What genes do mothers pass on?

Genetics plays a crucial role in determining many aspects of a child’s development and health. When conceiving a child, both parents contribute genetic material in the form of DNA that combines to create a new, unique individual. However, there are some distinct differences between the genetic contributions of mothers and fathers.

Mothers and fathers each provide 23 chromosomes to their offspring – 22 autosomes and 1 sex chromosome. Autosomes are numbered chromosome pairs that contain genes responsible for most bodily functions and development. The 23rd pair of chromosomes are known as the sex chromosomes, and these determine whether a child will be male or female.

Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). The mother always contributes an X chromosome, while the father contributes either an X or a Y chromosome. This determines the sex of the baby.

Mitochondrial DNA

In addition to the 23 chromosome pairs, mothers also contribute their mitochondrial DNA (mtDNA) to their children. Mitochondria are small structures within cells that generate energy. They have their own small circular strand of DNA separate from the DNA contained in the cell nucleus.

During conception, the sperm fertilizes the egg and contributes the father’s nuclear DNA. However, the sperm does not contain any mitochondria. Therefore, the mitochondria and mtDNA present in the fertilized egg all come directly from the mother. This mtDNA is passed on unchanged from mothers to both male and female children.

X-Linked Genes

One of the key genetic differences between males and females is that females have two X chromosomes while males have only one. The X chromosome contains approximately 5% of the total genes in the human genome. As such, females have two copies of each X-linked gene, while males only have one copy.

This has important implications for what genetic traits or disorders mothers can pass on to their sons versus their daughters. X-linked recessive disorders like color blindness, hemophilia, and Duchenne muscular dystrophy primarily affect males. Females may be carriers, but they typically do not show symptoms since their second X chromosome compensates. However, they will pass the defective gene onto 50% of their sons.

Autosomal Recessive Disorders

Autosomal recessive disorders are caused by mutations in genes on the 22 non-sex chromosomes. Both biological sexes have two copies of these autosomal genes. Recessive disorders only manifest if both copies of a gene are defective. If only one copy is faulty, the individual will be a carrier but unaffected by the disorder.

Carriers of autosomal recessive disorders can pass the mutation on to their children. If both parents are carriers, there is a 25% chance with each pregnancy of having an affected child. Examples of autosomal recessive disorders include cystic fibrosis, sickle cell disease, and Tay-Sachs disease.

Autosomal Dominant Disorders

In autosomal dominant disorders, only one copy of the defective gene is needed to cause disease. Individuals with just one faulty gene copy will manifest symptoms. Each child of an affected individual has a 50% chance of inheriting the mutation.

Examples of autosomal dominant disorders that can be passed from mother to child include Huntington’s disease, neurofibromatosis, and hereditary breast/ovarian cancers linked to BRCA mutations. Males and females are equally likely to inherit and be affected by these conditions.

Multifactorial Inheritance

Many common diseases and disorders do not follow simple Mendelian inheritance patterns. Instead, they are influenced by variants in multiple genes, often interacting with environmental factors. These multifactorial conditions include heart disease, diabetes, cancer, mental illness, and autoimmune disorders. The inheritability of these conditions is complex.

Mothers can still pass on genetic risk for multifactorial diseases. However, the child’s likelihood of developing the condition depends on a combination of genetic and lifestyle factors. Estimating the inheritance risk requires looking at family history and the effect sizes of implicated genetic variants.

Mitochondrial Inheritance

Because mothers pass on their mitochondria to children, mitochondrial genetic disorders exhibit maternal inheritance patterns. Mitochondrial disorders are complex, often involving both mitochondrial DNA mutations as well as nuclear DNA mutations that affect mitochondrial function.

Example mitochondrial disorders include Leber’s hereditary optic neuropathy (LHON), mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS), and myoclonic epilepsy with ragged-red fibers (MERRF). These conditions causes symptoms related to deficient energy production in the muscles, brain, heart, liver, and other systems.

Implications of Maternal Inheritance

The unique contribution of mtDNA and X chromosomes from mothers means they play an outsized role in passing on certain genetic conditions to their children. This has several important implications:

  • X-linked disorders disproportionately affect sons
  • Mitochondrial disorders are only passed from mother to child
  • Gene mutations interact differently in mothers vs fathers
  • Family history on the maternal side provides key insights
  • Genetic counselling may be approached differently based on sex

Overall, the sex-specific genetic contributions of mothers and fathers lead to some differences in the inheritance patterns of various diseases and traits. Understanding maternal inheritance provides key insights for genetic counselling, family planning, and personalized medicine.

Examples of Disorders Inherited from Mothers

Here are some specific examples of genetic disorders that can be passed down from mothers to their children:

Color Blindness

Color blindness is an X-linked recessive disorder causing inability to distinguish between certain colors. It primarily affects males. Mothers who carry one copy of the defective gene have a 50% chance of passing it on to their sons.

Hemophilia

Hemophilia A and B are X-linked recessive bleeding disorders caused by lack of clotting factor VIII or IX. They follow the same maternal inheritance pattern as color blindness. Women carriers pass the defective gene to half of their sons.

Fragile X Syndrome

Fragile X syndrome, the most common inherited form of intellectual disability, is caused by mutation of the FMR1 gene on the X chromosome. It exhibits complex inheritance, but the premutation form can expand and become a full mutation when passed from mother to child.

Duchenne Muscular Dystrophy

Duchenne muscular dystrophy results from mutations in the dystrophin gene on the X chromosome. It primarily affects boys and is passed on by mothers who are carriers of the defective gene.

Huntington’s Disease

Huntington’s is an autosomal dominant neurodegenerative disorder caused by a CAG trinucleotide repeat expansion in the HTT gene. Individuals affected with Huntington’s have a 50/50 chance of passing on the expanded allele to their children, including their daughters.

Lactic Acidosis and Stroke-like Episodes (MELAS)

MELAS is a mitochondrial disorder caused by point mutations in mtDNA. Because fathers do not contribute mtDNA, only mothers can pass on the genetic mutations that cause MELAS to their children.

Epidermolysis Bullosa

Epidermolysis bullosa is a group of blistering skin disorders caused by mutations in genes important for anchoring the epidermis to the dermis. Both autosomal dominant and recessive subtypes can be inherited from mothers.

Conclusion

Mothers contribute genetic material to their children in the form of nuclear DNA, mitochondrial DNA, and X chromosomes. This enables them to pass on a variety of genetic traits and disorders, including X-linked, autosomal dominant, autosomal recessive, and mitochondrial conditions. Genetic counselling and family planning may need to take into account the maternal pattern of inheritance.

While fathers also pass on genetic mutations, mitochondrial disorders and X-linked conditions disproportionately follow the maternal line. Understanding what mothers can transmit genetically provides insight into the health and development of their children.

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