When a sperm cell from the father meets an egg cell from the mother, the result is a new cell called a zygote which contains all the genetic information needed to develop into a human. This genetic information comes in the form of chromosomes which are passed down from each parent. But which parent gives which chromosomes to the zygote?
Chromosomes and Genes
Chromosomes are structures within cells that contain the cellular DNA. Human DNA is organized into 46 chromosomes found in 23 pairs. You inherit one chromosome per pair from each parent. The first 22 pairs of chromosomes are called autosomes. They control things like your height, eye color, blood type, and other physical features.
The 23rd pair of chromosomes are the sex chromosomes. 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, determining the sex of the child.
Genetic information is encoded in DNA molecules in the form of genes. Genes carry instructions for making proteins which have many different roles, such as building cells, breaking down toxins, and responding to environmental signals. You have two copies of each gene, one inherited from mom and one from dad.
Meiosis and Gamete Formation
In preparation for fertilization, reproductive cells undergo a special type of cell division called meiosis. During meiosis, the chromosome pairs separate and the number of chromosomes is reduced by half. This results in gametes (sperm or egg) containing only 23 chromosomes.
During meiosis in females, the two X chromosomes line up and one X chromosome is randomly distributed to each egg. All eggs contain a single X chromosome. During meiosis in males, the X and Y chromosome pair up and segregate randomly so that some sperm receive the X while others receive the Y chromosome. This explains why fathers determine the sex of the baby.
Fertilization and Early Development
At fertilization, the sperm and egg fuse to form a zygote containing 46 chromosomes – 23 from the father and 23 from the mother. The zygote will undergo many rounds of mitosis as it develops into an embryo and fetus. During mitosis, all 46 chromosomes are duplicated so that every cell contains two copies of each chromosome.
Because the chromosomes are mixed randomly during meiosis, each gamete is genetically unique. Therefore, the zygote formed from the union of a particular sperm and egg contains a unique combination of genetic information from each parent. Even siblings receive a different assortment of chromosomes and genes from their parents.
Dominant vs Recessive Genes
Some traits are controlled by genes that are dominant or recessive. If you inherit two different versions of a gene (one from mom, one from dad), the dominant gene will be expressed while the recessive gene will be masked. This explains why some traits run in families.
For example, free earlobes that hang down are dominant while attached earlobes are recessive. If one parent contributes a free earlobe gene and the other contributes an attached earlobe gene, the child will have free hanging earlobes since that trait is dominant. But if both parents contribute recessive attached earlobe genes, the child will have attached earlobes.
Some genetic diseases are carried on the X or Y chromosomes and are referred to as sex-linked conditions. Females have two X chromosomes so they have two chances to inherit such a condition while males have only one X chromosome. If the X chromosome inherits a disease allele, it will be expressed in males since they lack a second unaffected copy. Some examples are red-green color blindness, hemophilia, and Duchenne muscular dystrophy.
Sometimes abnormalities occur during meiosis or fertilization leading to missing, extra, or damaged chromosomes. Down syndrome is caused by an extra copy of chromosome 21. Turner syndrome results when all or part of an X chromosome is missing in a female. Klinefelter syndrome occurs when males inherit an extra X chromosome (XXY). Depending on the specific defect, chromosomal abnormalities can cause miscarriage, developmental disorders, or infertility.
Mitochondria are small organelles that generate energy in cells. They contain a small amount of DNA with just 37 genes. Mitochondria are inherited maternally through the egg cell, so mitochondrial genetic diseases are passed from mother to child. Leber hereditary optic neuropathy and myoclonic epilepsy are examples.
For most genes, the copy inherited from mom and the copy from dad are expressed equally. But some genes undergo imprinting which means they are expressed preferentially from either the maternally or paternally inherited chromosome. Imprinting helps distinguish certain parental roles that are important for normal development.
For example, insulin-like growth factor 2 promotes growth of the embryo and is only expressed from the paternal allele. Meanwhile, H19 regulates growth and is only expressed from the maternal allele. Errors in imprinting can lead to imprinting disorders like Prader-Willi, Angelman, and Beckwith-Wiedemann syndromes.
While a child inherits equal genetic contributions from both parents, the inheritance patterns for different traits and disorders can vary:
- Autosomes are inherited randomly from both parents.
- Sex chromosomes (XX or XY) are determined by the father.
- Dominant traits will be expressed when inherited from just one parent.
- Recessive traits require inheritance from both parents.
- Sex-linked traits are carried on X or Y chromosomes.
- Chromosomal abnormalities are caused by errors in meiosis and fertilization.
- Mitochondrial genes are inherited maternally.
- Imprinted genes are expressed preferentially from either the mother or father.
By understanding the complex patterns of genetic inheritance, we gain insight into human development, health, and disease.
|Chromosome Type||Number of Chromosomes||Inheritance Pattern|
|Autosomes||22 pairs||One copy inherited randomly from each parent|
|Sex Chromosomes||1 pair (XX or XY)||X from mother, Y from father determines sex|
|Mitochondrial||37 genes||Inherited maternally|
Examples of Genetic Inheritance Patterns
- Huntington’s disease
- Achondroplasia (dwarfism)
- Marfan syndrome
- Cystic fibrosis
- Sickle cell anemia
- Phenylketonuria (PKU)
- Red-green color blindness
- Duchenne muscular dystrophy
- Fragile X syndrome
- Heart disease
- Cleft lip/palate
- Leber hereditary optic neuropathy
- Myoclonic epilepsy with ragged red fibers
- Mitochondrial encephalomyopathy, lactic acidosis, stroke-like episodes (MELAS)
The wide variety of inheritance patterns across different genes and chromosomes demonstrates the complexity and diversity of human genetics. While we each inherit a unique combination of genes from both parents, certain traits, disorders, and conditions follow more predictable inheritance patterns based on the underlying biology.