What Does Homozygous Mean?

In genetics, "homozygous" means that a cell has two identical alleles for a single trait from both the mother and father cell. Alleles are present for each particular gene that exhibits a trait. The trait may be in appearance, such as hair color or eye color in human diploid cells, or it may be a mannerism such as a habit like biting your fingernails, which may have a hereditary link. When both parent cells contain the same allele, this particular allele will be in the same location on the two pairs of chromosomes such as in human diploid cells. Genes may be recessive or dominant in homozygous cells.

A homozygous cell in the human body, animals, insects and some bacteria has two sets of chromosomes, and the cell is referred to as a diploid cell. Homozygous cells have identical alleles located on the same area of the pair of chromosomes to exhibit a given trait. The alleles are received from both parent cells upon fertilization when the alleles pair up as homologous chromosomes. For example, a human cell has 23 pairs of chromosomes, with one pair from each parent for a total of 46 chromosomes. The alleles on the chromosomes give a child their traits or in the case of an organism, their characteristics.

Homozygous dominant cells contain two dominant alleles to express the dominant phenotype or expressed physical trait in humans and organisms. Homozygous recessive cells have an allele from each parent that carries the same recessive trait and expresses the recessive phenotype.

In a homozygous dominant organism, the organism has two copies of the same exact allele that is the dominant one. The dominant allele is usually written with a capital letter, such as Q, and the recessive allele for a gene is lowercased, such as q. So, a homozygous dominant organism will be expressed as QQ with two copies of the dominant genetic makeup. For example, in the case of human eye color, if the allele for brown is dominant and expressed as B, and the allele for blue eyes is recessive and expressed as b, a homozygous dominant person will have brown eyes with the genotype of BB.

In the case of a homozygous recessive human using the same information for eye color, they would have two copies of the same recessive gene, and it would be expressed with the genotype of bb with blue eyes.

A monohybrid cross is the study in which two parental (P generation) organisms are bred together and they have a different trait from each other. The P generations are homozygous, but they have different alleles for that particular trait.

For example, in the case of bean pod color in Gregor Mendel's garden experiments, the dominant allele for green pod color is G, and the recessive allele for yellow pod color is g. The genotype for the green pod color is GG and for the recessive pod color is gg. When the two cross-breed as a monohybrid cross, all pod colors will be expressed as Gg, and the cross results in all green pod colors because the dominant allele is the one that is expressed.

In 1905, many years after Mendel's garden experiments, mathematician Reginald Crundall Punnett developed what is known today as a Punnett square. This is a method for determining the probability of inheritance of traits from the cross of two organisms, which can be plants or humans with diploid cells.

The diploid cells carry two copies of the genes, which can be identical and homozygous or have variations in the alleles, making them heterozygous.

To use a Punnett square, draw a diagram with two parallel horizontal lines and two parallel vertical lines that intersect at a 90-degree angle. Basically, draw a graph like you would to play tic-tac-toe. Leave the top left corner square empty and enter a genotype with one letter for each pair of alleles for one parent in the next two boxes across the top of the diagram. Enter the genotype in the boxes to the left for the pair of alleles for the same gene in the other parent.

For example, both parents are carriers of a mutation for a disorder, and the genotype for each parent is Aa. The three squares in the top line would be blank, A and a; the left column would be blank, A and a. Fill in the other four squares with the potential combinations – you'd have AA in one square, Aa in two different squares and aa in the remaining square – and count each of the four boxes with two letters as a 25 percent chance to get the probability of a child in each pregnancy obtaining the disorder, being a healthy carrier or not even having the recessive gene disorder.

The chance of having the recessive disorder (aa) is 25 percent, the chance of being a healthy carrier (Aa) is 50 percent and the chance that each child will be healthy and not even carry the recessive allele (AA) is 25 percent.

Mutations of a DNA sequence occur when the chromosomes have genetic changes. It can be the loss or gain of portions of DNA and cause the cells to malfunction or not function at all. If identical gene mutations occur on both alleles of a homologous chromosome, then the mutation is called a homozygous mutation. Homozygous mutations are also called recessive mutations in which each allele from each parent organism contains abnormal versions of the same gene.

Gene mutations can be inherited from one or both parent organisms. In humans, this can produce gene disorders in the phenotype and can lead to a genetic disease. There are six different types of gene mutations: autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, Y-linked inheritance, and maternal or mitochondrial inheritance.

