pattern of inheritance. This is because, in general, males carry one X and one Y chromosome (XY), while females carry two X chromosomes (XX). Therefore, females carry two copies of each X-linked gene, but males carry only one copy of X-linked genes.

X-linked genes show a crisscross pattern of inheritance. In the case of X-linked dominant inheritance, a woman passes one of her X chromosomes to each child. Therefore, a woman with an X-linked dominant disorder has a 50% chance of having an affected daughter or son with each pregnancy. In the case of X-linked recessive inheritance, a woman who carries an altered gene for X-linked recessive has a 50% chance of having sons who are affected and a 50% chance of having daughters who carry one copy of the altered gene.

A striking characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons; fathers pass X chromosomes to their daughters and Y chromosomes to their sons. In contrast, mothers pass X-linked genes to both sons and daughters.

Sex-linkage can be identified through reciprocal crosses

In genetics, a reciprocal cross is a breeding experiment designed to test the role of parental sex on a given inheritance pattern. Reciprocal crosses can be used to detect sex-linked inheritance.

To perform a reciprocal cross, two sets of crosses are conducted, with the sexes reversed in the second set. In other words, a male and a female with different phenotypes are crossed, and then a second set of crosses are performed with the phenotypes reversed relative to the sex of the parents in the first cross. This method can be used to determine whether a trait is autosomal or sex-linked, and whether it follows a pattern of complete or incomplete dominance.

For example, in Drosophila melanogaster, a white-eye mutation was found to be sex-linked by Thomas Hunt Morgan. He found that a cross between a white-eyed male and a red-eyed female produced only red-eyed offspring, while a cross between a red-eyed male and a white-eyed female produced male offspring with white eyes and female offspring with red eyes. This was because the white eye allele is sex-linked (specifically, on the X chromosome) and recessive.

Mendel's laws are manifestations of the formation of gametes through meiosis

Mendel's laws are indeed manifestations of the formation of gametes through meiosis. Mendel's First Law, also called the Law of Equal Segregation, states that during gamete formation, the two alleles at a gene locus segregate from each other, with each gamete having an equal probability of containing either allele. This is underpinned by the process of meiosis, where the mother's and father's genes are separated, and the character alleles are separated into two distinct gametes.

Mendel's laws are also reflected in chromosome movement during meiosis. For example, the principle of segregation occurs in anaphase I and II of meiosis, where homologous chromosomes are segregated into two daughter nuclei with their various versions of each gene. During meiosis, the behaviour of homologous chromosomes contributes to the separation of alleles into distinct gametes for each genetic locus.

Mendel's laws are not always applicable, and there are exceptions, referred to as non-Mendelian inheritance. For instance, sex-linked traits explain why some traits are commonly found in either males or females. Mendel's work was based on hermaphroditic pea plants, so his laws did not recognise the potential role of sex-limited or sex-linked inheritance. However, in many multicellular eukaryotes, sex is determined by the presence of sex chromosomes, and sex-linked inheritance provides a simple extension of the principles of Mendelian inheritance.

In summary, Mendel's laws are indeed underpinned by the formation of gametes through meiosis, and they provide a foundation for understanding inheritance patterns. However, there are exceptions to these laws, particularly when considering sex chromosomes and sex-linked inheritance.

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