Autosomal dominance is noted when the abnormality or abnormalities appear in every single generation. The defective gene is located on a specific numbered chromosome. In the example that a female has autosomal dominance, every child born to her would have a 50 percent chance of inheriting the same genetic malfunction. The same percentage of chance is associated with the mutation whether the parent is male or female.

A child that carries one copy of the genetic disorder or mutation from one parent is considered a carrier of the disorder. They usually don't have symptoms of the disease, but carriers can pass the allele along to their children, making the children carriers themselves.

Autosomal recessive inheritance occurs when both parents do not show any signs of the disease, as they are both carriers, but they both pass the recessive allele to a child, who then expresses the recessive trait. The chance that two carrier parents may have a child with a disease is 25 percent for each and every pregnancy and are equally distributed between both male and female children. If a child were to have the symptoms of the disease, that child would have to receive the recessive genes from both of the carrier parents.

In X-linked recessive inheritance, it is more likely for males to get a gene defect than females. This is because the abnormal gene is on the X or female chromosome, so males don't transmit it to their sons, because the sons get the Y chromosome from the father. However, males can transmit it to their daughters. Females have two X chromosomes in which one may be abnormal and the other normal, and the normal one masks the abnormal gene. Therefore almost all daughters born to a man infected by a genetic disorder may seem normal and have no outward symptoms of the disease, but they are carriers. Each time the daughter as a carrier has a son, there is a 50 percent chance for her to pass on the abnormality to him.

X-linked dominant inheritance is very rare, and one example is vitamin D-resistant rickets.

Chromosomal disorders are a defect that is due to either an excess or a lack of the genes in a segment of a chromosome or in the entire chromosome.

Multifactorial disorders contribute to most of the common diseases seen today. The interactions of several genes in the environment or an illness or medication is responsible for these disorders. They include asthma, cancer, coronary heart disease, diabetes, hypertension and strokes.

Mitochondrial DNA-linked disorders are malfunctions in the small structures of mitochondria that are present in most of the cells in a human body. These types of genetic mutations are passed down to the children from the mother because mitochondrial DNA transfers to children through the female egg only. Nuclear DNA occurs in the nucleus of cells, but mitochondrial DNA is only a small portion of DNA in a cell. These disorders can appear at any age and not only at birth with a wide variety of signs and symptoms. The disorders include blindness, a delay in development, gastrointestinal upset, loss of hearing, heart rhythm malfunctions, metabolic problems and shortness in stature.

Zygosity in an organism is the level of similar genetic alleles for a particular trait. In the case of homozygous vs. heterozygous cells, the homozygous cells contain the exact same allele on the same area of both pairs of chromosomes with one from each parent cell. Diploid organisms contain two sets of chromosomes. Homozygous traits will be displayed in the resulting cell that is a cross between both parents' cells. In heterozygous cells, the two alleles for a particular trait in the same area on the homologous pair of chromosomes will have a differentiation and not be identical. Homologous pairs of chromosomes code for the same genes. Homologous pairs of chromosomes are found in all chromosomes except for the sex chromosomes, where the male sex chromosome is the Y chromosome and in the female sex chromosome it is the X chromosome.

Homozygous organisms have identical alleles and may have dominant or recessive alleles in a pair, but may not contain both dominant and recessive alleles. Heterozygous organisms contain distinct and dissimilar alleles that result in offspring with diverse genotypes and have both a dominant and a recessive allele.

In humans, there are large amounts of hereditary traits, which can be heterozygous or homozygous and dominant or recessive due to the alleles for each gene from each parent. In the case of the appearance of human ears, they may be attached ear lobes (e) or detached ear lobes (E). The actual genetics behind earlobe attachment are more complex than a simple Punnett-square example can address, but for the sake of this article, say that the dominant trait is detached earlobes, expressed as homozygous dominant as EE for the genotype. The heterozygous genotype is Ee and the homozygous recessive genotype is ee. In this example, the genotype of Ee will still have the same dominant trait with detached earlobes.

Other dominant and recessive human traits include eye color, in which brown is dominant over all other colors of gray, green, hazel and blue. In the case of vision, farsightedness is dominant over normal vision, so this explains why so many humans require corrective lenses. Nearsightedness, night blindness and color blindness are all recessive to the normal vision traits.

Human hair has several dominant traits as well. These include dark hair color, non-red hair, curly hair, a full head of hair and a widow's peak. The corresponding recessive traits for each of these dominant traits are blonde, light or red hair color, straight hair, baldness and a straight hairline.